Session Q01: Poster Session III

Photoionization of the n$=$4 subshells along the Xe isonuclear sequence

Inner shell photoionization cross sections angular distributions of 4s, 4p and 4d subshells are studied along the isonuclear sequence Xe, Xe$^{\mathrm{6+}}$, Xe$^{\mathrm{8+\thinspace }}$using the relativistic random phase approximation RRPA [1] and the RRPA \textit{with} relaxation [2]. Photoionization time delay [3, 4] is calculated as well. The effect of the increasing nuclear charge on the Cooper minimum [5] of the n $=$ 4 subshells is studied in some detail. Photoionization time delay is found to be sensitive to electron correlations and is addressed both the relaxed and unrelaxed relativistic RRPA. The atomic time delay measured in the XUV/IR two-photon ionization experiments consists of two components: the Wigner-Eisenbud-Smith (WES) component [6-8], and the Coulomb laser coupling component. The WES component is associated with the XUV photon absorption [9], and is the main focus of the present study. Results obtained from the RRPA and the RRPA-R are compared to understand the effects of relaxation on the photoionization dynamics. [1] W. R. Johnson and C. D. Lin, Phys. Rev. A \textbf{20}, 964 (1979); [2] V. Radojevi\'{c} and W. R Johnson, Phys. Rev. A \textbf{31, }2991 (1985); [3] A. S. Kheifets, Phys. Rev. A \textbf{87}, 063404 (2013); [4] S. Saha \textit{et al.,} Phys. Rev. A \textbf{90}, 053406 (2014); [5] J. W. Cooper, Phys. Rev. \quad A \textbf{47, }1841 (1962); [6] E. P. Wigner, Phys. Rev. \textbf{98}, 145 (1955); [7] L. Eisenbud, Ph.D. thesis, Princeton University, 1948; [8] F. T. Smith, Phys. Rev. \textbf{118}, 349 (1960).   

Photoemission time delay in quadrupole ionization channels from free and confined xenon

Nondipole effects in photoionization in the past were believed to be insignificant below \textasciitilde 5 keV. However, evidence of quadrupole (E2) transitions on the photoelectron angular distribution at rather low photon energies has been reported [1, 2]. Experimental studies [3, 4] have confirmed this. Studies of the quadrupole ionization from different subshells of xenon, both free (Xe), and confined (Xe@C$_{\mathrm{60}})$ [5, 6] indicate the presence of Cooper minima in E2 transitions, and important of interchannel coupling and confinement effects have been found. Photoionization time delay is of high interest, but studies of time delay in E2-channels is scanty [7, 8]. This present study reports E2 photoionization time delay for free Xe and Xe@C$_{\mathrm{60}}$ to investigate temporal photoionization dynamics in nondipole transitions. [1] W. R. Johnson and K. T. Cheng, Phys. Rev. A~\textbf{63}, 022504 (2001); [2] M. Ya. Amusia, A. S. Baltenkov, Z. Felfli and A. Z. Msezane, Phys. Rev. A~\textbf{59}, R2544 (1999); [3] O. Hemmers, \textit{et al},~Phys. Rev. Lett.~\textbf{91}, 053002 (2003); [4] O. Hemmers, \textit{et al},~Phys. Rev. Lett.~\textbf{93}, 113001 (2004); [5] P. C. Deshmukh, T. Banerjee, K. P. Sunanda and H. R. Varma Radiat. Phys. Chem.~\textbf{75}, 2211 (2006); [6] K. Govil and P. C. Deshmukh J. Phys. B~\textbf{42}, 175003 (2009); [7] A. Mandal, S. Saha, T. Banerjee, P. C. Deshmukh, A. Kheifets, V. K. Dolmatov and S. T. Manson, ~J. Phys. Conf. Ser. \textbf{635}, 092097 (2015); [8] A. Kumar, H. R. Varma, P. C. Deshmukh, S. T. Manson, V. K. Dolmatov and A. Kheifets,. (2016). Phys. Rev. A~\textbf{94}, 043401 (2016).   

Photoionization of Molecular Endohedrals.

We calculate the photoionization cross-section of a molecular endohedral M@C$_{\mathrm{N}}$. We limit ourselves to two-atomic molecules. The consideration is much more complex than for atomic endohedrals because the system even for almost spherical C$_{\mathrm{N}}$ has only cylindrical instead of spherical symmetry. But M@C$_{\mathrm{N\thinspace }}$is more interesting since the interelectron interaction in molecules is relatively stronger than in similar atoms. We present results of calculations of molecular hydrogen H$_{\mathrm{2}}$ stuffed inside almost spherical fullerene C$_{\mathrm{60}}$ -- H$_{\mathrm{2}}$@C$_{\mathrm{60}}$. For comparison, we perform calculations also for atomic endohedral He@C$_{\mathrm{60}}$. The results are obtained both in single-electron Hartree-Fock approximation and with account of multi-electron correlations in the frame of so-called random phase approximation with exchange -- RPAE. The presence of the fullerenes shell results in prominent oscillations in the endohedrals photoionization cross section. The role of interelectron correlations becomes clear by comparing HF and RPAE results for H$_{\mathrm{2}}$@C$_{\mathrm{60}}$ and He@C$_{\mathrm{60\thinspace }}$on the one side with that for H$_{\mathrm{2}}$ and He, on the other.   

On Demand

1. Determination of accurate iron opacity at the boundary of the radiative and convection zones in the sun as been under considerable investigation as it will provide the information on elemental abundances in the sun and a test ground for both the theory and experiment. Opacity depends on the radiation absorption via photoiozation and photo-excitations of the constituent ions in the plasmas. The on-going work for photoiozation of Fe~XVII and Fe~XVIII, 2 of the 3 main contributors to iron opacity at the boundary, using large wavefunction expansions that include 218 levels for Fe~XVII and 276 for Fe~XVII in close coupling approximation have been completed. The $\Delta n$=1 excitations in the core introduce huge resonant absorption in photoionization increasing the opacity significantly. This reduces the discrepancy between the predicted and expected opacities. The relavant features will be illustrated. 2. Phosphorus is a basic element of life and is abundant in the solar system. However, its presence in other astronomical objects has been undetectable until recently. JWST is expected to obtain high resolution infrared spectra of exoplanets. Predicted spectra of P~II will be presented for possible detection by JWST   

Relativistic effects in the photoelectron dynamics of Z$=$118

High-Z atoms are excellent laboratories to study the combination of relativistic and many-electron correlation effects in their electronic structure and dynamics. Radioactive Z$=$118 is very difficult to study experimentally, but. there are theoretical studies [1,2]. In the present work, the relativistic-random-phase approximation (RRPA) [3] at different levels of truncation is employed to explore the final-state correlation effects in the photoelectron dynamics of Z$=$118 to illustrate the relativistic effects resulting from coupling different photoionization channels arising from spin-orbit split subshells, termed the spin-orbit interaction activated interchannel coupling (SOIAIC) effect [4,5]. Coupling of channels arising from spin-orbit split subshells can cause significant changes in the energy dependence of the photoionization parameters in the near-threshold region. Comparison between photoelectron dynamics of Rn on a qualitative level is also carried out in this work, since Z$=$118 is a homologue of Rn with regard to photoionization dynamics. Photoelectron dynamics of 7p, 7s and 6d subshells are investigated and comparison between Z$=$118 and Rn is made. [1] V. Pershina \textit{et al}, J. Chem. Phys. \textbf{129,} 144106 (2008); [2] E. Eliav E \textit{et al}, Phys. Rev. Lett. \textbf{77}, 5350 (1996); [3] W. R. Johnson and C. D. Lin, Phys. Rev. A \textbf{20}, 964 977 (1979); [4] M. Ya. Amusia \textit{et al}, Phys. Rev. Lett. \textbf{88}, 093002 (2002); [5] S. S. Kumar \textit{et al}, Phys. Rev. A \textbf{79}, 043401 (2009).   

Relativistic effects in the photoelectron angular distribution of $s$-states of superheavy elements

The influence of relativistic effects on the electronic structure of heavy and superheavy atoms has been discussed [1,2]. However, the effect of relativity on photoionization dynamics from these elements is seldom studied. The angular distribution of photoelectrons is known to be sensitive to relativistic effects [3]. In the present work, the relativistic-random-phase approximation (RRPA) [4] at different levels of truncation is employed to find the relativistic and correlation effects in the angular distribution of photoionization from \textit{ns} subshells of superheavy elements up to Z$=$118. We find that relativistic interactions in the final continuum states are strong enough to engender a substantial of the value of the dipole angular distribution $\beta $ parameter from its non-relativistic value of 2. Z$=$118 is a homologue of Rn and a qualitative comparison is made with photoelectron angular distribution of Rn [7], in which the relativistic effects are smaller relative to Z-118. [1] P. Pyykko, Adv. Chern. Res. \textbf{11}, 353 (1978); [2] \textit{Relativistic Effects in Atoms, Molecules, and Solids}, edited by G. L. Malli (Plenum, New York, 1983); [3]S. T. Manson and A. F. Starace, Rev. Mod. Phys. \textbf{54}, 389 (1982); [4]W. R. Johnson and C. D. Lin, Phys. Rev. A \textbf{20}, 964 (1979); [5] V. Pershina \textit{et al}, J. Chem. Phys. \textbf{129}, 144106 (2008); [6] E. Eliav \textit{et al}, Phys. Rev. Lett. \textbf{77}, 5350 (1996); P. C. Deshmukh, V. Radojevic, and S. T, Manson, Phys. Rev. A \textbf{45}, 6339 (1992).   

Photoionization of C$_{\mathrm{60}}$ Well Above Threshold

Calculations of the photoionization cross section and asymmetry parameter, $\beta $, are performed at the density functional (DFT) and time dependent density functional (TDDFT) levels for all 32 valence levels of C$_{\mathrm{60}}$ well above threshold for the isolated molecule in icosahedral symmetry with the aim of delineating how these cross sections fall off with energy. An accurate description of the molecular structure is required because the dipole matrix element must be generated close to a carbon nucleus at the higher energies to satisfy the conservation of momentum; model jellium potentials, which are useful at the photon energies near threshold, are not accurate at the higher energies. Included in the rich phenomenology exhibited by the results is the observation that confinement resonances extend out at least to 1 keV for virtually all of the valence levels.   

Inner-shell electron spectroscopy using hard x rays

Photoelectron and Auger electron spectroscopies excited by tunable synchrotron radiation are sensitive to electronic structure, photoionization dynamics, and core-hole decay mechanisms. These topics attract the interest of theorists who seek to explain and model the observed phenomena. The intense, polarized, tunable, narrow bandwidth x rays produced by beamlines at the Advanced Photon Source (APS) are ideal for Hard X-ray Photoelectron Spectroscopy (HAXPES). We are developing a HAXPES instrument for experiments at the APS using a high-resolution, high-collection-efficiency electron analyzer. The first experiments will explore two topics. The first is to characterize inner-shell resonance and threshold effects by tuning the x-ray energy across \textit{K}-edges of atoms and small molecules. Our second goal is to characterize double-core-hole states in molecules in which hollow core shells are produced by single-photon absorption and generated by electron correlation. The electron spectra will record states with one core-ionized electron and one core-excited electron [1]. [1] D. Koulentianos \textit{et al.}, J. Chem. Phys. \bf{149} 134313 (2018).   

Photoionization of atom-fullerene hybrid levels in F@C$_{\mathrm{60}}$ versus F$^{\mathrm{-}}$@C$_{\mathrm{60}}^{\mathrm{+}}$.

A density functional study to model the ground state structure and to calculate the photoionization properties of the F@C$_{\mathrm{60}}$ endohedral molecule is performed. The method goes beyond the local density approximation by employing the Leeuwen-Baerends (LB) exchange-correlation functional [1] and the interaction of the molecule with the radiation field is described in a linear response framework [2]. The study primarily focusses on the photodynamics of atom-fullerene hybrid levels of $p $angular momentum character. It is found that this hybridization is weak for F@C$_{\mathrm{60}}$ as well as for F$^{\mathrm{-}}$@C$_{\mathrm{60}}^{\mathrm{+}}$ formed after an electron transfers to F from most any of the C$_{\mathrm{60}}$ levels. However, when this electron originates from the C$_{\mathrm{60}}$ hybridizing level itself, the hybridization is strengthened dramatically. This contradicted the hole-level universality found for Cl$^{\mathrm{-}}$@C$_{\mathrm{60}}^{\mathrm{+}}$ earlier [3]. Consequently, significant variations in photoionization cross sections at the plasmonic energies of the spectrum and beyond emerge. Detailed results will be presented in the conference. [1] R. Van Leeuwen and E. J. Baerends, \textit{Phys. Rev. A} \textbf{49}, 2421 (1994); [2] M. E. Madjet \textit{et al}., \textit{J. Phys. B} \textbf{41}, 105101 (2008); [3] D. Shields, R. De, M. E. Madjet, S. T. Manson, and H. S. Chakraborty (submitted) \underline {arXiv:1907.04881}~ [physics.atm-clus].   

On Demand

We calculate and study the properties of subshell photoelectron spectra of fullerene molecules endohedrally doped by noble metals. The cases of Cu@C$_{\mathrm{60}}$ and Ag@C$_{\mathrm{60}}$ have been considered. The numerical methods include the application of a gradient corrected Leeuwen-Baerends (LB) exchange-correlation functional [1] within density functional theory. The dipole coupling of the molecule with the photon is described in a linear response treatment, called the time dependent local density approximation (TDLDA) augmented by LB [2]. Metal-fullerene ground state hybridization predicted for outer electrons of $s$, $p$ and $d$ angular momentum character significantly affects the structures of photoionization cross sections of these hybrid levels. The hybridization character is not found to change appreciably for likely more stable configurations Cu$^{\mathrm{-}}$@C$_{\mathrm{60}}^{\mathrm{+}}$ and Ag$^{\mathrm{-}}$@C$_{\mathrm{60}}^{\mathrm{+}}$ after a fullerene electron transfers to the metal to form closed shell atomic anions. In fact, the general behavior of the hybrid cross sections also appear qualitatively similar to those of closed shell Zn@C$_{\mathrm{60}}$ and Cd@C$_{\mathrm{60}}$ in the periodic table, aside from shifts in energy. [1] R. Van Leeuwen and E. J. Baerends, \textit{Phys. Rev. A} \textbf{49}, 2421 (1994); [2] M. E. Madjet \textit{et al}., \textit{J. Phys. B} \textbf{41}, 105101 (2008).   

Application of tunable infrared pulses for pump-probe study of XUV photoionization and dissociation dynamics

We use tunable near-infrared and short-wave infrared pulses in conjunction with extreme ultraviolet (XUV) attosecond pulse trains to study the photoionization and photodissociation dynamics in atoms and molecules. The tunability of IR probe fields allows us to control the interferences between photoionization pathways of XUV excited Rydberg states. In an argon atom, we use tunable IR to control the outgoing electron energy relative to spin-orbit split ionization thresholds to understand the contributions of different angular momentum states. In another experiment, we explore changes in angular distributions of XUV ionized photoelectrons due to couplings introduced by an IR field. We also extend our tunable IR spectroscopy approach to study the photoionization and photodissociation of polyatomic molecules.   

Theoretical Studies of ground state and photoionization dynamics of Na$_{\mathrm{\mathbf{x}}}$ inside C$_{\mathrm{\mathbf{n}}}$ (x=20 and n=240)

A number of studies has already been reported on the photoresponse of Na [1] clusters and fullerenes [2]. The hollow space inside fullerene cages can accommodate small metal clusters [3, 4] to form exotic cluster endofullerene molecules. In this work, the ground state of such an endohedral systemis modelled in a jellium-based density functional method with a gradient corrected exchange-correlation functional (LB94) [5] before calculating the system's photoionization in a linear response framework called time-dependent local density approximation (TDLDA) [2]. Plasmon and confinement resonances are found to be the key features of the photoionization cross-sections of the system. Comparisons with free systems, Na$_{\mathrm{x}}$and C$_{\mathrm{n}}$, facilitate detailed understandings of the results. \textbf{References} [1] Chunlei Xia, Chunrong Yin, and Vitaly V. Kresin, Phys. Rev. Lett. 102, 156802 (2009). [2] J. Choi et al., Phys. Rev. A 95, 023404 (2017). [3] Cabrera-Trujillo et al., Phys. Rev. B 53, 16059 (1996). [4] Cabrera-Trujillo et al., Current Problems in Condensed Matter(1998), pp 133-141, Springer, Boston, MA [5] R. van Leeuwen and E. J. Baerends, Phys. Rev. A 49, 2421 (1994).   

Resonant inter-Coulombic decay (ICD) of innershell photoexcitations in halogen@C$_{\mathrm{60}}$ endohedral molecules

Considering halogen@C$_{\mathrm{60}}$ endohedral molecules, we study the transfer-decay of photoinduced innershell vacancies of the atom or the fullerene through the ``other'' ionization continuum, that is, respectively, \textit{via} the fullerene or atomic continuum. This process, driven by the Coulomb-type long range coupling between the members of the `dimer' and augmented by various degrees of wavefunction hybridizations, is generally known as the inter-Coulombic decay (ICD). A density functional study is employed to model the ground state structure of the molecules and their ionization response to the radiation field is treated in a linear response framework [1]. Previous predictions of ICD based were reported for various noble gas endofullerenes [2,3]. The current research, on the other hand, investigates effects of open-shell structures of the halogen atoms on ICD resonances. Focus has also been given to observe how the ICD features evolve when a fullerene electron fills the halogen outer vacancy. Detailed results will be presented in the conference. [1] Madjet \textit{et al}., \textit{Phys. Rev. A} \textbf{81}, 013202 (2010); [2] Javani \textit{et al}., \textit{Phys. Rev. A} \textbf{89}, 063420 (2014); [3] De \textit{et al.}, \textit{J. Phys. B} \textbf{49}, 11LT01 (2016).   

Intermolecular Relaxation Process in Carbon Dioxide Dimers and Oxygen Dimers

Excited systems embedded in an environment can efficiently transfer their energy to neighboring species via an ultrafast de-excitation mechanism known as the Inter-Coulombic Decay (ICD). Significantly more studies have been carried out on atomic clusters versus molecular clusters. Compared to atomic clusters, new questions arise in the investigation of molecular clusters, e.g., how does the higher structural complexity of molecular clusters affect ICD? What other ultrafast relaxation processes can be present, and how do they compete with ICD? To tackle these questions, we have investigated the fragmentation dynamics of CO$_2$ dimers and O$_2$ dimers after single XUV photon absorption. Specifically, we focused on the investigation of the symmetric fragmentation channels of the doubly charged dimers using COLTRIMS to measure the particles 3D momenta. We found that the direct dissociation or autoionization of CO$_2^{+*}$ is suppressed due to the fast relaxation of the dimer via ICD. For O$_2$ dimers the relative emission angle between the two electrons showed contributions from ICD as well as knock-off processes \footnote{W. Iskandar et al., Phys. Rev. A \textbf{99}, 043414}.   

Resonances in Valence Photodetachment of Li- below the Opening of the K-shell

Calculations of the photodetachment cross sections for the valence 2s$^2$ shell of Li$^-$ have been performed using the Belfast R-matrix code (Berrington, et al. Computer Physics Communications, 1995). Both single-electron detachment and detachment-plus-excitation channels are included; specifically, channels leaving the Li atom in the 1s$^2$2s and the 1s$^2$2p states are included. A single strong resonance is predicted just below the opening of the K-shell threshold and is a 1s$\rightarrow$2p resonance leading to a 1s2s$^2$2p resonance state. This resonance decays to both the 1s$^2$2s and the 1s$^2$2p states of neutral Li, and it is found that it appears most strongly (by and order of magnitude or so) in the detachment-plus-excitation channel leaving the atom in the excited 1s$^2$2p of Li. This is explained in terms of the structure of the 1s2s$^2$2p resonance state in which the interaction of the two 2s electrons is much larger than the interaction between the 2s and 2p electrons.   

Multi-photon Single Ionization of Helium

This project explores the multiphoton single ionization of helium, for intermediate state energies that are either below or above the first ionization threshold. The approach is based on the eigenchannel R-matrix method and multichannel quantum-defect theory (MQDT), and is an application of methods introduced by Robicheaux and Gao to solve two-photon ionization problems. The electronic radial and angular correlations are elucidated by calculating multiphoton ionization cross sections and photoelectron angular distributions for an extended energy range. We consider different polarization configurations and bichromatic light sources to study the interference between different pathways. We also consider the ionization process triggered by a pair of entangled photons. Our results for two-photon ionization of helium for photon energies near the range of 25-32 eV are compared with some earlier theoretical treatments. The detailed behavior of the Rydberg series near the ionization threshold and the Fano lineshape properties in multiphoton ionization processes will also be discussed.   

Investigation of Relaxation Process in D$_2$O$^{2+}$ leading to D$^+$+O$^+$+D fragmentation channel

We present investigation of relaxation dynamics on heavy water molecules after single photo double ionization. The experiments were performed at beamline 10.0.1.3 at the Advanced Light Source in Berkeley using XUV of 61 eV. A COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) was employed to measure in coincidence the 3D momenta of the emitted ions and electrons. Among multiple fragmentation channels arising from the dissociation of D$_2$O dication, a very weak channel D$^+$+O$^+$+D is present. For this channel, we were able to identify and separate the autoionization process from the direct two electrons emission process. For the direct one, preliminary results show two distinct electronic states contributing to the fragmentation channel and that these D$_2$O$^{2+}$ states undergo sequential dissociaiton into D$^+$+O$^+$+D.   

Exploring the effects of exchange-correlation functionals on the photoionization dynamics of Na$_{\mathrm{x}}$ = $(x=20, 40 and 92)$ clusters

The photodynamics of metal clusters modelled as super-atoms can be studied using a linear-response density-functional method known as the time-dependent local-density approximation (TDLDA) [1]. In this method, the proper choice of exchange-correlation functional (xcf), that appropriately models bound and continuum electron's long-distance properties, is crucial. In the current work, two approximation schemes of xcf with Gunnarsson-Lundqvist parametrization [2] are employed: (i) the electron self-interaction correction scheme [3] and (ii) the Leeuwen-Baerends (LB94) model based on the electron density-gradient [4]. Results determine the role of xcf in the ground state and photoionization description of Na$_{\mathrm{x}}$ (x$=$20, 40 and 92) clusters. Comparisons of the ionizing residue of plasmon resonances obtained by these xcf schemes with available experimental data will be presented. \textbf{References } [1] J. Choi et al., Phys. Rev. A 95, 023404 (2017). [2] O. Gunnarsson and B. I. Lundqvist, Phys. Rev. B 13, 4274 (1976). [3] J.P. Perdew and A. Zunger, Phys. Rev. B 23, 5048 (1981). [4] R. van Leeuwen and E. J. Baerends, Phys. Rev. A 49, 2421 (1994).   

On Demand

We report on a new experiment studying light-atom interactions, where an all-optical, near-resonant laser atom trap is used to prepare an electronically excited and polarized gas target at mK-temperature for complete photo-ionization studies. As a proof-of-principal experiment, lithium atoms in the 2p (m$_l$=+1) state are ionized by a 266 nm laser source, and emitted electrons and Li$^+$ ions are momentum analyzed in a COLTRIMS spectrometer. The excellent resolution achieved in the present experiment allows not only to extract the relative phase and amplitude of all partial waves contributing to the final state, but also to characterize the experiment regarding target and spectrometer properties. Photo-electron angular distributions are measured for five different laser polarizations and described in a one-electron approximation with excellent agreement. The study shows that the all-optical trap along with the momentum spectrometer allow to obtain high-resolution and high-quality data providing insights into detailed structures of the final momentum space, which can be used in future measurements where multiphoton or strong-field ionization of polarized lithium atoms, molecules, or ultra-cold atomic samples will be investigated.   

Observation of double-core-hole continua and ionic fragments of formamide upon irradiation by intense X-ray pulses

Doubly core-ionized states of molecules, in which two different atoms have an ionized K shell each, are characterized by enhanced chemical shifts compared to their singly core-ionized counterparts. In this way the chemical environment of an atom in a molecule can be probed in detail [Cederbaum \textit{et al}., J. Chem. Phys. \textbf{85}, 6513 (1986)]. The elusive experimental observation of such states became feasible thanks to the development of third generation synchrotron radiation facilities [Eland \textit{et al}., Phys. Rev. Lett. \textbf{105}, 213005 (2010)] and X-ray free electron (XFEL) lasers [Berrah \textit{et al}., PNAS \textbf{108}, 16912 (2011)]. Using the high-flux, high-intensity, femtosecond X-ray pulses of the Linac Coherent Light Source (LCLS) at Stanford, the doubly core-ionized states of formamide (HCONH2) have been recorded and identified, using an experimental setup which consists of five time-of-flight (TOF) electron spectrometers mounted in different orientations with respect to the polarization of the incoming light, along with an ion TOF detector. This setup allowed us to observe the photoelectron peaks associated with the formation of double-core-hole states involving all three sites of the molecule (C,N,O), as well as the different ionic fragments.   

Relativistic Effects in the Photoionization of Spin-Orbit Doublets Well Above Threshold

In the absence of relativistic effects on the radial wave functions, the branching ratios of the photoionization cross sections of spin-orbit n$l$ doublets must go to the nonrelativistic (statistical) value of $l$/($l+$1) at high energies where the energy splitting of the $j = \quad l $\textpm 1 becomes negligible in comparison. Thus, deviations from the statistical values provide a quantitative measure of relativistic interactions. To study the situation, a theoretical investigation of the photoionization cross section branching ratios of all spin-orbit doublets from the ground states of the noble gases Ne, Ar, Kr and Xe has been initiated. The relativistic-random-phase approximation (RRPA) [1]is employed in the calculations since it is based upon the Dirac equation, so it includes relativistic effects \textit{ab initio}, and it also includes significant aspects of correlation in both the initial and final states of the photoionization process. Preliminary results suggest that, even in Ne, the branching ratios are decreasing at high energy, indicating that relativity plays a role even for light elements. [1] W. R. Johnson and C. D. Lin, Phys. Rev. A \textbf{20}, 964 (1979).   

Photoionization and Dissociation of Dichloroethene (C$_{\mathrm{2}}$H$_{\mathrm{2}}$Cl$_{\mathrm{2}})$: Towards Molecular Frame Photoelectron Angular Distributions

An experiment was performed using soft X-rays at the Advanced Light Source (ALS) in the Lawrence Berkeley National Lab on Beamline 9.0.1. Photoionization and dissociation of 1,1-Dichloroethene and trans-1,2-Dichloroethene at the Chlorine L-edge was performed with photon energy of 211.9 eV. Chlorine ionization threshold energies are 207.9 eV and 206.26 eV. Data was collected to examine the correlated momenta of the molecular fragments and the photoelectron in coincidence using the COld-Target-Recoil-Ion Momentum Spectroscopy (COLTRIMS) method. Preliminary results will be shown.   

Dissociative ionization dynamics of Na$_{\mathrm{2}}$ by non-resonant multiphoton ionization

We present the results from a series of experimental studies on dissociative ionization behavior of sodium molecules with the aid of a mild supersonic molecular beam and linear time of flight (TOF) mass spectrometer. We have studied the production of Na$^{\mathrm{+}}$ and Na$_{\mathrm{2}}^{\mathrm{+}}$ via a non-resonant delayed pump-probe multiphoton ionization using 355 nm, 532 nm, and 1064 nm photons. Upon absorption of two 355 nm photons from the pump laser, sodium molecules are promoted into neutral states, which converge to the repulsive 1$^{\mathrm{2}}\Sigma_{\mathrm{u}}^{\mathrm{+}}$ potential of Na$_{\mathrm{2}}^{\mathrm{+}}$. These dissociative Rydberg states are analogs to the well-known sates in the hydrogen molecule but much less studied for alkalis. The TOF spectra reveal a significant enhancement in both Na$^{\mathrm{+}}$ and Na$_{\mathrm{2}}^{\mathrm{+}}$ ion yield with the absorption of another photon from the probe laser even after long delays like tens of microseconds.   

Energy-resolved photoion angular distributions from ion-pair formation in O$_2$ following 2-photon absorption of a 9.3 eV femtosecond pulse

We present a combined experimental and theoretical study on the photodissociation dynamics of ion-pair formation in O$_2$ following resonant 2-photon absorption of a ~35 femtosecond 9.3 eV pulse produced via 400 nm driven high harmonic generation, where the resulting O$^+$ ions are detected using a 3-D momentum imaging spectrometer. Ion-pair formation states of $^3\Sigma_g$ and $^3\Pi_g$ symmetry are accessed through predissociation of continuum molecular Rydberg states that are resonantly populated via a mixture of both $\parallel$-$\parallel$ and $\parallel$-$\perp$ 2-photon transitions, where this mixture varies with the kinetic energy release (KER) of the dissociating ion-pair. The variation in the mixture's composition is captured by the KER dependent photoion angular distribution, which helps clarify the underlying 2-photon absorption dynamics involved in the ion-pair formation mechanism and indicates that the propensity towards undergoing a $\parallel$-$\parallel$ or $\parallel$-$\perp$ transition varies with the molecular structure.   

Time-dependent photoelectron angular distributions from multiphoton ionization of molecules with broad rotational wave packets

Rotational dynamics of impulsively aligned N$_{\mathrm{2}}$ and CO$_{\mathrm{2}}$ molecules are used to study the photoelectron angular distributions from multiphoton ionization by 266 nm pulses. Electron momentum distributions are recorded using a velocity map imaging spectrometer, Abel-inverted using the pBasex algorithm, and then fitted to delay-dependent moments of the molecular axis distributions. The analysis provides access to high-order asymmetry ($\beta )$ parameters of the photoelectron angular distributions.   

Deep minima in the Coulomb-Born triply differential cross section for electron and positron ionization of hydrogen and helium

Previous measurements [1] of a deep minimum in the triply differential cross section (TDCS) for electron-impact ionization of helium at 64.6 eV have been connected by Macek \textit{et al.} [2] to a vortex in the velocity field associated with the ionization amplitude. In the experiment, a gun angle of 67.5\textdegree and energies of 54.6 eV, 64.6 eV and 74.6 eV were considered; the deepest minimum occurred at 64.6 eV [1]. Theoretical calculations have been compared to the experimental results, for instance the time-dependent close coupling and the 3DW calculations [3]. We applied the Coulomb-Born (CB1) approximation to the experimental geometries and obtained minima in the TDCSs [4]. However, we had to vary the gun angles used in the experiment to obtain deep minima that correspond to zeros in the CB1 transition matrix element. We applied the CB1 from 44.6 eV to 79.6 eV (in 5 eV steps) and determined the gun and polar angles to obtain deep minima in the TDCSs. Corresponding to each zero in the CB1 transition matrix element there is a vortex in the velocity field associated with this element. Using the CB1 method we also obtained a deep minimum in the TDCS for electron-impact ionization of hydrogen as well as for positron-impact ionization of hydrogen and helium. All calculations were done in a double symmetric geometry [1].   

Charge Transfer in Ne$^{10+}$ + H Collisions

A time-dependent lattice method is used to calculate Ne$^{9+}$ (nl) n=1-9 capture cross sections at incident energies of 1.00, 3.00, and 5.00 keV/amu. Using standard radiative transition rates, Lyman line ratios are calculated in support of Clemson CUEBIT experiments.   

L-changing through very-long-range interactions in collisions between high-n, n$=$300, n$^{\mathrm{1}}$F$_{\mathrm{3}}$ strontium Rydberg atoms.

Studies of state-changing in collisions between high-$n$ Rydberg atoms are analyzed. While close collisions result in ionization of an atom, even at large distances the dipole-dipole interaction generates an effective electric field at each atom triggering Stark precession and L-changing which is examined using both classical and quantum theory. The theoretical predictions are tested experimentally using high-$n$ n$^{\mathrm{1}}$F$_{\mathrm{3\thinspace }}$strontium atoms and the results show that, even for impact parameters as large as 40 $\mu $m, collisions can lead to rapid L-changing, highlighting the long-range nature of the interactions responsible.   

Probing Dynamic Processes in the Formation of Temporal, Electronic, Vibrational and Rotational Quantum States of Antibonding-Intermediate States of $\left( {H_{3}^{+} } \right)^{\ast \ast }$that Lead to its Three-Body Breakup

The excitation energy of over 175 states of temporal $\left( {H_{3}^{+} } \right)^{\ast \ast }$have been determined with 3-5 milli-electron volt resolution, by measuring the center of mass energies of H$^{\mathrm{+}}$, H$^{\mathrm{+}}$, and H$^{\mathrm{\mathunderscore }}$ resulting from single-dissociation events produced in collisions of 4.0 keV $H_{3}^{+} $with He. This was achieved by using triple coincidence techniques.[1] We then formed a Linear Dalitz Plot by graphing the fractional center of mass (c.m.) energies of the ions as a function of the total c.m. energy to display and interpret the data.[2] We found that multiple sets of triple coincidence events form patterns in the projectile frame energy interval 4.5 eV to 10eV. This form of graph elucidates many temporal features of the dissociation process that will be presented. [1] Lisa Marie Wiese Ph,D. thesis, University of Nebraska-Lincoln, 1998. [2] B. Jordon Thaden, Ph,D. thesis, University of Nebraska-Lincoln, 2005.   

Magnetic Field Dependent Collisional Dynamics of NaLi Molecules in the Triplet Ground State

Ultracold gases of molecules offers a new platform to study short-range chemical reactions, many-body systems, and quantum information science. The NaLi molecule, the lightest bi-alkali molecule, in the triplet ground state have long collisional lifetime which allow us to investigate the complexity of chemical reactions by finding links to scattering theory. We have previously observed internal state dependent collision of Na-NaLi mixture and have seen favorable collisional properties in their fully stretched states enabling collisional cooling of NaLi molecules. We report results of recent studies with our triplet state molecules including the observation of magnetically controlled collision of Na-NaLi mixture. This can give positive influence on understanding molecular collisions in the quantum regime and discovering more efficient way of sympathetic cooling of molecules.   

Jahn-Teller Effect in Three-Body Recombination of Hydrogen Atoms

The three-body recombination rate coefficients of the H+H+H$\to$H$_2$+H process for different final rovibrational levels of H$_2$ are determined using a fully-quantum mechanical approach at zero total angular momentum. The Jahn-Teller coupling between the lowest electronic states of the H$_3$ system is accounted for. It is found that the Jahn-Teller effect substantially enhances the recombination rates for deeply bound dimers at room temperature, but only leads to a 12\% increase of the total three-body recombination rate. It is also found that the nascent population of the H$_2$ molecules, formed in the recombination process, is dominated by highly excited rovibrational levels, which should have a substantial impact on astrophysical models of environments where atomic hydrogen is present.   

Alkali-metal spin-destruction rates measured with pulsed EPR.

Determination of the efficiency of spin-exchange optical pumping (SEOP) depends on careful measurement of both the rate of spin exchange between the alkali-metal and noble-gas atoms and the rate of alkali-metal spin destruction from all sources. Spin-destruction rates are not well understood [1], particularly for Cs and for the range of parameters that characterize $^{\mathrm{129}}$Xe SEOP. Spin-exchange efficiency is relevant to determining the appropriate alkali-metal partner (Rb or Cs) for efficient production of hyperpolarized $^{\mathrm{129}}$Xe for applications such as lung imaging. We have developed an optically detected pulsed-rf EPR technique that allows us to measure the line width [2] and spin relaxation ``in the dark'' of alkali-metal hyperfine resonances with significantly improved SNR over cw-rf techniques. We present preliminary results of Rb and Cs spin-destruction measurements as a function of Xe pressure. [1] I. A. Nelson and T. G Walker, Phys. Rev. A \textbf{65}, 012712 (2001). [2] A. Ben-Amar Baranga, et al., Phys. Rev. A \textbf{58}, 2282 (1998).   

Magnetic-field stability in unshielded Helmholtz coils: Update

Last year's poster [1] compared several current-stabilization techniques for driving a Helmholtz coil pair, a common setup employed in spin-exchange optical pumping (SEOP) and other table-top experiments. We previously achieved 100-ppm stability on time scales between a few seconds out to an hour by employing an external comparator driving the gate of a FET in series with the coils. We refined this design by (a) adding a cover to reduce ambient temperature fluctuations, (b) attaching a capacitive filter to the output of the comparator to limit the high-frequency response, and (c) placing temperature-sensitive external circuitry (sensing resistors, FET, comparator feedback resistor) onto a water-chilled aluminum block, temperature-controlled at a few degrees below room temperature to \textpm 0.1 \textdegree C. We now achieve 2-ppm current stability on time scales ranging from few seconds out to an hour, which allows us to use a fluxgate magnetometer (max. field 1 G) \textbf{outside} the coils to separately correct ambient-magnetic-field shifts and drifts, e.g., from the current-carrying wires powering the building elevator. If we use the magnetometer to correct for these additional ambient fluctuations, the magnetic-field stability approaches the 2-ppm current stability over similar time scales in an unshielded environment. [1] D.J. Morin, et al., Poster Abstract S01 16, Bull. Amer. Phys. Soc. \textbf{64}(4), 191 (2019).   

Quantum dynamics of energy transfer for H collisions with water

Modeling of molecular emission spectra from the interstellar medium requires the calculation of rate coefficients for excitation by collisions with abundant species. Water is an abundant molecule in a variety of astrophysical environments, and has been the focus of countless theoretical astrophysical studies and observations. In this work we report a full-dimensional (6D) potential energy surface (PES) and scattering calculations for the H$_2$O-H collision system. The 6D PES was calculated using the high-level ab initio RCCSD(T)-F12b method. A two-component invariant polynomial method was applied to fit the PES analytically in 6D. The pure rotational state-to-state cross sections and rate coefficients from selected initial states of H$_2$O were compared with previous theoretical results. We also consider the calculation of rovibrational state-to-state cross sections and rate coefficients of H$_2$O in collision with H for the fundamental vibrational modes of water, $\nu_1$ (symmetric stretching), $\nu_2$ (bending mode), and $\nu_3$ (antisymmetric stretching).   

Momentum Imaging of the Dissociation Dynamics for Double Photoionization of NH$_{\mathrm{3}}$

We have measured the dissociation of Ammonia, NH3, in the gas phase, following double photoionization of a 61.5eV soft X-Ray photon produced by the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The momentum of the resulting fragments and the photoelectrons were measured in coincidence using a Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) technique. The dissociation dynamics of both the two and three body break up corresponding to several potential energy surfaces were observed. These different states show distinct differences in the electron energy, kinetic energy release, and bond angles during the fragmentation. Using this technique, we are able to determine the orientation of the molecule in three dimensions following both two and three body breakup. We show the results using a combination of analysis software presenting both energy and momentum figures.   

On Demand

COLTRIMS measurements of ethane molecules exposed to 23-fs, 1$\times$10$^{14}$-W/cm$^2$, 780-nm laser pulses are used to obtain the two-body fragmentation branching ratios, kinetic energy release, and angular dependence of the resulting photofragments with an emphasis on examining D$_3^+$ formation. D$_3^+$ + C$_2$D$_3^+$ is the most likely two-body double ionization channel. These measurements are contrasted with velocity map imaging studies of D$_3^+$ and D$_2$H$^+$ production in interactions between shaped ultrafast laser pulses and the D$_3$C-CH$_3$ isotopologue of ethane, which selects between trihydrogen monocations formed from atoms on one or both sides of ethane. When an adaptive learning algorithm supplied with 3D momentum-based feedback is used to identify intense laser pulse shapes that enhance the D$_2$H$^+$/D$_3^+$ ratio from D$_3$C-CH$_3$, the observed D$_2$H$^+$ angular distribution is altered significantly.   

Coherent Attosecond Control of Photoelectron Emission

Coherent control of electron dynamics in matter is a growing research field in ultrafast science, which has been mainly driven over the last two decades by major advances in laser technology. Recently, the advent of extreme-ultraviolet (XUV) light pulses in the attosecond time scale (1 as $=$ 10$^{\mathrm{-18}}$ s) has opened new avenues for experimentalists to manipulate the electronic dynamics with unprecedented temporal precision. In this work, attosecond pulses with controlled temporal profiles were used to guide the electron emission from an atomic target. Attosecond pulse trains made of odd and even harmonics were used to ionize the target in the presence of a weak IR field. An asymmetric photoelectron emission resulting from the interference between one- and two-photon transitions is produced. We show that the direction of photoemission can be varied along the polarization axis of the driving field by tailoring the spectral components of the attosecond pulse.   

Shortcuts to Adiabatic Passage in Particle Slowing

We analyze theoretically a method for slowing particles by laser fields that potentially has the ability to generate large conservative forces, but without the associated momentum diffusion that typically results from the random direction of spontaneously scattered photons. We implement a shortcut to adiabaticity approach that is based on Lewis-Riesenfeld invariant theory. We apply a laser that addresses an ultranarrow electronic transition, and periodically modify its detuning so that it cycles through repeated stimulated Raman transitions between motional states. This affords our scheme the advantages of adiabatic transfer, where there can be an intrinsic insensitivity to the precise strength and detuning characteristics of the applied field, with the advantages of rapid transfer that is necessary for obtaining a short slowing distance. As an application of this scheme, we demonstrate how the adiabatic speedup protocol can be applied to reduce the typical slowing distance in a Zeeman slower by one to two orders of magnitude. This would result, for typical parameters, in slowing an atomic beam from a thermal oven source in only a few centimeters.   

Taking advantage of noisy spectra through machine learning

Traditionally, noise is considered a limiting factor in (experimental) spectra. Here we demonstrate with two rather different examples how noise can be turned into an asset with the help of Machine Learning. In the first example we purify successfully noisy photo electron spectra generated with intense XFEL pulses. The single shot spectra contain in the combination of non-linear light-matter coupling and broad-band noise much more information about the target than a clean spectrum. In the second example we construct a system of networks with different levels of noise trained to recognize periodic signals. The system identifies correctly the periodicity as well as the noise level in a spectrum, as demonstrated with intrinsically noisy high harmonic spectra.   

Two-color few-photon ionization of helium: comparison of theory and experiment.

In this joint theoretical and experimental project, we investigate the response of the He atom subjected to a combination of two short and intense laser pulses, namely 1) an eXtreme Ultraviolet (XUV) pulse from a free-electron laser (FEL) and 2) an infrared (IR) pulse. The XUV frequency can be varied between the excitation energy of the $\rm (1s2p)^1P$ state ($\approx 21.8\,$eV) and the ionization threshold ($\approx 24.6\,$eV), thereby making it possible to hit or miss resonant $\rm (1s{\it n}p)^1P$ Rydberg states and also to investigate light-induced states [1,2] that are generated through the combined interaction of the two pulses. The appearance and disappearance of various pronounced features can further be observed by varying the delay between the pulses. Our calculations are based on the single-active electron (SAE) approximation, in which one electron is kept frozen in the 1s orbital while the other one is moving in the partially screened Coulomb potential of the nucleus and subjected to the laser pulses. Overall, very satisfactory agreement between theory and experiment is obtained. [1] M.~Reduzzi {\it et al.}, Phys. Rev. A {\bf 92} (2015) 033408. [2] S.~Chen {\it et al.}, Phys. Rev. A {\bf 86} (2012) 063408.   

Resonant propagation of x-rays from linear to nonlinear regimes

The modification of strong x-ray fields propagating through a resonant medium of gaseous neon is studied via simulation. The simulation is based upon the solution of a 3D time-dependent Schrodinger-Maxwell equation, with the incident x-ray photon energy on resonance with 1s-3p transition. We solved for the evolution of the combined incident and medium-generated fields -- which includes stimulated emission, absorption, ionization and Auger decay, as a function of the input pulse energy and duration. Self-induced transparency and self-focusing of strong x-ray free-electron laser (XFEL) pulses were revealed. These effects are important to understand and potentially applicable as control variables for XFEL pulse properties. Pulse reshaping and spectral modulation is also of interest for x-ray optics and spectroscopy.   

High-resolution Fluorescence Imaging with X-ray Free-electron Pulses

Intensity correlation of x-ray fluorescence, based on the principle introduced by Hanbury Brown and Twiss, has been proposed for high-resolution imaging of a 3D arrangement of atoms. To explore the applicability of this imaging approach, we theoretically investigate fluorescence dynamics of non-periodic systems subject to femtosecond XFEL pulses over a range of pulse parameters from the linear to non-linear x-ray absorption regimes. In particular, we present the fluorescence intensity correlation computed from the angular distribution of the fluorescence patterns and discuss the impact of sample damage on retrieving high-resolution structural information and elemental contrast in heterogeneous systems.   

Real-time dynamics of the formation of hydrated electrons upon irradiation of water clusters with XUV light

Here, we report on the formation of the hydrated electron in real-time using XUV-UV pump-probe photoelectron spectroscopy. The XUV pulse, from a free electron laser, initially ionizes the water clusters resulting in the creation of low kinetic energy electrons. Through elastic and inelastic scattering, some of the electrons are trapped within the cluster forming bound, hydrated states. With a second UV pulse, we probe the process in time recording the resulting electron kinetic energy distribution. By varying the UV pulse, we map out the hydration process in time and determine the formation as well as decay times in the low picosecond range and simultaneously observe the formation of free, excited hydrogen atoms as a fast, dominant radiation product. This work was funded by the Carl-Zeiss-Stiftung and the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, grant No. DE-SC0012376.   

Optical imaging of molecular rotational wave packets

Short laser pulses are widely used for controlling molecular rotational degrees of freedom and inducing molecular alignment, orientation, unidirectional rotation, and other types of coherent rotational motion. The present work offers and demonstrates a novel non-destructive optical method for direct visualization and recording of movies of coherent rotational dynamics in a molecular gas {[}1{]}. The technique is based on imaging the time-dependent polarization dynamics of a probe light propagating through a gas of coherently rotating molecules. The probe pulse continues through a radial polarizer, and is then recorded by a camera. We illustrate the technique by implementing it with two examples of time-resolved rotational dynamics: alignment-antialignment cycles in a molecular gas excited by a single linearly polarized laser pulse and unidirectional molecular rotation induced by a pulse with twisted polarization. This method may open new avenues in studies on fast chemical transformation phenomena and ultrafast molecular dynamics caused by strong laser fields of various complexities. {[}1{]} arXiv:1912.03045 [physics.optics], 2019   

Distinguishing geometrical conformers using Coulomb Explosion Imaging

We report the results of an experimental study on distinguishing molecular conformers using coincident ion momentum imaging. This work extends our earlier study on identifying \textit{cis }and \textit{trans} isomers of 1,2-dibromoethene (C$_{\mathrm{2}}$H$_{2}$Br$_{2})$ using Coulomb explosion imaging (CEI). The experiment was performed on 1,2-dibromoethane (C$_{\mathrm{2}}$H$_{4}$Br$_{2})$ using 140 eV photons at the Advanced Light Source (ALS). Our results suggest that CEI can distinguish between anti and gauche conformal isomers, which are only distinguished by rotation around single bond. Moreover, we can observe a change in the ratio between \textit{anti }and\textit{ gauche }conformers as a function of temperature. The observed breakup patterns show similarities to the related cis-trans isomers but indicate a higher fraction of sequential breakup.   

Coulomb Explosion Imaging of C-S and S-S Bond Breaking with X-rays

Photofragmentation of dimethyl sulfide (DMS) can produce reactive radicals by cleavage of C-S bonds, while dimethyl disulfide (DMDS) can additionally undergo cleavage of the S-S bond. Owing to this S-S bond, the DMDS backbone has an open-book geometry. In the present investigation, Coulomb explosion imaging (CEI) was used to study the C-S and S-S fragmentation processes after inner-shell ionization in the photon energy range of 130-300 eV by measuring the momenta of coincident ions. Preliminary analysis showed that the dissociation has both sequential and concerted contributions. We also look towards exploiting CEI to identify the geometry of molecules in their initial states. Simulations are being performed to get further insight into the experimental results. These results will pave the way for future time-resolved experiments.   

Developing a camera-based 3D momentum imaging system capable of 1 Mhits/s.

A camera-based three-dimensional (3D) imaging system with a superb time-of-flight (TOF) resolution and multi-hit capability was recently developed for electron/ion imaging [Lee et al. J. Chem. Phys. 141, 221101 (2014)]. In this work, we report further improvement of the event rate of the system by adopting an event-driven camera, Tpx3Cam, for detecting the 2D positions of electrons, while a high-speed digitizer provides highly accurate ($\sim $30ps) TOF information for each event at a rate approaching 1 Mhits/sec..   

The Ultimate Limit of High Harmonic Rescattering Radiation from Atoms and Ions in Ultrastrong Laser Fields

The discovery of laser driven rescattering and the 3.17 times the ponderomotive energy rule in strong laser fields led to attosecond pulse generation, coherent x-rays, and high energy photoelectron production. As the laser field drives the interaction to higher energies, relativistic effects and the Lorentz force from the laser magnetic field enter into the dynamics and deflect the photoelectron continuum wave function. Recent studies of laser rescattering \footnote{M. Klaiber, et al, \textbf{Phy. Rev. Lett} 118, 093001} have included these relativistic effects and the Lorentz force from the laser magnetic field to give a quantitative description of rescattering dynamics in the high energy limit, i.e. recollision energies of order 1000 Hartree. In this high energy limit, we treat the emitted high harmonic radiation from rescattering as a Bremsstrahlung process using the relativistic Larmor formalism. We report the radiated power, time dependent electric fields, and energy spectra for recollision near the ultimate cutoff for rescattering. The numerical results including the ion core electrons are shown compared to traditional Bremsstrahlung radiation results for an electron scattering from the Coulomb field of a bare nucleus.   

A Nano-Tip THz Field-Probe

We introduce a new technique for non-invasive THz field characterization using high-energy electron field-emission from tungsten nano-tips. The scheme exploits the large electric field enhancement in the vicinity of nano-tip to enable the measurement of electric-field transients with sub-micron spatial and sub-picosecond temporal resolution. The technique employs an intense single-cycle THz ``source'' pulse to create a sub-picosecond burst of keV electrons from a nano-tip with a radius of \textasciitilde 100 nm [1]. A second ``signal'' pulse is also incident on the tip. The signal field modulates the net instantaneous local field, resulting in a proportional change in the electron yield and maximum energy. We have used two, near single-cycle THz pulses, produced via optical rectification of 100 fs 800 nm laser pulses in LiNbO$_{\mathrm{3}}$, to demonstrate the method. We vary the time-delay between the two THz pulses, and raster scan the location of the nano-tip within the focused signal beam in vacuum, while measuring the emitted electrons with a MCP detector. Changes in the time-dependent waveform of the signal field throughout its focus, including the Gouy phase-shift, are readily observed. [1] S. Li and R.R. Jones, Nature Comm. \textbf{7}, 13405 (2016).   

Development of an Extreme Ultraviolet Light source and Beamline for Femtosecond Nonlinear Spectroscopy

Recently, we have demonstrated that the laser-ablation technique is an effective method to produce intense and broadband high-order harmonic (HH) laser light with linear and rotated polarization (Singh, M. \textit{et al.}, APL \textbf{115}, (2019) 231105.). These HH laser pulses are predestined to produce intense attosecond pulse trains or investigate ultrafast nonlinear dynamics in gases or materials with a single harmonic, having narrower spectral width (femtosecond pulse). We are developing an ultrafast light source with an expected energy of \textasciitilde 5 \textmu J per pulse per harmonic in the spectral region from 4.7 eV to 26 eV, sub-ten fs pulse length, tunable intensity and energy, coherent, and stable at high repetition rates (\textgreater 1 kHz), employing high-order harmonic generation (HHG) of intense laser light from laser-ablated plumes (LAP). HHG from LAP has the advantage to produce extreme ultraviolet (XUV) photons (up to \textasciitilde 156 eV) with near-infrared driving fields. This light source will be used to study the 3rd order nonlinear response of excited molecules by ultrafast transient polarization spectroscopy. We report recent developments of experimental tools to split, delay and focus two femtosecond XUV pulses and one or more near infrared pulses in a gas cell, and to analyze the polarization and spectrum of the XUV light generated by four-wave mixing.   

Thermal Effects in Molecular Gas-Filled Hollow-Core Fibers

High average-power ultrafast laser sources based on Yb technology are a promising avenue for the generation of isolated attosecond pulses at high repetition rates. So far, compression to few-cycle durations has largely relied on employing large levels of nonlinearity, prolonged interaction lengths, or multiple compression stages due to their \textgreater 100 fs output laser durations. At these durations, pulses can be more efficiently compressed through nonlinear propagation in molecular gases, where the field-driven rotational alignment on timescales shorter than the laser pulse duration enhances the optical nonlinearity. However, the thermal effects associated with the additional internal degrees of freedom in molecular gases, and their impact on spectral broadening in gas-filled hollow-core fiber, have not been explored. Here we present measurements of the average power dependent spectral broadening in an N$_{\mathrm{2}}$O-filled hollow-core fiber. We find that high rotational temperatures severely limit the broadening at high average powers, and we demonstrate the effect buffer-gas cooling has on mitigating the thermal effects, leading to an improved transmission and beam profile without active cooling.   

Impulsive Stimulated X-ray Raman Scattering in Molecular Systems

The field of nonlinear X-ray interactions is growing with technological advances at free-electron lasers. The development of intense attosecond X-ray pulses has opened a new regime for studying nonlinear X-ray interactions in the impulsive limit. In an experiment conducted at the Linac Coherent Light Source, we made the first observation of impulsive stimulated X-ray Raman scattering~(ISXRS) in a molecular system, finding a two-photon cross section of~$\left(3\pm2\right)\times10^{-55}$~cm$^4$s/photon. ISXRS is a versatile process for studying electronic charge motion and X-ray-matter interactions. Simulations matching the observed signal show that ISXRS coherently populates multiple valence-excited electronic states. This is a significant step toward direct observation of coherent electronic wavepackets using the planned 2-color attosecond X-ray mode of the LCLS.   

Exploration of Enhanced X-ray Scattering by Transient Atomic Resonances

Intense x-ray free-electron laser (XFEL) pulses hold great promise for imaging function in nanoscale and biological systems with atomic resolution. However, the achieved spatial resolution obtained from the scattering signals of single shot experiments is currently limited by the induced electronic and structural damage in intense XFEL pulses. Our calculations show that, by exploring resonant scattering in the vicinity of atomic resonances of transient electronic states, enhanced scattering cross section and signal can be achieved without the need of extreme pulse intensity, where ultrafast structural damage can take place. Our results predict that scattering hotspots exist over a range of pulse parameters and show that attosecond XFEL pulses will enable the exploration of this resonant scattering scheme in both the soft and hard x-ray regimes.   

Attosecond Science at the Linac Coherent Light Source

We report the first results using isolated attosecond soft X-ray pulses from an X-ray free-electron laser (XFEL)\footnote{Duris, Li \textit{et al.}, Nat. Phot. \textbf{14}, 30 (2020)}. This new attosecond source produces peak powers on the gigawatt scale,and opens the door for a suite of X-ray spectroscopies probing few- to sub-femtosecond dynamics. High peak power pulses facilitate nonlinear spectroscopies such as attosecond X-ray pump/attosecond X-ray probe, and wave mixing. Moreover, the inherent tunability of an XFEL source allows the selective probing of different core-to-valence transitions at disparate atomic site in a molecule, providing an atomic site-specific probe of valence electron dynamics. We present single-shot attosecond pulse characterization, the preparation of a coherent electronic wavepacket \textit{via} stimulated X-ray Raman scattering, time-resolved photoemission studies of pre-edge~(resonant Auger) and post-edge~(direct) \textit{K}-shell ionization and two-color, two-pulse operation.   

Dichroism in Chiral Free Electrons Scattering from Plasmonic Nanostructures

The spatial amplitude and phase of a free electron can be efficiently shaped with the use of off-axis material holograms similar to the way spatial light modulators are used in light optics. These holograms can be used to apply an azimuthally varying phase to the electron wavefront imparting quantized amounts of orbital angular momentum. We show simulations and experiments to measure the energy dependent transfer of orbital angular momentum from a chiral free electron to plasmonic modes of metallic nanoparticle clusters and how these spectra can exhibit dichroism with respect to the cluster's orientation and chirality of the incident electron. Furthermore, we provide an outlook for the future use of singular electron beams probing local inelastic excitations in an orbital angular momentum basis.   

Magnetic dichroism in the few-photon ionization of un-polarized and polarized lithium atoms

In the last decade, the interaction of atoms with circular and elliptically polarized light has been studied extensively. For tunnel-ionization in intense and CEP stabilized laser pulses, it was found that the electron angular distribution exhibits an angular shift. This shift was interpreted to be caused by the finite tunneling time of the active electron that translates into a shift of the mean emission angle due to an angular streaking mechanism often referred to as ‘attoclock’. Here, we report on an experiment where the angular distribution in the two- and three-photon ionization of lithium is investigated. The lithium target can be prepared un-polarized in the 2s ground state or polarized in the 2p (m$_l$=+1) state. A shift in the photo-electron angular distributions is observed that remains even for a fully linearly polarized light if the target atoms are initially excited with a polarization direction perpendicular to ionizing electric field. The observed angular shift is a magnetic dichroism that can interpreted in terms of non-vanishing phase angles between contributing partial waves with different orbital angular momentum. The resemblances and differences of the presently observed angular shift and the attoclock mechanism will be discussed.   

On Demand

Recently it became possible to experimentally create macrodimers in a lattice through coupling of an ultracold bosonic quantum gas to high lying Rydberg states [1]. As a follow-up study, it was proposed to use the coupling to macrodimer states for the enhancement of Rydberg dressing schemes, which might lead to a rich phase diagram of non-trivial quantum phases. We theoretically study a bosonic quantum gas trapped in an optical lattice with weak Rydberg dressing, resulting in an effective next-nearest neighbor interaction. In this work we consider both attractive and repulsive interaction. In order to capture additional quantum fluctuations and the expected broken translational symmetry, we treat the system with the Cluster Gutzwiller method [2]. We find various quantum phases, such as Mott insulating and superfluid phases as well as density wave phases between the Mott lobes that are stabilized by hopping processes. We propose how to access these phases with a range of experimentally feasible parameters.\\ \big[1\big] S. Hollerith et al., Science 364, 664 (2019)\\ \big[2\big] D. L\"uhmann, Phys. Rev. A 87, 043619 (2013)   

Improving the measurement of squeezed states using noise subtraction techniques.

Ponderomotive squeezing produced in an optomechanical cavity with a strong optical spring has some advantages over squeezed light sources that use nonlinear crystals. However, the cavity requires feedback to maintain stability, and excess noise is injected as a result. The excess noise may be removed from the squeezing measurement by time-domain subtraction. Here, we present a noise subtraction technique that relies on measuring the coherence between the feedback signal and the squeezed state to purify the squeezed state. The experimental setup consists of the optomechanical system and a subsystem used to detect transmitted light from a Fabry-Perot cavity with a 1064 nm Nd: YAG NPRO laser. A beam splitter is used to pick off 15{\%} of the transmitted cavity light to a photodetector for locking the cavity, and the remaining 85{\%} is used for combining with a local oscillator to detect squeezing. Our results at different quadratures show that the budgeted noise agrees with the measured subtracted noise, and that if this subtraction technique was not applied, no squeezing would be seen in any quadrature.   

Cooperative mirror of artificial atom dipoles

Recently, it was shown that 2D dipolar arrays could be employed as a near perfect frequency dependent mirror that could shape the emission pattern from an individual quantum emitter into a well-defined, collimated beam. In our calculations we investigate the implementation of such a system using nanorods or similar shapes with the goal to reach into short-wavelength regimes.   

dynamic phonon laser in a cavity magnomechanical system

We have studied a new type of phonon laser that works based on the mechanical vibrations caused by the magnetostrictive force in a cavity magnomechanical system beyond the steady state. The system includes a small yttrium iron garnet (YIG) sphere that is placed in a microwave cavity. Also, a uniform external bias magnetic field $H$ is applied in the vertical direction and a drive magnetic field is established perpendicular to $H$. The uniform field produces a uniform magnon mode. Excitation of the magnons in the YIG sphere by the drive field leads to varying magnetization and hence geometric deformation of the surface (magnetostriction), and it results in the magnon-phonon coupling. One can control the frequency of the collectively excited magnons by adjusting $H$. The cavity has two supermodes corresponding to the ground and excited states, like a two-level atom. Once there is a population inversion between the cavity supermodes, a coherent phonon field is amplified. In contrast to the steady-state case, in the dynamical case, the phonon laser operates under the blue-detuned pump rather than red-detuned. Moreover, the quantum and thermal noise significantly contribute to the supermode population inversion and hence the stimulated emitted phonons.   

A Keldysh formalism approach to superradiance of two-level system

We use a non-equilibrium Keldysh formalism to develop a method of describing superradiance. A time evolution operator on the Schwinger-Keldysh contour is written in terms of the field and atomic operators. The quantized field’s degrees of freedom are formally eliminated to obtain an effective two-atom master equation. Using non-equilibrium Green’s function method and Dyson equation formalism, we derive a self-consistent expression for superradiant decay rates. We consider a homogeneous gas of initially inverted two-level atoms, and numerical results are obtained by solving the closed form of its equation of motion. The effect of optical depth on super- and sub-radiance is also discussed.   

Enhanced on-resonance squeezing from four wave mixing via additional optical beam

Squeezed light is an essential resource in quantum-enhanced metrology, particularly in optical based devices that take advantage of its reduced noise properties. In order to take advantage of squeezed light in atomic systems, it is necessary to generate resonant squeezed light to enable an efficient light-atom interaction. While we have previously generated squeezed light resonant with the D1 line of $^{87}$Rb using four-wave mixing (FWM) in $^{85}$Rb, generating resonant squeezed light on the D2 line would be useful to trap or probe cold atoms in optical lattice experiments. However, it has been shown that the more complicated energy level structure of the D2 line makes it difficult to achieve squeezing on resonance with this transition. To enhance off-resonance FWM on the D2 line, previous experiments have dressed the atomic states with an additional co-propagating beam and demonstrated an improvement in their level of squeezing. We combine the use of a counter-propagating beam with our previous configuration for the FWM in $^{85}$Rb to generate squeezed light on resonance with the D2 line of $^{87}$Rb. The counter-propagating beam is resonant with the D1 line of $^{85}$Rb and leads to an increase in FWM gain as well as 1 dB of squeezing.   

Analyzing Two Photon Interference Between Trapped Ion and Rydberg Ensemble Systems in a Hybrid Quantum Network

Future quantum networks are likely to be hybrid in nature, relying on the strengths of disparate quantum systems. One way to entangle different quantum memories is through the interference of photons entangled with each matter system [1,2]. Here, we analyze the temporal and spectral factors contributing to the measured interference signal between 780-nm photons from a Rb-Rydberg ensemble and frequency-converted photons originating from a trapped Ba+ ion [3]. Additionally, we project potential entanglement rates that could be achieved using this interference in a hybrid quantum network. [1] C. K. Hong, Z. Y. Ou, and L. Mandel, PRL, 59, 2044 (1987) [2] Y. H. Shih and C. O. Alley, PRL, 61, 2921 (1988) [3] A. N. Craddock, J. Hannegan, D.P. Ornelas-Huerta, et. al, PRL, 123, 213601 (2019)   

Control in a spin-orbit mixed four-level molecular system coupled by three lasers

In laser spectroscopy, spin-orbit mixed singlet-triplet rovibrational molecular levels are commonly used as a gateway to access some normally spin forbidden transitions to higher excited triplet electronic molecular states since the mixed states can carry both triplet and singlet characteristics. By coupling the mixed states to an auxiliary quantum state with lasers, the spin-orbit mixing coefficient of two mixed levels can be modified by ac Stark effect via varying the Rabi frequency of the coupling lasers and the detuning of the laser frequency. We use density matrix equations and a four-level molecular model to show that coupled spin-orbit mixed singlet-triplet rovibrational levels can be used as a channel to enhance the probability of accessing target quantum states.   

Information transfer by quantum matter wave modulation

Classical communication techniques by electromagnetic wave modulation and secure quantum communication schemes with photons revolutionized our modern society. Here, we demonstrate a fundamentally new information transfer scheme based on the quantum matter wave nature of electrons. It allows a signal transmission by a non-trivial quantum modulation of electron wave packets. The data is encoded in a biprism electron interferometer with a rather simple element, the Wien filter. It introduces a longitudinal shift of the separated wave packets that leads to a change in the fringe contrast without changing the beam position, total intensity or phase. We transmitted a message by binary encoding the information in the interference. The readout on the receiver side is done by a dynamic contrast measurement. Our scheme has no analog in light optics. It relies on the Aharonov-Bohm effect for charged matter waves and can therefore not be performed with photons. We discuss the high level of transmission security and demonstrate it by introducing a semiconducting plate close to the separated beam paths. It is equivalent to an eavesdropper attack which immediately destroys the interference pattern due to decoherence and stops the transmission.   

High-harmonic generation in lanthanides--containing plasmas

High-order harmonic generation (HHG) in laser-produced plasmas (LPPs) from the very beginning provided a promising route for generating coherent extreme ultraviolet radiation using different targets. Among new potentially interesting elements of periodic table, the group of non-radioactive solid materials with large atomic mass, such as lanthanides, can be considered. This also allows studying the high-order nonlinear optical properties of those elements thorough HHG in LPPs. We have analyzed the high-order harmonic spectra produced during propagation of the femtosecond pulses through the lanthanide-contained LPPs. The species under investigation (praseodymium, terbium, lanthanum, and ytterbium) are among the heaviest non- radioactive solid elements of periodic table. We analyzed HHG in the plasmas produced on the surfaces of these lanthanides and their oxides (La, Yb, Pr$_{\mathrm{6}}$O$_{\mathrm{11}}$, Tb$_{\mathrm{4}}$O$_{\mathrm{7}})$ using different techniques (two-color pump, application of chirped pulses, variation of the parameters of heating and driving pulses, etc.). We have shown that Yb LPP is the efficient medium for the harmonic generation up to the 73rd order, which is one of the largest harmonic cutoffs generated in the laser-produced plasmas.   

Coherence-based cross-phase modulation of arbitrarily weak fields in disordered molecular ensembles

Nonlinear optical signals such as cross-phase modulation can be coherently enhanced in multilevel atomic gases under conditions of electromagnetically induced transparency. The quality of these coherent signals can dramatically decrease in presence of inhomogeneous broadening. In atomic gases, coherently enhanced cross-phase modulation can still be achieved with inhomogeneous broadening, but only over a narrow range of system parameters and specific laser geometries. In solids, inhomogeneous broadening is unavoidable and is believed to be an intractable obstacle to coherence-based nonlinearities. We analyze the cross-phase modulation of two arbitrarily weak classical fields in a resonant cavity with molecules under conditions of vacuum-induced transparency (VIT) subject to strong inhomogeneous broadening. We show that for a specific family of molecules, the VIT-enhanced cross-phase modulation signal can surpass the detection limit despite disorder. Our work shows that coherence-based non-linear optics for weak optical fields is feasible in strongly disordered organic materials, extending the realization of vacuum-enhanced cross-phase modulation beyond the gas phase.   

On Demand

High-order harmonic generation (HHG) of ultrashort laser pulses in laser-produced plasmas (LPPs) requires the optimally formed plasma plumes during laser ablation of different targets. Plasma dynamics characterization for improvement of HHG in LPPs allows determining the best conditions of laser-plasma interaction. We analyze the temporal dynamics of the plasma characteristics suitable for HHG using ICCD camera and delay-dependent variations of the yield of harmonics in the case of application of low- (C), medium (Ti), and heavy-weight (Au) atoms. Role of the nanoparticles appearing during laser ablation on the HHG efficiency is discussed. We present the time-resolved spectra and images of C, Ti, and Au plasmas. The velocities of the LPPs produced on different targets are determined and compared with those defined from the HHG studies at optimal delays between the heating and driving pulses.   

Scattering in Optical Nanofibers

Optical nanofibers (ONFs) have provided a robust platform for strong interactions between light and atoms. More recently, studies have investigated the optomechanical interaction between the torsional motion of an ONF's tapered region and guided light with linear or angular momentum. Here we investigate the optical scattering inside an ONF as a function of wavelength and its impact on torsional motion. This work hopes to guide research into the suppression of torsional motion, which is understood to be a heating source for atoms trapped around the ONF.   

Four-Wave Mixing in a Hot Sodium Vapor Cell in the Large Single-Photon Detuning Regime

Four-wave mixing (4WM) is a non-linear process that can produce correlated twin beams of light, which are useful for quantum-enhanced sensing and interferometry below the shot-noise limit. We are interested in generating entangled twin beams at 589 nm to enhance density measurements of our sodium Bose-Einstein condensate. We use 4WM in a double-lambda configuration in a hot sodium vapor cell to generate twin beams of light, known as probe and conjugate. While twin beams with a large degree of quantum correlations have been previously generated in rubidium via 4WM, this is not the case for sodium. Sodium has a smaller hyperfine splitting, such that the Doppler-broadened absorption lines overlap, which leads to a significant absorption for the twin beams when operating close to the resonant regime. To reduce the loss of photons, we operate our 4WM system in a large single-photon detuning regime where the frequency of the twin beams falls in a region of the spectrum where there is no absorption. This is done by adjusting the single-photon and two-photon detunings. We present our experimental progress and characterize the gain dependence on various experimental parameters.   

Quantum-enhanced Spectroscopy Using Squeezed Light on Raman Two-Photon Absorption

We investigate the possibility to extend the applicability of two-mode intensity squeezed twin beams to improve the sensitivity of the weak absorption measurements to different optical frequencies using Raman two-photon absorption resonances. Normally, the twin beams can be used to achieve the sub-shot noise sensitivity only for the narrow spectral range determined by the FWM gain line. By using a strong pump field to connect one of the squeezed beams to desired higher excited state, and then performing the usual differential measurements, it becomes possible to measure various characteristics of very weak transitions with sub-shot noise precision. We conduct proof-of-principle demonstration using spectroscopy of 5D states of Rb.   

Demonstration of multiple round-trip waveguide atom interferometer

Atom interferometers in which the atoms move inside waveguides can be much smaller than devices in which the atoms move in free space. A potential issue with this approach is that fringe contrast can drop rapidly as the distance traveled inside the waveguide increases because of effects associated with residual curvature in the waveguide. We have found that the loss of contrast is much smaller in an interferometer in which the atoms make many small-amplitude round trips during the measurement cycle instead of a single large-amplitude round trip. We use a weakly-interacting gas to reduce scattering when the wavepackets pass through each other. In this way we have demonstrated a linear interferometer with interrogation time longer than any previous waveguide interferometer. For this experiment, we produce a condensate with \textasciitilde 5,000 atoms of $^{\mathrm{39}}$K \textbar F, m$_{\mathrm{F}}$\textgreater $=$\textbar 1, -1\textgreater by exploiting a magnetic Feshbach resonance at \textasciitilde 562 G in an optical dipole trap formed by crossed horizontal and vertical 1064 nm beams. The s-wave scattering length is set to a desired value by sweeping the magnetic field, and the vertical beam is turned off to put the atoms into a one-dimensional waveguide. Retro-reflecting a blue detuned 767 nm laser forms a Bragg grating along the waveguide axis that imparts a \textpm 2$\hbar $k momentum kick. The interferometer uses a $\pi $/2-[-$\pi $-]$^{\mathrm{n}}$-$\pi $/2 pulse sequence with up to n$=$50 $\pi $-pulses. The pulse sequence could be useful for an atomic Sagnac interferometer if the waveguide is synchronously shifted during interrogation in order to form an enclosed area.   

Analytic Treatment of Bragg Diffraction Phases for Atom Interferometry

High-fidelity Bragg pulses operate in the quasi-Bragg regime in which no simple analytic description of the diffraction process exists. Whilst such pulses enable enabling an efficient population transfer essential for state-of-the-art atom interferometers, the diffraction phase and its dependence on the pulse parameters are currently not well characterized despite playing a key role in the systematics of these interferometers. We develop an analytic theory for such pulses based on the adiabatic theorem. We provide an intuitive understanding of the Bragg condition and derive a unitary scattering matrix in case of driving with adiabatic pulses in the sense of the adiabatic theorem. We find, that perturbations of the adiabatic solution are well described by Landau-Zener physics. Furthermore, we include the effects of linear Doppler shifts applicable to narrow atomic velocity distributions on the scale of the photon recoil of the optical lattice. As an illustration, with our comprehensive microscopic model we study diffraction phase shift fluctuations caused by laser intensity noise affecting the sensitivity of a Mach-Zehnder atom interferometer.   

Bloch-band Approach for Precision Atom Interferometry with Yb Bose-Einstein Condensates

We report on progress towards a precision measurement of the fine-structure constant, $\alpha$, via a photon recoil measurement in our ytterbium (Yb) Bose-Einstein condensate (BEC) interferometer deployed through a pulsed standing wave optical lattice. Building on earlier work applying a Bloch-band picture for atom diffraction and interferometry [1] we have now devised and characterized a method using excited-band Bloch oscillations (BOs) to minimize diffraction phases and demonstrated high efficiency $40\hbar k$ momentum transfer in an atom interferometer using this technique [2]. We are currently implementing a contrast interferometer [3] with large momentum separation through BOs in a vertical geometry. The vertical geometry increases our interferometer time thereby increasing sensitivity for metrological applications. [1] D. Gochnauer et al, Phys Rev A 100, 043611 (2019). [2] K. McAlpine et al, arXiv:1912.08902 [3] B. Plotkin-Swing et al, Phys. Rev. Lett. 121, 133201 (2018).   

Nonvolatile atomic memory in the spontaneous scattering of light from cold atoms

We report an experimental investigation of the spontaneous scattering of light at a certain angle from an ensemble of cold two-level atoms, obtained from a magneto-optical trap of Rubidium 87 atoms. We report the observation of correlations between photons scattered by consecutive laser pulses detuned from the atomic resonance. Such correlations lead to an enhancement of probability to detect other photons in the same direction once a first one is observed, and after storage in the atomic system. They last considerably longer than the excited state lifetime and are robust to changes in the reading process, pointing to the crucial role of recoil-induced resonances in such simple systems. We propose a theoretical model for the mechanism of this memory that allows a more concrete interpretation for its nature.   

Ion-Ion Entanglement for Quantum Networking at the Air Force Research Laboratory

Quantum networking is a vital area of research that may provide distributed quantum computing capabilities and may ultimately offer tamper-proof and tamper-evident communications. One method to achieve this is to entangle trapped ions in distant network nodes. Ytterbium 171 is a near-ideal candidate for memory due to its internal properties that allow for long-lived quantum bit states. The first step towards a viable network is to reliably entangle two ions trapped in separate vacuum chambers. This poster will focus on the progress to date to entangle two remote ions. ~In addition, we will discuss the longer-term quantum networking goals towards compact network nodes and distributing entanglement across a multi-node network.   

Deterministic Shaping and Reshaping of Single-Photon Temporal Wave Functions

Single photons are the most used carrier to communicate within a quantum network. All degrees of freedom of these photons need to be well controlled to use them in quantum applications. The characterization of the temporal mode function is typically reduced to its Hong-Ou-Mandel interference and its temporal intensity profile as measured via single photon counters. However, the connection of different systems in a quantum network requires the ability to control the phase profile of the temporal mode. Here, we investigate thoroughly the possibilities offered by a cavity quantum electrodynamics (QED) system, namely a single Rb87 atom in a high finesse optical resonator. Starting from previous theoretical work [1], we developed a comprehensive and exhaustive model of our system [2]. Thanks to this, we experimentally demonstrate a very high control of the temporal mode of a single photon in amplitude and phase. This opens up various possibilities as for instance modifying the temporal shape by 3 orders of magnitude in time and bandwidth. It also shows that our platform can be compatible with many others and can even be used as a photon mode converter. [1] A. Gorshkov et al., Phys. Rev. A 76, 033804 (2007). [2] O. Morin et al., Phys. Rev. Lett. 123, 133602 (2019).   

Crossed fiber cavities with single trapped atoms

Single atoms strongly coupled to the optical mode of high-finesse, fiber-based Fabry-Perot cavities (FFPCs) represent a progressing system in the field of quantum information processing. FFPCs allow for small mode volumes\footnote{Hunger et al., \textbf{New J. Phys.} 12, 065038 (2010)} \footnote{Uphoff et al., \textbf{New J. Phys.} 17, 013053 (2015)} which increase the atom cavity mode coupling strengths and enable new cavity geometries due to the small lateral size of the mirrors. We have set up a new cavity experiment that exploits the experimental advancement of high coupling strength and small mirror size. A single neutral atom is trapped at the center of two crossed FFPCs. Each cavity mode can be independently tuned to an atomic transition which enables the implementation of quantum information schemes based on two-mode cavity QED. We will present in detail the cavity fibers fabrication results, the assembly of the experimental setup, the atom loading scheme and the atom imaging system. Furthermore, we show data that proves strong coupling between single atoms and both cavity modes for long trapping times and the coherent microwave manipulation of the atomic states.   

Progress Toward a Three-Node Multispecies Ion Trap Quantum Network

To address the challenge of scaling trapped ion quantum computing systems, we utilize a modular architecture consisting of separate traps, each containing $^{171}$Yb$^+$ memory qubits and $^{138}$Ba$^+$ communication qubits. Single photons from the Ba$^+$ ions provide links between nodes of the network for remote entanglement. The main challenge in this system is achieving high remote entanglement generation rates. Here we present the design of and progress toward construction of a three-node network with increased modularity and nearly quadrupled potential photon collection rates. In particular, we focus on the design and testing of in-vacuum high numerical aperture (NA=0.8) aspheric lenses as replacements for our current NA=0.6 multi-component objective. Additionally, we discuss a protocol for generating full remote entanglement among three traps utilizing both atomic species.   

Bose polaron and bi-polaron in 1D: condensate depletion and generalized phonons

We discuss the interaction of mobile impurities with a surrounding condensate in 1D. For a large impurity-condensate coupling the usual approach of linearizing around a constant condensate density (extended Fröhlich model) does not account correctly for deformations of the BEC and is therefore no longer applicable. We give an alternative approach, taking into account the condensate deformation already on a mean field level, which can be solved analytically [1]. The energy and effective mass of the polaron quasi-particle agree well with quasi exact quantum Monte-Carlo calculations [2] and the agreement is further improved by including phonon-like excitations of the deformed background. We present the mean-field solution for one as well as two impurities immersed in the condensate from which an effective impurity-impurity interaction potential, mediated through the condensate, is derived. \newline [1] V. Hakim Phys. Rev. E 55, 2835-2845 (1997) \newline [2] F. Grusdt, G. Astrakharchik, E. Demler New J. of Phys. 19.10, 103035 (2017)   

Creating quantum many-body scars through topological pumping of a 1D dipolar gas

Quantum many-body scars, long-lived excited states of strongly correlated quantum chaotic systems that evade thermalization, are of great fundamental and technological interest. We create novel scar states by quantum-quenching the short-range interactions of a repulsively dipolar near-integrable 1D bosonic dysprosium gas from strongly repulsive to strongly attractive. Stiffness and energy measurements show that repulsive long-range interactions render the resulting dipolar super-Tonks-Girardeau gas dynamically stable against collapse and thermalization. As the short-range interactions are subsequently made weak, the system remains in a nonthermal excited state. Cycling the interactions from weakly to strongly repulsive, then strongly attractive, and finally weakly attractive implements a quantum holonomy that offers an unexplored topological pumping method for creating scars.   

Spin-charge separation in a strongly interacting 1D Fermi gas

We propose to measure the response of a 1D Fermi gas to both density- and spin-mode excitations as a function of interaction strength via Bragg spectroscopy. We realize a pseudospin-1/2 system with two ground-state hyperfine levels of $^6$Li. We confine the atoms in an array of 1D tubes created by a 2D optical lattice and use a Feshbach resonance to tune the interactions between atoms in different spin-states. In a previous work\footnote{ T. Yang et al., Physical Review Letters \textbf{121}, 103001 (2018).} the excitation spectrum for the density-mode in this system was successfully measured. Spontaneous emission poses the largest challenge to measure the spin-mode excitations. We address this issue by performing Bragg spectroscopy on the narrow-linewidth 2S-3P transition as well as by probing a mixture of the lowest- and third-to-lowest, $|1 \rangle$-$|3\rangle$, hyperfine states. With these improvements we expect to directly observe the difference in the propagation speeds for the density and the spin modes as a function of the interaction strength, as predicted by the Tomonaga-Luttinger liquid theory.   

Exploring Conditions for Dynamical Fermionization in 1D Spinor Gases

Dynamical Fermionization is a unique quantum phenomena in one dimensional bosonic gases with hard core contact interactions. When quenched from a harmonic trap to free expansion, the gas dynamically approaches the momentum distribution and real space density distribution of a non-interacting spinless fermionic TG gas. We present our study of this phenomena in 1D spinor gases with harcdcore and strongly repulsive interactions, as well as its extension to quenches from different initial trapping potentials.   

Many-body quantum dynamics, induced correlations and bound states of Bose polarons

We unravel the ground state properties and the nonequilibrium dynamics of spinor impurities immersed in a bosonic gas following an interspecies interaction quench. For the ground state of non-interacting impurities we reveal signatures of attractive induced interactions and the formation of bipolaron states, whilst a direct impurity-impurity repulsion forces the impurities to stay apart. Turning to the quench dynamics we inspect the time-evolution of the contrast unveiling the existence, dynamical deformation and the orthogonality catastrophe of Bose polarons. It is shown that for an increasing postquench repulsion the impurities reside in a superposition of two distinct two-body configurations while at strong repulsions their corresponding two-body correlation patterns show a spatially delocalized behavior evincing the involvement of higher excited states. For attractive interspecies couplings, the impurities exhibit a tendency to localize and remarkably for strong attractions they experience a mutual attraction on the two-body level which signals the formation of a bipolaron state. To analyze and quantify induced impurity-impurity correlations, we construct an effective two-body Hamiltonian identifying the mean-field attraction and the corresponding correlations effects.   

Transport coefficients for bosonic and fermionic strongly correlated, disordered nanowires

Cold lattice gases offer an isolated platform for measuring the intrinsic transport properties of quantum systems. We propose a series of transport measurements in both bosons ($^{87}$Rb) and fermions ($^{40}$K) to study the relationship between mass, heat, and spin transport coefficients. We present a scheme for isolating a system of 1D tubes within a 3D optical lattice system using a tightly focused beam. Tunable perturbations, interactions, and disorder allow for the study of transport properties of non-trivial phases, e.g., glassy and asymptotically localized states.   

Towards Non-Equilibrium Interfaces of Strongly Interacting Fermi Gases

Strongly interacting Fermi gases out of equilibrium provide an ideal platform for quantum simulation of transport and non-equilibrium dynamics of strongly correlated fermions. Here we describe our apparatus and proposed experiments utilizing lithium-6 atoms in multi-region atomic traps for non-equilibrium and transport studies. We will employ thin light sheet potentials to prepare two- and three-region homogeneous-density atomic traps in which the spin composition of each region can be independently initialized, enabling the controlled production of non-equilibrium normal-superfluid interfaces. The transport properties of fermionic atoms across such interfaces will shed light on transport in strongly correlated normal-superconductor and ferromagnet-superconductor solid state junction devices. We describe our experimental setup, which includes a novel high-power RF antenna design for implementing spin-dependent forces, and high-field magnetic bias coils with field curvature that can be tuned through zero to maintain a homogeneous trapping potential.   

Non-equilibrium quench dynamics and observation of Townes solitons in two-dimensional Bose gases

We experimentally study non-equilibrium dynamics of two-dimensional atomic Bose gases confined in a box potential \footnote{Cheng-An Chen and Chen-Lung Hung, \textbf{arXiv:} 1907.12550 (2019)}. When the atomic interaction is quenched from repulsive to attractive values, we observe the manifestation of modulational instability that, instead of causing collapse, fragments a large two-dimensional superfluid into multiple wave packets universally around a threshold atom number, leading to the formation of unstable Townes solitons. Our density measurements in space and time domain reveal detailed information about this process, from the hyperbolic growth of density waves, the formation of solitons, to the subsequent collision and collapse dynamics, demonstrating multiple universal behaviors in an attractive many-body system in association with the formation of a quasi-stationary state. In a second quench experiment, we analyze the generation and distribution of entangled phonon pairs in the early-time evolution of the modulational instability by monitoring the time evolution of the density-density correlation and the static structure factor.   

Mardia’s Coefficient of Multivariate Kurtosis as a Measure of Thermalization

The thermalization of strongly interacting, disordered quantum systems is not fully understood. Differentiating between pre-thermalized, metastable, and statistically thermalized states is challenging. We have developed a technique to use Mardia’s coefficient of multivariate kurtosis to quantify if an atomic gas has fully thermalized. We measure the in-situ density profile of thermal Bose gas for variable hold times after a perturbation has been applied using a repulsive optical potential. We find that Mardia’s coefficient is sensitive to small deviations from equilibrium.   

Dynamics of Homogeneous Bose Gases

Quantum gases are an ideal platform to investigate out-of-equilibrium processes in real time. Using a 3D homogenous Bose gas with tunable interactions, we study effects ranging from density waves in linear response, via purely non-linear damping of the fundamental mode, to far-from-equilibrium processes in strongly driven and quenched systems. In particular, we present our microscopic study of the first and second sound mode in a weakly driven superfluid at finite temperatures and our observation of third-harmonic generation for a strong drive of the fundamental BEC mode. For a longer drive, a turbulent cascade emerges with a universal power-law distribution in quasi-steady-state. Finally, we demonstrate self-similar scaling in space-time by strongly quenching the systems through the BEC transition.   

Single-Atom-Resolved Imaging in a Triangular Optical Lattice

Frustrated spin systems belong to one of the most demanding problems of magnetism and condensed matter physics. The simplest geometrical spin frustration occurs in the triangular structure with antiferromagnetic interactions. A large variety of characteristic spin configuration can arise due to competition between the interactions and the geometry of the lattice. Recently, there have been considerable advances in the direction of simulating quantum magnetism by using a quantum gas microscope (QGM) technique. In our group, we are engineering an experimental setup of $^{87}$Rb atoms in an optical triangular lattice with QGM, which will enable us to acquire insights into real-space properties in the frustrated spin system. The QGM measurement is associated with heating of atoms due to photon scattering and therefore requires a cooling mechanism. We adopt a Raman sideband cooling (RSC) scheme for that purpose. As a result of machine-learning optimization of RSC parameters, we achieved a long lifetime during the fluorescence imaging. We also picked up a single layer of the triangular lattice planes by using a combination of a magnetic field gradient and a microwave transition. We will report on detecting fluorescence signals from atoms in a single-atom-resolved manner.   

Thermodynamics and Magnetism across the Hubbard Hamiltonian 2D-3D Crossover

Understanding the mechanisms behind quantum magnetism in lattices of ultracold fermionic atoms, which are well described by the Fermi Hubbard Model (FHM), is a major objective of optical lattice emulation. A central question is the interplay between the lattice geometry and the appearance of magnetic correlations. A particularly important aspect of geometry to understand is dimensionality, as ground states fundamentally differ in different spatial dimension, and the crossover from 2D to 3D in an anisotropic lattice is relevant to the physics of layered high-temperature superconducting cuprates. We have studied an anisotropic FHM in which the interplane hopping amplitude $t_{\perp}$ is unequal to the intraplane hopping amplitude $t$. We find that the interaction strength $U$ where nearest-neighbor correlations and structure factor are maximized depends non-trivially on $t_\perp$, but their maximum value is always largest for $t=t_\perp$.   

\textbf{Production and Control of Subradiant States in Optical Lattices}

The study of dense atomic systems with photon-photon mediated interactions are of uttermost interest in the fields of atomic physics and quantum information science. Subradiant states in dipole-dipole interacting optical lattices are of interest, since, due to collective effects, they possess decay rates far smaller than that of a single atom. Here, we present strategies for subradiant state preparation and analyze the conditions necessary for steady state subradiance in a dissipative driven optical lattice both in vacuum and inside an optical cavity. Our work paves the way towards control of long-lived atomic excitations for the storage of quantum information.   

Fermi Gas Microscope with Full Density Readout

Quantum gas microscopy has been proven to one of the most useful tools to understand quantum many-body physics ranging from high-temperature superconductivity to topological phase of matter. However, most of the experiments are using a parity projected measurement which prevents us from extracting the full information in the charge base. In this work, we demonstrate an imaging scheme for measuring the full density of a single-band Fermi-Hubbard model. By utilizing the Feshbach resonance, the atoms in doubly occupied lattice site will be coherently loaded into different layers of a vertical lattice. Due to the small lattice spacing, both layers could be in focus simultaneously and we can extract the occupation in a single image. With the high fidelity imaging technique, we measure the density fluctuation and further the temperature of the cloud using the fluctuation-dissipation theorem. Also, we measure the correlation of doulons and holons and show its correlation with the underlying spin ordering of Fermi-Hubbard model.   

Realization of anyonic Hubbard and lattice gauge models with Rydberg atoms

We propose a scheme to realize a one-dimensional Hubbard model for anyons with tunable statistical exchange phase. The scheme utilizes the density-dependent Peierl's phase in the hopping amplitude of excitations of Rydberg atoms in a zig-zag lattice, as was recently realized experimentally in [1]. The obtained Hamiltonian for hard-core anyons contains nearest-neighbor hopping as well as next nearest neighbor density-dependent hopping that results from the combination of the direct and second-order dipole-dipole exchange interactions between the atoms in the Rydberg $ns$ and $np$ states. We show how the effective anyons in the lattice can be braided to reveal their exotic exchange statistics. As a second application of the same setup, we propose the realization of a lattice gauge theory using the density-dependent second-order hopping of the Rydberg excitations.\\ [1] V. Lienhard \textit{et al.}, \textit{"Realization of a density-dependent Peierls phase in a synthetic, spin-orbit coupled Rydberg system"}, arxiv:2001.10357   

Synthetic magnetic monopole field created by two-photon Raman process in a pseudo-spin-1/2 spinor Bose-Einstein condensate

Since the beginning of the systematic study of electromagnetism hundreds of years ago, magnetic monopole have been an interesting problem in many research areas, especially in high energy and condensed matter physics. The search for naturally existing magnetic monopole has been going on for decades, and no convincing evidence for the natural magnetic monopole has been found in the laboratory so far. However, in the study of spinor quantum gases, it is possible to construct a synthetic monopole magnetic field by engineering the electromagnetic field interacting with the atoms. The Dirac monopole generated by superposition of quadrupole magnetic field and by the artificial gauge field in the dark state manifold has been studied in a spin-1 Bose-Einstein condensate (BEC). In this work, we present a new method of creating a synthetic magnetic monopole field by a modified two-photon Raman process in a pseudo-spin-$\frac{1}{2}$ BEC. We also explore the ground state spin texture of the pseudo-spin-$\frac{1}{2}$ BEC in a spherically symmetric harmonic trap by numerically solving the Gross-Pitaevskii equation.   

A Bose-Einstein condensate (BEC) on a synthetic Hall cylinder: quantum transport, emergent lattices, and topological effects

Interplay between matter and gauge fields in physical spaces with nontrivial geometries can lead to unexpected phenomena. However, most experimental studies in atom-based quantum systems have focused on spaces with relatively simple geometries. Here, we realize a BEC on a synthetic cylindrical surface (composed of a real spatial dimension and a curved synthetic dimension formed by cyclically-coupled atomic spin states) subject to a net radial synthetic magnetic flux. A lattice with a topological band structure emerges on such a Hall cylinder but disappears in the 2D plane counterpart. Applying a force to the BEC allows for studying its transport in such a lattice. We observe Bloch oscillations of the BEC with doubled period of the band structure, analogous to traveling on a Mobius strip in the momentum space, reflecting the topological band crossings protected by a nonsymmorphic symmetry. We further apply a symmetry-breaking perturbation to induce a topological transition with gap opening at the band crossings. We also study possible effects of inter-particle interactions in the quantum transport. Our work opens the door to engineering synthetic gauge fields in synthetic spaces with nontrivial geometries and observing intriguing phenomena inherent to such spaces.   

Topological effects in Floquet-engineered ultracold matter

Ultracold atoms in optical lattices constitute a versatile platform to study the fascinating phenomena of gauge fields and topological matter. Periodic driving can induce topological band structures with non-trivial Chern number of the effective Floquet Hamiltonian and paradigmatic models, such as the Haldane model on the honeycomb latticce, can be directly engineered. In recent experiments, we realized new approaches for measuring the Chern number in this system and map out the Haldane phase diagram. This includes time-resolved Bloch-state tomography allowing for the observation of a dynamical linking number after a quench as well as the application of machine learning techniques to analyze experimental data. In the future, the combination of gauge fields with a quantum gas microscope will allow accessing new regimes such as fractional Chern insulators.   

Gram Matrices, Coherent States, and Hofstadter Butterfly with Flat Band

We propose a new principle using which Hamiltonians supporting flat band can be systematically constructed. The principle is built upon the properties of the Gram matrices. Especially, the Gram matrices of certain subsets of coherent states can be interpreted as Hamiltonians describing a charged particle hopping on a two-dimensional lattice subjected to a gauge field. The massive degeneracy of the ground states in these models is a universal property guaranteed by the (over)completeness of the coherent states, independent from the geometry of the lattice. We study the ground state wave functions and the band structure of these models. Experimental realization of the model is promising because the essential features can be seen on very small lattices.   

Gauge fields and superfluid dynamics of ultracold atoms

Using the toolbox of ultracold quantum gases, we study the effects of Abelian gauge fields which possess a simultaneous time- and spatially-dependent character. GPU-accelerated numerics allow us to investigate the real-time dynamics of the Gross-Pitaevskii equation, subject to these unique vector potentials. In certain parameter regimes, we observe not only vortex nucleation, but also vortex precession around the condensate. We also report recent progress towards experimental quantum simulation of Abelian and non-Abelian gauge potentials in a $^{\mathrm{87}}$Rb Bose-Einstein condensate.   

Topology of a quantum gas with Rashba spin-orbit coupling

Topological order can be found in a wide range of physical systems, from crystalline solids and even atmospheric waves to optomechanic, acoustic, and atomic systems. Topological systems are a robust foundation for creating quantized channels for transporting electrical current, light, and atmospheric disturbances. These topological effects are quantified in terms of integer-valued `invariants', such as the Chern number. We engineered Rashba spin-orbit coupling for a cold atomic gas giving non-trivial topology, without the underlying crystalline structure that conventionally yields integer Chern numbers. We validated our procedure by spectroscopically measuring both branches of the Rashba dispersion relation which touch at a single Dirac point. We then measured the quantum geometry underlying the dispersion relation and using matter-wave interferometry to implement a form of quantum state tomography, giving a Berry's phase with magnitude $\pi$. When the Dirac point is opened, both resulting dispersions (bands) have a half-integer Chern number. This is in contrast to crystalline materials, where topological indices take on integer values, potentially implying new forms of topological transport.   

Charge and heat transport in an atomic Fermi-Hubbard system

Strongly interacting quantum systems frequently exhibit unusual transport properties that are poorly understood. In previous experiments, we used quantum microscopy to study diffusive charge transport in a Fermi Hubbard system [1], revealing a strange metal phase. We imposed density modulations on a uniform system of $^6Li$ atoms in a 2D lattice and observed their decay due to charge diffusion as a function of wavelength and temperature. In our most recent work, we added an external linear potential (a ``tilt'') and observe subdiffusive charge dynamics. The tilt couples mass transport to local heating through energy conservation. Due to this coupling the system quickly heats up to near infinite temperature in the lowest band of the lattice. We study the high-temperature transport and thermalization in our system as a function of tilt strength and find that the associated decay time $\tau$ crosses over as the tilt strength is increased from characteristically diffusive to subdiffusive with $\tau\propto\lambda^4$. In order to explain the underlying physics and emphasize its universal nature we develop a hydrodynamic model that exhibits this crossover. [1] P. T. Brown et al, Science 363, 379-382 (2019)   

Doping a Hubbard Antiferromagnet in and out of Thermal Equilibrium

Understanding how dopants interact with antiferromagnetism is key to deciphering the mysteries of the Hubbard model. Quantum gas microscopy enables progress towards this goal by allowing for site-resolved measurements of density and correlation functions. We use such a platform to first create a doped 2D Hubbard system in thermal equilibrium. Using novel string observables, we examine how the dopants scramble the spin background in non-trivial ways that are distinguishable from elevated temperatures. We then prepare a system at half-filling with elevated potential at a single site, creating a pinned dopant. We quench the system by releasing the dopant and examine the dynamics in a time-resolved manner. The dopant exhibits a transition from ballistic to sub-ballistic motion; measurements of the spin correlation function show that the dopant motion scrambles the surrounding spin environment during this process. Examination of the Hubbard model both in and out of equilibrium may shed light on the mechanisms behind the highly correlated phases that may exist in the ground state of the doped Hubbard model.   

Progress towards Cold Molecule Synthesis and Single Molecule Detection with a Micro-resonator

Ultracold molecules have wide applications in ultracold chemistry, quantum computation and quantum many-body physics. Photoassociation (PA) is a powerful approach to the assembly of molecules from cold atoms into their deeply bound molecular states, even the rovibrational ground state. However, traditional ways to determine molecular state such as resonance enhanced multiphoton ionization method require large number of molecules to be tested. To detect the final state of PA-assembled molecules, we propose to make use of a high-Q micro-resonator supporting a whispering gallery mode to achieve state-sensitive detection of molecules. Although it is difficult to find a close cycle in the complex energy structure of a molecule, the weak photon signal could be detected via the high-cooperativity coupling of a molecule to a micro-resonator, which prolongs the interaction time by way of modifying photon mode density in resonator. Here, we discuss an apparatus for cold molecule assembly and present theoretical simulation on single molecule detection with a microcavity.   

Towards Optical Dipole Trapping of SrF Molecules with High Capture Efficiency

The ability to directly cool and trap molecules in conservative traps, such as optical dipole traps (ODT) and magnetic quadrupole traps, has been demonstrated recently for both CaF and SrF molecules. This is a critical step towards further cooling and compression of molecules, as is needed to study molecular collisions in the quantum regime and to potentially reach quantum degeneracy. Currently, only $N\sim$10$^{4}$ molecules can be collected in magneto-optical traps (MOTs) of $\sigma\sim 1$\,mm in size, which are the starting point for these experiments. Thus, capturing as large a fraction of these molecules as possible into an ODT is critical. As has been demonstrated in CaF (Cheuk et al., PRL 121, 083201 (2018)), $\Lambda$-enhanced gray molasses can cool molecules within an ODT, enhancing loading fractions tenfold compared to `un-enhanced' gray molasses cooling. This poster will report on several other methods aimed at efficient loading of SrF molecules into an ODT.   

Progress towards large, dense samples of laser-cooled molecules

The extension of laser cooling and trapping techniques to molecules promises access to new research directions from quantum simulation to improved precision measurements. To date, inefficient trap loading has been a key barrier preventing the production of large, dense samples of ultracold molecules using molecular magneto-optical traps (MOTs). Our experiment aims to remove this barrier by producing brighter beams of cold molecules and by working with species with closed electronic shells in $^{\mathrm{1}}\Sigma ^{\mathrm{+}}$ ground states. These molecules have favorable properties for laser cooling including a lack of spin-rotation structure and the presence of strong optical transitions for efficient trap loading. We will present an update on experimental progress including the conditions required to produce quasi-continuous beams of cold and slow molecules, a background-free fluorescence imaging scheme and a new laser system projected to produce \textasciitilde 1 W at 261.5 nm.   

Laser cooling and trapping CH radicals

Molecular laser cooling and trapping offers a general technique to produce ultracold molecules and is applicable to a variety of species with different internal structures. Our latest experiment aims to capitalize on this generality by laser cooling and trapping CH radicals for tests of ultracold organic chemistry. The low mass and blue optical transitions in this species lead to high recoil velocities which can significantly reduce the number of scattered photons required to slow and trap a molecular beam from a cryogenic buffer gas beam source. Our proposed optical cycling schemes use bichromatic laser light on the X$^{\mathrm{2}}\Pi $-A$^{\mathrm{2}}\Delta $ transition to apply coherent stimulated forces and the radiative force from optical cycling on the X$^{\mathrm{2}}\Pi $-B$^{\mathrm{2}}\Sigma^{\mathrm{-}}$ transition. We will present the challenges associated with rotational branching in this species alongside an update on our experimental progress.   

Single weakly-bound NaCs molecule in optical tweezers

Ultracold polar molecules have long-range, anisotropic, and tunable interactions providing a versatile platform for studying quantum many-body physics, quantum information, and quantum simulation. Optical tweezers allow us to trap atoms and molecules in flexible configurations and fully control their quantum states. The formation of weakly bound molecules in optical tweezers is an important intermediate step towards strongly interacting ground state molecules. I will present the two approaches we took to create the weakly bound molecules from atoms in the optical tweezer. I will discuss the challenges and results on creating Feshbach molecules and progress towards optical transfer to the vibrational ground state.   

Quantum state dependent chemistry of ultra-cold $^6$Li$_2$ dimers

Starting from an ultra-cold ensemble of $^6$Li$_2$ Feshbach molecules, we produce deeply bound molecules by STIRAP in the lowest energy levels of the $v=0,\; 5,\; 8$ and $9$ vibrational manifolds of the $a(1^3\Sigma_{u}^+)$ potential. The ensemble lifetime is found to be limited by two-body collisions with a loss rate near the universal rate for three of these states and, remarkably, below universality for the $|v=9, N=0\rangle$ state. In addition, unlike all prior experimental work with ultra-cold molecules, we observe a rotational state dependence of the reaction rate. We observe that molecules in the absolute lowest triplet level are unstable. Because of the suppression of spin-changing collisions and absence of other inelastic collision channels, we conclude this instability is primarily due to trimer formation, consistent with theoretical predictions.   

Large molasses-like cooling forces for molecules using polychromatic optical fields: A theoretical description

Recent theoretical investigations have indicated that rapid optical cycling should be feasible in complex polyatomic molecules with diverse constituents [1], geometries and symmetries [2]. However, as a composite molecular mass grows, so does the required number of photon scattering events necessary to decelerate and confine molecular beams using laser light. Utilizing coherent momentum exchange between light fields and molecules can suppress spontaneous emission and significantly reduce experimental complexity for molecular slowing and trapping. Working with BaH as a test species, we have identified a robust, experimentally viable configuration to achieve large molasses-like cooling forces for molecules using polychromatic optical fields addressing both X-A and X-B electronic transitions simultaneously. Using direct numerical solutions of the time-dependent density matrix, we demonstrate that creation of optical molasses-like forces with large capture velocities is generically feasible for polyatomic molecules of increasing complexity that have an optical cycling center. Both numerical results and progress towards experimental proof-of-principle implementation with BaH will be described. [1] Klos and Kotochigova, arXiv:1912.09364 [2] Augenbraun et al., arXiv:2001.11020   

Fingering instabilities and pattern formation in a two-component dipolar Bose-Einstein condensate

We study fingering instabilities and pattern formation at the interface of an oppositely polarized two-component Bose-Einstein condensate (BEC) with strong dipole-dipole interactions (DDIs) in three dimensions. It is shown that the rotational symmetry is spontaneously broken by fingering instabilities when the DDIs are strengthened. Frog-shaped and mushroom-shaped patterns have been shown with different strengths of the DDIs. A Bogoliubov analysis gives a qualitative understanding of the interfacial instabilities of the two dipolar BECs, and a dispersion relation similar to that in classical fluids is obtained. Spontaneous density modulation and dipolar domain growth in the dynamics have also been demonstrated, in which we have analyzed the characteristic sizes of the dipolar domains corresponding to different patterns at the initial and later times in the evolution. We have also investigated the parameter dependence of the ground states, and found that the droplet patterns are formed due to the population imbalance in the two components. Labyrinthine patterns grow as the trap ratio increases, and a striped phase appears as the angle of tilted polarization increases. Our findings may establish further connections between superfluids and classical fluids.   

Many Body Energy Transfer in an Ultracold Rydberg Gas

In a resonant dipole-dipole interaction, Rydberg atoms exchange energy such that they transition to different states. We investigate the $n$-dependence of many-body dipole-dipole interactions. We present experimental and simulated results on the line shape of two, three, and four-body interactions.   

Efficient production of Bose-Einstein Condensates of $^{168}$Er

Recently lanthanide atoms such as dysprosium and erbium have attracted significant attention in quantum simulation with ultracold atoms due to their large magnetic moment and richness of Feshbach resonances. In this poster, we demonstrate the achievement of Bose-Einstein Condensates of $^{168}$Er atoms in a new apparatus. Using the technique of two-stage slowing, we operate a narrow-line magneto-optical trap (MOT) with more than 10 times improvement of loading rate, followed by the optical evaporation in a crossed optical dipole trap, and produce BECs every 10~s. The experimental scheme in our apparatus lays a good foundation to conduct experimental studies of supersolidity or interacting topological phase in the future.   

Extended coherently delocalized states in a frozen Rydberg gas

The long-range dipole-dipole interaction can create delocalized states due to the exchange of excitation between Rydberg atoms. We show that even in a random gas many of the single-exciton eigenstates are surprisingly delocalized, composed of roughly one quarter of the participating atoms. We identify two different types of eigenstates: one which stems from strongly-interacting clusters, resulting in localized states, and one which extends over large delocalized networks of atoms. These two types of states can be excited and distinguished by appropriately tuned microwave pulses, and their relative contributions can be modified by the Rydberg blockade and the choice of microwave parameters.   

Numerical studies of few-body dipole-dipole interactions in rubidium

Ultracold Rydberg atoms can exchange energy through resonant dipole-dipole interactions. This includes a class of recently discovered few-body interactions. In concert with recent experimental work in our group, we present computational studies of the density-dependence and lineshape of two-, three-, and four-body resonant dipole-dipole interactions in rubidium. We also investigate the prospects for studying thermalization and many-body localization in these systems.   

Rydberg-State-Resolved Resonant Energy Transfer in Cold Collisions of Ammonia Molecules with Rydberg Helium Atoms

The large electric dipole moments associated with transitions between Rydberg states of atoms make them ideal model systems with which to study Förster resonance energy transfer in collisions with other Rydberg atoms or molecules, or polar ground state molecules. Here we report Rydberg-state-resolved measurements of the resonant transfer of energy from the ground-state inversion sublevels in NH$_3$ to He atoms in triplet Rydberg states with principal quantum number $n = 38$. These intrabeam collision studies were performed at translational temperatures of ~1 K in seeded pulsed supersonic beams. Electric fields of up to 15 V/cm were used to tune individual Rydberg-state-to-Rydberg-state transitions into resonance with the NH$_3$ inversion transitions. Resonant energy transfer in the atom-molecule collisions was identified by Rydberg-state-selective electric-field ionization. The experimental data, with resonance widths of ~500 MHz, have been compared to a theoretical model of the resonant dipole-dipole interactions between the collision partners based on the impact parameter method.   

Engineering Tunable, Nonlocal Heisenberg Spin Models in an Optical Cavity

Photon-mediated interactions between atoms in an optical cavity promise to enable the study of quantum spin models with complex, nonlocal interaction graphs. In our system, the sign, strength and spatial structure of these interactions are controlled by magnetic and optical fields, and in situ imaging enables visualization of local spin dynamics. We have recently implemented a family of XXZ Heisenberg models with continuous tunability between XY (spin-exchange) and Ising interactions. This tunability enables access to a rich phase diagram, in which we explore a paramagnetic-to-ferromagnetic phase transition predicted by a collective spin model. We show that ferromagnetic XY interactions protect the coherence of the collective spin, opening prospects for designing robust squeezing protocols. Conversely, breakdowns of the collective spin model create complex many-body dynamics of theoretical interest. To better explore these dynamics, current directions include trapping single or small groups of atoms with optical tweezers and improving control over the spatial structure of interactions through the spectrum of a drive field. These advances will allow exploration of new forms of integrability, fast scrambling, and simulation of hard computational problems.   

Dipole blockade between symmetric combinations of Rydberg Stark states

Quantum information protocols relying on neutral atom based architectures rely on the Rydberg (Ry) blockade mechanism to mediate conditional logic. The leading order interaction, in this case, is the dipole-dipole interaction between Ry states of opposite parity. One problem in using the first-order dipole interactions between Ry states of definite parity, such as $|ns\rangle$ and $|np\rangle$, is that the interaction is anisotropic, introducing errors in two-qubit gate operations relying on Ry blockade. We will demonstrate that simple linear combinations of Ry Stark states can be employed to partially mitigate this issue. We consider linear combinations of Ry Stark states that are symmetric about the middle of the Ry Stark manifold, such as $|R\rangle \pm |B\rangle$ where $|R\rangle$ and $|B\rangle$ are the red-most and blue-most states in a Stark manifold. Such states have decompositions in terms of the $l$-basis states that only include either the even or the odd parity states. As states closer to the middle of the Stark manifold contain more $l$-states in their decomposition, one can in principle attain better isotropy in the dipole blockade while retaining its order of magnitude interaction strength and $1/R^3$ behavior.   

Rydberg-Dressed Ising Interactions for Quantum Simulation and Metrology

Rydberg dressing provides a means of realizing optically controllable long-range interactions between neutral atoms. Theoretical proposals envision leveraging these interactions to simulate frustrated quantum magnets, access new non-equilibrium phases of matter, or engineer metrologically useful entangled states. Here, we present experimental results on the emulation of a transverse-field Ising model using a cold bulk gas of Rydberg-dressed cesium atoms (\emph{arXiv:1910:13687}). We observe Ising dynamics which manifest as one-axis twisting, holding promise for creation of squeezed spin states, and detect dynamical signatures of the paramagnetic-ferromagnetic phase transition. We additionally discuss progress towards combining our current Rydberg-dressing capabilities with the spatial control of optical tweezers, to enable applications in quantum simulation and combinatorial optimization.   

In situ imaging of Quantum Degenerate $^{133}$Cs-$^6$Li Bose-Fermi Mixture

We report our experimental progress on the investigation of Bose-Einstein condensates of $^{133}$Cs atoms immersed in degenerate Fermi gases of $^6$Li atoms. We employ a high resolution imaging system and a digital micro-mirror device (DMD) in order to detect and control Bose-Fermi mixtures. The microscope with numerical aperture of 0.6 is expected to reach a resolution below 1 micron for both species. In addition, the DMD is capable of preparing generic optical potentials at the same length scale. The system enables novel schemes to characterize the long range nature of the fermion-mediated interactions between bosons and offers a versatile platform to resolve new quantum phases and dynamics of interacting Bose-Fermi mixtures.   

Effect of finite size cavity on $nS$ Rubidium Rydberg state lifetimes

In this work, we present lifetime measurements of $nS$ states of Rb as a function of the principal quantum number (40 $\leq$ n $\leq$ 70) using a sample of cold atoms held in a magneto-optical trap, which is performed in a finite size metal vacuum chamber. The Rydberg states are excited through a two-photon transition, and detected by pulsed field ionization. Our measurements are larger than the predictions by well established theoretical model. We have implemented a theoretical model, which considers the vacuum chamber as a lossy Fabry-Perot cavity with a discrete spectrum, and compared with experimental results. Such comparison indicates that the blackbody radiation contribution on Rydberg state lifetime can be decreased by using a small size metal cavity, without the need of cryogenic environment. This effect may have application in experiments where longer Rydberg lifetimes are required.   

Rotons and Bose condensation in Rydberg-dressed Bose gases

We investigate the ground-state properties and excitations of Rydberg-dressed bosons using the hypernetted-chain Euler-Lagrange approximation, which accounts for correlations and thus goes beyond the mean-field approximation. The short-range behavior of the pair distribution function signals the instability of the homogeneous system with respect to the formation of droplet crystals at strong couplings and large soft-core radius. This tendency to spatial density modulation coexists with off-diagonal long-range order. The contribution of the correlation energy to the ground-state energy is significant at large coupling strengths and intermediate values of the soft-core radius while for a larger soft-core radius the ground-state energy is dominated by the mean-field (Hartree) energy. We have also performed path integral Monte Carlo simulations at selected system parameters to verify the performance of our hypernetted-chain Euler-Lagrange results. In the homogeneous phase, the two approaches are in very good agreement. Moreover, Monte Carlo simulations predict a first-order quantum phase transition from a homogeneous superfluid phase to the quantum droplet phase with face-centered cubic symmetry for Rydberg-dressed bosons.   

Apparatus improvements for experiments on ultracold neutral plasmas

This poster will describe recent improvements in our apparatus for experimental investigations of ultracold neutral plasmas (UNPs). Specifically, we have implemented a new technique for observing the UNP expansion by stripping the electrons with a fast rise-time electric field pulse ($\sim10$ ns rise-time, $\sim10$ V/cm amplitude) and projecting the ions towards our micro-channel plate detector (MCP). By careful calibration of the ion time-of-flight signal, the spatial profile of the UNP can be recovered. By observing how this changes as a function of delay between the plasma creation time and the application of the fast pulse, the UNP expansion velocity can be obtained. Additionally, we have installed a tapered amplifier (TPA) on our vapor-cell MOT cooling and repumper laser beams to increase the cold atom density. Finally, we have installed a Pockels cell switch to switch off the MOT cooling and repumper beams in $\sim10$ ns. This will be used in conjunction with a narrow bandwidth 780 nm laser pulse applied after the MOT laser beams have been switched off to suppress the thermal atom background in experiments on cold Rydberg plasmas.   

Modeling Launched Cold Atoms in Distinguishable Quantum States Traversing a Cylindrically-Symmetric Electric Field

We continue to model the trajectories of launched laser-cooled Rydberg atoms as we prepare to steer atoms in the electric field of a charged wire. Our iterative calculations account for acceleration derived from the spatially-dependent Stark shift for slow-moving rubidium atoms in Rydberg states. We model atoms with $n=35$ and $|m_j|=7/2$, identifying paths for each of 63 individual quantum sublevels that are distinguishable in an external electric field. Our model of a launched cold-atom cloud is seeded from a distribution with vertical (launch) velocity $v_y=4.0\pm0.4$ m/s, horizontal velocity $v_x=0.0\pm0.2$ m/s, and a spatial spread of 100 $\mu$m in the vertical direction. These conditions correspond to our experiments using the two-photon pathway $5S\rightarrow5P\rightarrow5D$ to excite atoms in flight and prepare us for three-photon excitation to selected sublevels of the Rydberg state with $n=35$.   

Observation of a large, resonant, cross-Kerr nonlinearity in a cold Rydberg gas and progress towards quantum non-demolition measurement of photon number

We report an experimental observation of a dispersive cross-Kerr nonlinearity based on resonant Rydberg EIT. We observe that the phase shift acquired by a resonant optical pulse propagating through a cold cloud of atoms under EIT conditions depends linearly on the intensity of a second optical pulse. Our observations are consistent with a simple theoretical treatment based on van der Waals interactions, which provides an intuitive explanation for the origin and scaling of the cross-Kerr nonlinearity. Finally, we discuss various ways to increase the per-photon phase shifts and decrease measurement uncertainty, both of which will be required in order to realize a quantum non-demolition measurement of photon number.   

RYDBERG Excitation of Laser-colled Potassium Atoms in the AC-MOT

The operation of a new cold atom trap (the AC-MOT) and its application in Rydberg atom spectroscopy experiments is described. A significant limitation of magneto optical trapping (MOT) techniques has been the requirement to eliminate the magnetic fields prior to the interaction occurring. The AC-MOT is a pulsed trap, so that the magnetic fields are eliminated prior to the excitation to Rydberg states. The excitation of potassium atom to different Rydberg states is discussed, the excitation proceeding in a stepwise manner using a combination of infrared radiation and radiation from a tunable blue laser. Precise energy levels of high-n Rydberg states of potassium have been measured using stepwise-excitation of cooled, trapped atoms in the AC-MOT, with the intermediate state being the $4^{2}P_{1/2} $state.   

The resonant Energy transfer Rb ns $+$Rb ns $+$2h$\nu \to $Rb np$_{\mathrm{1/2}} \quad +$ Rb np$_{\mathrm{3/2}}$ in a frozen Rydberg gas

We have observed the process Rb ns $+$Rb ns $+$2h$\nu \to $Rb np$_{\mathrm{1/2}} \quad +$ Rb np$_{\mathrm{3/2}}$ from n$=$34 to n$=$41 in a frozen gas of Rb Rydberg atoms. It is resonant when the microwave frequency is halfway between the ns$\to $np$_{\mathrm{1/2}}$ and ns$\to $np$_{\mathrm{3/2\thinspace }}$frequencies. The frequencies range from 57 to 106 GHz. The process can not occur in isolated atoms, nor can it occur if the magnetic quantum numbers are unchanged, an implicit assumption of one dimensional models. A Floquet-Forster model shows that the coupling between the initial and final states involves the absorption of two microwave photons and the dipole-dipole interaction, which leads to a coupling proportional to the product of the density, the microwave field squared, and n$^{\mathrm{14}}$. We have experimentally verified these dependences. The observed resoannces are asymmetric, with a low frequency tail, which we attribute to the van der Waals shift of the final np$_{\mathrm{1/2}}$np$_{\mathrm{3/2}}$ state due to its dipole-dipole interaction with the nearby ns(n$+$1)s state. While the van der Waals shift is negligible for most of the atoms in the Rydberg gas, it is not for the pairs of close atoms which undergo this transition. *permanent address: Quaid-i-Azam University   

Interactions in ultracold Rydberg gases

Rydberg atoms are highly excited atoms. The radius of Rydberg atoms is proportional to n$^{\mathrm{2}}$, where n is the principal quantum number. Therefore, the interactions between such atoms are much stronger than the interactions between ground state atoms. Room temperature Rydberg atoms have very high kinetic energies, and it is difficult to study such interactions. Laser cooling and trapping made it possible to study such interactions. In this presentation, we report on recent studies on interactions within an ultracold Rb gas.   

Comparing multiple theory treatments applied to off-resonant radiofrequency heating of ultracold neutral plasmas with varying electron magnetization

We have recently conducted experiments measuring ultracold neutral plasma electron off-resonant radiofrequency heating rates. These measurements were performed at low (weak) and high (extreme) degrees of magnetization. Multiple theoretical treatments can be adapted to be applied to these measurements, including binary collision, stopping power, and AC conductivity theories. We discuss the applicability of these theories and compare their predictions to experimental results.   

Excitation of high-angular-momentum Rydberg states in a deep Ponderomotive Optical Lattice

A Rydberg-atom ponderomotive optical lattice (POL) differs from a conventional optical lattice trap in that the trapping force is mainly derived from the ponderomotive force of the light field on the quasi-free Rydberg electron, and in that the size of the Rydberg atom may be comparable to the lattice spacing. Thus, the effective potential is an average over the spatial density of the Rydberg electron wavefunction and no longer follows the shape of the lattice intensity. In the deep-lattice regime of interest in this work, the POL requires a non-degenerate treatment that accounts for POL-induced state mixing within large Hilbert spaces. The resultant adiabatic energy levels are a combination of rotational and vibrational energy series near the intensity minima and maxima, and Stark-like level series near the inflection points, leading to rich unusual spectra. In the present experimental implementation, a deep cavity-generated 1064-nm POL mixes the Rydberg F-state with the high-angular-momentum states to allow three-photon excitations of the type 5S$_{1/2}\rightarrow$5P$_{1/2}\rightarrow$5D$_{3/2}\rightarrow$n($\ell\geq$3) that would otherwise be suppressed by electric dipole selection rules.  The theoretical analysis will be reviewed and experimental progress will be presented.   

Development of reinforced learning optimal control based on ML-MCTDHX

In this poster, we present our recent development of the optimal control based on the reinforced learning, on top of the numerical engine of ML-MCTDHX. The optimal control has become a powerful tool in various fields for improving the fidelity of the state transformation. An efficient optimal control mainly relies on two key ingredients: One is the powerful numerical engine which are cable to perform numerical simulation on a complicate setup, and the other is the efficient optimization of the parameters of the Hamiltonian. In our routine, we combine ML-MCTDHX, which is an ab-initio numerical tool capable to handle strongly correlated multiple degrees of freedom systems, and the reinforced learning optimization scheme, which has been proved to efficiently perform optimization in a high-dimension parameter space. We will demonstrate the efficiency of the routine with a state preparation in an ultracold atom setup.   

Gausian processes for system-agnostic construction of high-dimensional PES with sparse ab initio data

Gaussian process (GP) regression has recently been proposed as a system-agnostic tool for building global potential energy surfaces (PES) of polyatomic systems. As with other machine-learning tools, the accuracy of GP regression can be improved by increasing the number of training points. However, this presents significant challenges for applications of GPs in physics. Here, we show that the accuracy of GP models of PES can be improved by increasing the complexity of GP kernels instead of the number of ab initio points. This allows us to build accurate global PES for molecular systems with 4 and 19 atoms, using 500 and 5000 quantum chemistry calculations, respectively. We show that GP models of the PES thus constructed have generalization power, allowing us to extrapolate global PES from low energies to high energies. We illustrate an algorithm to enhance the accuracy of PES by simultaneously optimizing the distributions of ab initio points and the complexity of GP kernels. References: arXiv:1907.08717; arXiv:2001.07271   

Machine-learning-corrected quantum dynamics calculations

Quantum scattering calculations must generally rely on approximations. All approximations introduce errors. The impact of these errors is often difficult to assess because they depend on the Hamiltonian parameters and the particular observable under study. In this work, we illustrate a general, system and approximation-independent, approach to improve the accuracy of quantum dynamics approximations. The method is based on a Bayesian machine learning (BML) algorithm that is trained by a small number of rigorous results and a large number of approximate calculations, resulting in ML models that accurately capture the dependence of the dynamics results on the quantum dynamics parameters. Most importantly, the present work demonstrates that the BML models can generalize quantum results to different dynamical processes. Thus, a ML model trained by a combination of approximate and rigorous results for a certain inelastic transition can make accurate predictions for different transitions without rigorous calculations. This opens the possibility of improving the accuracy of approximate calculations for quantum transitions that are out of reach of rigorous scattering calculations.   

On Demand

Quantum Neural Networks incorporating Quantum Variational Monte Carlo have become a new tool to study quantum 1D spin chains. We discuss our work in studying the response of a deep convolutional neural networks hidden layers to the symmetries and structure of 1D spin chains such as an SU(N) model, and our analyses of modifying the architecture design on the learning rate of the model. We also discuss the response of the hidden layers to the symmetries of the SU(N) Hamiltonian, and extensions of the model to other 1D spin chains in inhomogenous traps.   

Measuring Magnetic Fields with Magnetic-Field-Insensitive Transitions

Atomic sensing is typically performed by tracking an accumulating dynamical phase, which appears due to the dependence of the energy on the quantity to be sensed. Contemporary high-sensitivity atomic magnetometers operate by tracking the phase difference between Zeeman-split atomic states. Here, we use clock states, atomic states that have a magnetic-field insensitive energy difference. These states are, in leading order, robust to magnetic field noises and therefore have long coherence time. They are typically used in atomic clocks. One would expect that clock states would be unable to sense magnetic fields. We show that, surprisingly, clock states can indeed acquire a phase, which is proportional to the magnetic field magnitude. We measure this effect on an ensemble of trapped Rb87 atoms. We propose a new magnetic field sensing method which uses magnetic-field-insensitive transitions. We show that our measurement’s sensitivity scales inversely with the coherence time of the clock subspace, which is typically much longer than in a Zeeman-split subspace. This implies that our proposed method may be used to improve upon the sensitivity of Zeeman-splitting based magnetometry methods. Our findings have been recently published on Phys. Rev. Lett. 123, 133204 (2019).   

Imaging Nematic Transitions in Iron-Pnictide Superconductors with a Quantum Gas

The SQCRAMscope is a recently realized Scanning Quantum CRyogenic Atom Microscope that utilizes an atomic Bose-Einstein condensate to measure magnetic fields emanating from solid-state samples. Here, we combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface of a sample to study iron-pnictide superconductors, where the relationship between electronic and structural symmetry-breaking resulting in a nematic phase is under debate. We conduct simultaneous and spatially resolved measurements of both bulk and surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing local measurements of emergent resistivity anisotropy in iron pnictides, we observe sharp, nearly concurrent transport and structural transitions. More broadly, these measurements demonstrate the SQCRAMscope’s ability to reveal important insights into the physics of complex quantum materials.   

Progress Toward a Free Precession Hg-Cs Co-magnetometer for Measurements of Long-Range Spin-Spin Interactions

We report progress on the development of a Hg-Cs co-magnetometer for use in an experimental search for long-range spin-spin interactions (LRSSI) using the Earth as a polarized spin source. Our new apparatus uses a pump-then-probe geometry in which the light beams are perpendicular to the applied field such that no first-order light shifts should arise. These light shifts are believed to have limited the LRSSI bounds established by our previous experiment [1]. We have demonstrated that the statistical sensitivity of this new scheme is sufficient to achieve an order of magnitude improvement over previous limits. Investigations of systematics associated with variations of the frequency and power of the light beams are also reported. [1] L.R. Hunter, J.E. Gordon, S.K. Peck, D. Ang, and J.-F. Lin, Science \textbf{339}, 928 (2013).   

Properties of alkali atoms trapped in solid parahydrogen

Alkali atoms trapped in solid parahydrogen exhibit excellent spin coherence properties at high electron spin densities. We have studied potassium, rubidium, and cesium atoms trapped in parahydrogen. Different species exhibit order-of-magnitude differences in optical pumping and ensemble spin dephasing times. Using dynamical decoupling techniques, the spin coherence time can be extended by orders of magnitude. These properties and other measurements in parahydrogen will be presented. Future applications in quantum sensing and precision measurement will be discussed.   

Pulsed gradiometry in Earth's field

Measuring ambient (without shielding) magnetic field gradients from a single optical signal is an important problem in atomic magnetometry. We report on the development of an atomic gradiometer based on the hyperfine splitting in two vapor cells of warm $^{\mathrm{87}}$Rb atoms. The gradiometer takes advantage of a process similar to resonant Raman scattering, where a pulsed microwave field resonant with the hyperfine ground state splitting prepares an atomic coherence and generates sidebands offset from a weak (carrier) beam incident the two vapor cells. From the sidebands, a single optical beat note is produced, with the frequency of the beat determining the magnetic field gradient between the two cells. Operation of the gradiometer in multiple field orientations is discussed, along with current research investigating the feasibility of single laser operation, where one beam acts as both a pump and carrier. Applications of this research, including Magnetoencephalography (MEG), where multiple sensor channels are positioned around the human skull, would benefit from the compactness and simplicity of a single laser setup. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology {\&} Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.   

Characterization of a simple RF magnetometer for the GNOME network

The Global Network of Optical Magnetometers to search for Exotic physics (GNOME) is a network of geographically separated, time-synchronized, optically pumped atomic magnetometers searching for correlated transient signals that might herald exotic physics [1]. We present a characterization of a phase-locked loop (PLL)-locked pump-probe RF-driven magnetometer that probes the $^{\mathrm{85}}$Rb F$=$2 hyperfine transition [2]. Our station is simple and low-cost, using a single 780 nm DBR laser and homemade electronics, instead of a lock-in amplifier, to measure the circular dichroism of the probe light. The magnetometer can run unattended for days at a time and reaches a sensitivity of 400 fT at 0.2 s with a bandwidth of \textasciitilde 100 Hz. ~ [1] S. Afach, D. Budker et al., Physics of the Dark Universe, 22, 162-180 (2018) [2] Groeger, S., Bison, G., Schenker, JL. et al. Eur. Phys. J. D, 38, 239-247 (2006)   

Enhanced Quantum Diamond Microscope

We present an enhanced 'Quantum Diamond Microscope' (QDM) that uses ensembles of nitrogen-vacancy (NV) defects in diamond for imaging magnetic fields of rock samples with micron-scale spatial resolution and mm-scale field-of-view. Hardware improvements of the laser beam profile, microwave delivery, and light collection increase the sensitivity and decrease the noise floor of the QDM. Additionally, new data analysis methods utilizing machine learning extend the range of rock samples that can be quantitatively studied.   

Two-photon vibrational transitions in ${\rm O}_2^+$

Vibrational overtones in the ${\rm O}_2^+$ molecule are electric-dipole forbidden and thus intrinsically narrow and immune from some systematic shifts.[1] They could serve as reference frequencies for optical clocks or as probes of new physics such as time-variation of fundamental constants. We report on our attempts to drive these transitions with two photons from a nanosecond pulsed laser. Our goal is to reduce the measurement uncertainty in the vibrational frequency by several orders of magnitude. In addition to an overview of our experiment, we present recent upgrades that reduce the temperature of our molecular beam and increase our signal.\\ \\ 1. R. Carollo, A. Frenett, D. Hanneke, Atoms v.7, 1 (2018)\\ \\ This work is supported by the NSF (RUI PHY-1806223).   

Searches for Physics Beyond Standard Model Using Polyatomic Molecules

Polar molecules are a robust platform for precision measurement searches of Charge-Parity (CP) violating physics beyond the Standard Model (BSM). Recent experiments on diatomic molecules have excluded BSM CP-violating leptonic physics at TeV energy scales. To further extend this range, it is desirable to combine advances in molecule laser cooling and trapping with these sensitive searches. However, diatomic molecules either do not have electronic structures amenable to optical cycling, or do not have innate strong systematic error rejection critical for these searches. Fortunately, certain polyatomic molecules have been identified to exhibit both laser-cooling capability and co-magnetometer states for error rejection, making them ideal candidates for advanced BSM searches. We report progress on two precision measurement experiments: a beam measurement probing hadronic CP violation via the magnetic quadrupole moment of the 173Yb nucleus using 173YbOH, and a measurement of laser-cooled and trapped 174YbOH to probe the electron EDM to search for new physics at the PeV scale. Recent advances include enhancement of molecule production by an order of magnitude, and spectroscopic studies of important energy levels of YbOH.   

Rotational spectroscopy of BaF

Barium fluoride (BaF) is one of the heaviest molecular candidates for effective laser cooling, and as such BaF is being employed in eEDM, anapole and a variety of other experiments in a number of laboratories worldwide. Beyond the relevant recent BaF optical spectroscopy study by Steimle, et al., the purpose here is to extend and complement the existing microwave spectroscopy data for BaF. We will present high-resolution (\textasciitilde 1 kHz) data for the N $=$ 1-0 and 2-1 transitions in the vibrational ground state for the five most abundant stable Ba isotopes. This reflects an improvement in resolution of a factor of 20 or more, and direct ablation of barium metal in the presence of a fluorine-containing buffer gas has enabled the first microwave observations of low abundance $^{\mathrm{135}}$BaF (6.6{\%}) and $^{\mathrm{134}}$BaF (2.4{\%}). A comparison of the resulting molecular parameters for the two odd barium isotopologues $^{\mathrm{137}}$BaF and $^{\mathrm{135}}$BaF (both I $=$ $^{\mathrm{3}}$/$_{\mathrm{2}})$ will be highlighted, in the context of the goal of a robust global fit of all the microwave data, which includes transitions up to N $=$ 22-21 and vibrational states up to v $=$4.   

Enhanced sensitivity to ultralight bosonic dark matter in the spectra of SrOH

The rich spectra of polyatomic molecules offer the possibility of enhanced sensitivity to variations in fundamental constants such as the proton-to-electron mass ratio, $\mu$, arising from coupling between Standard Model particles and theoretically well-motivated ultralight dark matter (UDM) candidates. Recent extension of direct laser cooling techniques to a few linear triatomic metal hydroxide radicals has potential to enable long measurement coherence times and high spectroscopic precision. We show that in SrOH, a near-degeneracy between rotational states in the $X(200)$ and $X(03^10)$ vibrational manifolds of different character leads to $~10^3\times$ enhanced sensitivity to $\mu$: a time-dependent change $\delta\mu$ in $\mu$ would lead to a change $\delta\nu$ in the resonance frequency $\nu$ according to $\delta\nu/\nu~10^3\delta\mu/\mu$. We propose to use laser cooling and trapping of SrOH molecules and an experimental approach to enable measurements of $\delta\mu/\mu$ with as low as $10^{-17}$ fractional uncertainty [1]. A preliminary investigation of potential systematic errors will be discussed as well as possible implications for UDM searches. [1] Kozyryev et al., arXiv:1805.08185   

Atom Interferometry Techniques for Improved Sensitivity, Stability and Dynamic Range

Cold-atom interferometers have demonstrated extremely high sensitivity as inertial sensors measuring gravity, gravity gradients, and accelerations and rotations. However, high performance operation outside the lab is generally challenging due to limited dynamic range and systematic effects. We will present new approaches to atom interferometry that tackle these limitations, including simultaneous quadrature-phase detection, extension of dynamic range by orders of magnitude using new, composite, interferometric fringes in a Moir\'{e}-like effect, and scale-factor stabilization in point-source interferometry using correlation with fringe contrast. We study all these schemes and demonstrate them experimentally in static and dynamic conditions.   

Rotation Sensing with Trapped Ion Interferometry

We report on work developing a precision rotation sensor through interferometry with a single trapped ion [1]. We demonstrate ultrafast manipulation of a Zeeman qubit in $138{\rm Ba}^+$ via Raman transitions with a picosecond pulsed laser. Qubit rotations with a single 20 ps pulse correspond to an instantaneous Rabi frequency above 30~GHz. We have used the same technique to perform ultrafast spin-motion entanglement. Work is ongoing to harness this spin-motion coupling to perform 1D interferometry. The long term goal of rotation sensing is expected to realize a precision which is competitive with commercial rotation sensors. A consideration of the associated systematic effects indicates that this goal is achievable with the current ion-trapping toolbox [2]. Implementation of SDKs in a Zeeman qubit may also provide a versatile technique of achieving large momentum transfer that could be broadly applicable to matter-wave interferometry.\\ \\References: \newline [1] W. C. Campbell and P. Hamilton, J. Phys. B \textbf{50}, 064002 (2017)\newline [2] A. West, Phys. Rev. A \textbf{100}, 063622 (2019)   

Progress towards Ultrasensitive Gravity Gradiometers using Macroscopically Delocalized Strontium

Recent precision measurements based on torsion balances and pendulums disagree on the gravitational constant G by nearly 40 times the smallest reported uncertainty [1,2]. To address this discrepancy, it is important to measure G with a variety of different methods. Quantum sensors based on atom interferometry have proven to be a powerful tool for measuring G, with a different set of systematic errors than the classical techniques used in most measurements [2]. Here, we discuss progress toward a new atom interferometric measurement of G that will leverage recent advances in ultrasensitive atomic gravity gradiometers. We will detail our designs and progress toward the construction of a 2 m fountain capable of delocalizing atomic wavefunctions on a macroscopic scale by utilizing recent advances in large momentum transfer on the strontium transitions. We intend to test our gravity gradiometer with two large single-crystal silicon proof masses. The masses will translate on thick, level granite slabs between measurements where a high-resolution atomic phase readout will allow the determination of G. In addition, the apparatus will be used to test the gravitational inverse square law in order to search for new particles beyond the standard model. [1] A. Mann. PNAS 113, 9949-9952 (2016); Q. Li et al., Nature 560, 582-588 (2018). [2] G. Rosi, F. Sorrentino, L. Cacciapuoti, M. Prevedeli, and G. M. Tino, Nature 510, 518 (2014).   

MAGIS-100: Fundamental Science with Atom Interferometry

MAGIS-100 is an atom interferometric sensor over a vertical baseline of 100 m currently under construction at Fermilab. By implementing clock atom interferometers at either end of this baseline, MAGIS-100 will be sensitive to potential signatures of ultralight dark matter with scalar and vector couplings. Additionally, by measuring the light travel time along the baseline, MAGIS-100 will serve as a pathfinder for a gravitational wave observatory in the mid-band, with frequencies from 0.1-1 Hz. With free-fall times up to nine seconds, MAGIS-100 will test quantum superposition at macroscopic scales of length (meters) and time (seconds). We present the science goals of the MAGIS-100 instrument and designs for its vacuum, magnetic, and laser systems.   

A DFT based method for energy and information-entropic analysis in quantum confined atomic systems

Atom trapped inside a cavity introduces fascinating changes in the observable properties. Here we report some preliminary theoretical results of the development and current status of a newly proposed DFT-based method in our laboratory, to address quantum confinement in atoms. This adopts a physically motivated non-variational, work-function-based exchange potential, along with a local parametrized simple Wigner functional and a nonlinear, gradient- and Laplacian-dependent functional. GPS method is used to construct an optimized non-uniformly discretized spatial grid for solving the non-relativistic KS differential equation. Exploratory results are presented for atoms and ions enclosed within impenetrable and penetrable spherical cages, along with other environments in both ground and excited states. That includes external potential in the form of harmonic confinement, atom/ion embedded in a plasma environment or in a fullerene cage. The exchange-only results are practically of Hartree-Fock quality. With inclusion of correlation; these are comparable to some of the multi-configurational calculations. Information theoretical quantities, such as Shannon entropy, Renyi entropy, Tsallis entropy, Fisher information,Onicescu energy, etc., are also presented in stated environment.   

Energy structure and transition data of doubly ionized mercury: Hg III.

The spectrum of doubly ionized mercury (Hg III) has been studied in the wavelength region of 400 - 2000{\AA}. The Hg III is Au II like ion with its ground configuration 5d$^{\mathrm{10\thinspace }}(^{\mathrm{1}}$S$_{\mathrm{0}})$. The outer electron excitations are of the type 5d$^{\mathrm{9}}$n$\ell $(n$\ge $5). Some low lying internal excitations 5d$^{\mathrm{8}}$6s$^{\mathrm{2}}$ and 5d$^{\mathrm{8}}$6s6p have also been reported along with 5d$^{\mathrm{9}}$ (6s, 7s {\&} 6p) configurations. We are investigating particularly 5d$^{\mathrm{9}}$6d and 5d$^{\mathrm{9}}$8s configurations for missing levels with the aid of experimental recordings made on a 3-m normal incidence vacuum spectrograph using a triggered spark source. The spectra of 5d ion(s) are composite not only quantitatively but also qualitatively, due to the overlap of configurations. Their study requires high-resolution spectral instruments in the ultra-violet wavelength region as well as reliable methods for the interpretation of the spectra. The \textit{ab-initio} calculations were carried out by means of R. D. Cowan's Hartree-Fock code with superposition of configurations and relativistic corrections to predict the energy eigenvalues as well as the associated wavelength and transition probabilities along with the cancellation factor. The entire analysis was freshly carried out and was found that earlier reported values were satisfactory except the levels of 5d$^{\mathrm{9}}$8s configuration. Only $^{\mathrm{3}}$D$_{\mathrm{3}}$ level of this configuration is being confirmed in the present work and the others were newly established. Final results were interpreted by least squares fitted parametric calculations. We have used Ritz extrapolation formula using a least squares fitting code RITZPL to calculate the ionization limit. With the three member 5d$^{\mathrm{9}}$ns series, the ionization potential was found to be 278200 \textpm 400 cm$^{\mathrm{-1\thinspace }}$(34.49 \textpm 0.05eV).   

Influence of atomic radiations in HID discharge Lamps

Radiations of plasma discharge lamps come from different mechanisms that take place inside the lamp. The knowledge of atomic data responsible for these radiations is essential to describe the radiations that take place inside them. A ray tracing method based on the resolution of radiative transfer equation is adopted. The discharge is divided into small cells responsible for launching rays in all directions. The calculations consider that the discharge has a cylindrical symmetry and assume the plasma is at local thermodynamic equilibrium (LTE). Hence, the only knowledge of temperature profile and pressure is sufficient to calculate the plasma composition and to account the mechanisms of broadening of spectral lines in the treatment of radiative transfer. In this work, the atomic data is the only resource to calculate both coefficients. We will show the results for a pure mercury HID lamp. For each spectral line, the local absorption and emission coefficients are strongly dependant on the broadening constants. Calculations reported in the literature use different values for these constants, leading to marked differences in output of the models.   

Large Scale Monte Carlo Simulations of Non-Maxwellian Collisions in Rydberg Plasmas

While a Maxwell-Boltzmann (MB) energy distribution for the charged particles in a plasma is customarily assumed, more general $\kappa$-distributions have been proposed as generalizations that apply for non-equilibrium space plasma physics, and other special situations. Specific to these distributions is the substantial power-like high-energy tail. The effects of these deviations from MB distribution on the rate of collisional rates are investigated by using large scale Classical Trajectory Monte Carlo simulations for electron-Rydberg processes. A novel methodology for running simulations in parallel uses Redis, the same in-memory database that is behind the powerful Tweeter engine. The computational results are compared with analytical results.   

A model of electromagnetically induced transparency and high energy charged particles in atomic media

We investigate the interaction of charged HEPs with 3-level lambda systems under the EIT regime. HEP particles emit Cherenkov radiation with a phase and group cone that strongly depend on coherence between the ground and excited states. We use the Lindblad master equation to determine a set of analytic equations that model the coherence terms and show the dependence of electric susceptibility on field and system parameters. The effects of smaller FWHM in the coherence term will translate to a lower group velocity of Cherenkov radiation near the resonant frequency which can be used to provide an effective control scheme for detecting HEPs.   

Femtosecond optical frequency combs and applications to quantum control of three-level atomic systems

Mode-locked frequency combs provide a powerful way to assemble a spectrum of finely spaced frequencies over the duration of the pulse train. Altering pulse parameters and introducing pulse modulation can be used to develop control protocols to drive atomic systems to targeted states. We investigate models of frequency combs with Gaussian envelopes and various phase modulation functions and their mathematical descriptions in the frequency domain. Considering three-level atomic systems, we study how a single train of ultrafast mode-locked pulses can be used to gradually develop coherences and populations. Numerical simulations as well as control protocols, in the case of picosecond pulses, were used to study the effect of changing laser parameters on system state evolution.   

Temporal compression of sub-1000 eV picosecond electron pulses with light fields for use in ultrafast electron microscopy

We discuss progress on using all-optical techniques to compress low energy free electron pulses from picosecond durations to tens of femtoseconds or shorter. Our numerical simulations show that electron pulses that have disbursed from femtosecond pulse durations to picosecond durations after propagation can be recompressed into individual and series of femtosecond duration pulses. The use of low energy electrons allows simpler optical setups and single laser wavelengths greatly decreasing experimental complexity. These results are guiding current experiments in the lab that if successful will improve the temporal resolution of low energy ultrafast electron microscopes by more than an order of magnitude, allowing the exploration of dynamic systems that have motions too fast for current technologies to be explored.   

Metastable Manipulation of the 133Ba+ Qubit

Trapped ions are attractive qubit hosts due to their long coherence times and straightforward manipulation via electromagnetic fields. Future fault-tolerant quantum computers will not only require ultra-high fidelity gate operations, which has been the focus of recent efforts, but also ultrahigh fidelity state preparation and measurement (SPAM), which is currently orders of magnitude lower. We present recent work with the synthetic trapped-ion qubit 133Ba+, a radioactive isotope of barium with a 10.5yr half-life. The spin-1/2 nucleus, visible wavelength electronic transitions, and long-lived 2D5/2 state make this trapped-ion qubit ideal for ultra-high fidelity work. We demonstrate manipulation through a stimulated Raman transition of the 2D5/2 state as a stable qubit.   

Engineering Interactions in Waveguide-QED

By drawing on spectacular advances on trapping with optical tweezers, it is now possible to create one-dimensional ordered chains with perfect filling fraction. With small enough lattice constant, these chains feature dark states that allow for dissipationless transport of photons, allowing one to consider the array as a waveguide. Here, I demonstrate how to use such an atomic waveguide to mediate interactions between qubits, and between photonic excitations. A qubit, or multiple qubits, can be strongly coupled to the guided modes of the waveguide, as the group velocity of the guided modes can be made to be slow. This produces non-Markovian interactions between qubits due to time-delay, and between the waveguide and qubit, resulting in population trapping and the formation of bound states. Moreover, due to the two-level nature of the atoms, atomic waveguides are necessarily quantum. I show how this feature can be used to “collide” counter-propagating photons. This non-linearity is tunable through the system parameters, and allows for the exploration of many-body physics between guided photons.   

Cavity QED of silicon vacancy centers with a silica optical micro resonator

Negatively charged silicon-vacancy (SiV) centers in diamond feature an inversion symmetry making the SiV optical transitions robust against charge fluctuations in the surrounding environment. SiV centers have been successfully integrated into photonic crystals for cavity QED studies and for the development of optical quantum networks. Diamond photonic crystals, however, feature a relatively broad optical linewidth (\textasciitilde 50 GHz), limiting this platform to the bad cavity limit in cavity QED. We have developed an experimental platform combining a thin (\textasciitilde 110 nm thick) SiV implanted diamond membrane with a tunable silica optical microresonator with a diameter near 50 micrometers. PLE spectra of SiV centers at 10 K show linewidths ranging from 200 to 300 MHz for membranes as thin as 100 nm. This composite system features an optical cavity linewidth as narrow as 40 MHz, enabling the achievement of the good cavity limit in cavity QED. For cavity QED studies, a diamond membrane is in contact with a silica microresonator. This composite cavity QED system provides a highly promising platform for pursuing cavity QED of SiV centers in the good cavity limit.   

Tweezer Arrays for Rydberg States in Erbium for Quantum Simulation

We present our design for a novel platform for quantum simulation based on Rydberg states in erbium confined in optical tweezers. Rydberg atoms are promising candidates for quantum simulation due to their extremely strong and long-range interactions, and have been applied already very successfully in alkali atoms. However, the simple electronic structure with only one valence electron can pose limitations to the tool box of control and manipulation of Rydberg states. Trapping, cooling or direct imaging cannot be done in a straigthforward way. Therefore, Rydberg states in multi-electron atoms are promising new platforms, as shown in recent studies in strontium and ytterbium. We plan to use erbium atoms, which have two valence electrons in their outer 6s shell and 12 electrons in an open, sub-merged, 4f shell. The properties of Rydberg states in such a complex system are not yet well understood and require intense spectroscopic effort. We here present our design for a new experiment dedicated to the study of these states in controllable arrays of optical tweezers.   

Simulating a Josephson charge qubit with coupled Bose--Einstein condensates

A Bose--Einstein condensate (BEC) is attractive for simulating the superconducting Josephson junction (SJJ), which is the basis for superconducting qubits. The SJJ comprises two superconducting plates, which are separated by a thin barrier that allows Cooper pairs to tunnel between the plates. To realize a Bosonic Josephson junction, we use two BECs that are coupled together in a double-well. We model this setup with the two-mode Bose--Hubbard Hamiltonian, where the parameters are set so that fluctuations in the atom number difference are suppressed (number squeezing). The system will thus be analogous to an SJJ charge qubit. We present an experimental protocol to realize a BEC charge qubit analog using an ultracold cloud of $^{\mathrm{87}}$Rb atoms confined in a planar, red-detuned optical dipole trap. The ground and excited state wavefunctions are calculated variationally, and a spectral method is used to numerically simulate the dynamics of the system when the potential is varied in time. Similar to the charge qubit, the BEC analog has a ground state corresponding to both wells having the same number of atoms, and a first excited state of one atom having tunneled between the wells. Any superposition of these states can be reached by controlling the barrier position.   

Demonstration of the QCCD trapped-ion quantum computer architecture

We report on the integration of all necessary ingredients of the trapped-ion QCCD (quantum charge-coupled device) architecture into a robust, fully-connected, and programmable trapped-ion quantum computer. The system employs $^{171}$Yb$^{+}$ ions for qubits and $^{138}$Ba$^{+}$ ions for sympathetic cooling and is built around a Honeywell cryogenic surface trap capable of arbitrary ion rearrangement and parallel gate operations in multiple zones. Using two of these zones in parallel, we can execute arbitrary four-qubit quantum circuits. We benchmark the architecture at both the component level and at the holistic level through a variety of means. State preparation and measurement errors, single-qubit gates, and two-qubit gates are characterized with randomized benchmarking. Holistic tests include parallelized randomized benchmarking showing that the cross-talk between different gate regions is negligible, a teleported CNOT gate utilizing mid-circuit measurement, and a quantum volume measurement of $2^4$.   

Spins in two dimensions for quantum sensing and simulation

Nitrogen vacancy (NV) centers in diamond can act as nanoscale sensors capable of measuring the magnetic field created by only a few nuclear spins. This makes the NV center an ideal local probe for studying nuclear spin dynamics in two dimensions. In this experiment we use an NV center combined with an external radio frequency field to locally initialize, control and readout nuclear spins inside hexagonal boron nitride (hBN). To achieve even more localized control, we further use electronic defects that are only angstroms away from the boron spins and are passivated beneath the hBN flake. The NV center provides optical access to these reporter spins and allows the detection of nuclear spins through dipolar coupling at distances an order of magnitude smaller than is possible with NV centers alone. These techniques open the door to room temperature studies of spin dynamics in many-body systems.   

Rydberg Excitation Microscopy

Probing atomic quantum gases at scales below optical wave lengths presents a major challenge, preventing direct observations of many-body dynamics at scales comparable to the interparticle distance. In this work we theoretically investigate a novel tool that employs Rydberg excitations as a means to convert spectroscopic data at optical wave lengths into information about correlations on UV length scales. At the core of such a Rydberg excitation microscope is the formation of ultra-long-range-Rydberg molecules. Since the spectroscopically probed formation of molecules depends crucially on the size of the Rydberg orbit, excitations with different principal quantum numbers probe correlations on tunable lengths scales. We show how the spectroscopic dimer line strengths can be related to correlation functions of fermionic quantum gases and how the probability distribution of Feshbach molecular wave functions can be observed at distances far below optical wave lengths. Our theoretical approach is based on the derivation of approximate sets of wave functions that can be generalized to complex many-body systems. This opens the perspective of Rydberg excitation microscopy as a new tool to study the real-time quantum dynamics of strongly correlated systems in a minimally destructive way.   

Quantum turbulence in effective low dimensional formulation

Cold atomic gases provides a new platform for understanding nonlinear fluid dynamics. If there is a non-linear term in the energy-density, then the high number-density region has a larger k-vector or moves faster than a low density region. This causes the high density fronts to form a very sharp slope as a wavepacket propagates outward which can cause the energy density to diverge. Such extreme behavior is regulated by the viscous term in a dissipative (viscous) theory and the gradient in the kinetic energy term in a dispersive theory. We show that an axial model, which turns it into an effective 2D simulation, can reduce computational time while providing agreement with the experiment.   

Thermodynamics and vortex physics in shell-shaped Bose-Einstein condensates

We present a study of thermodynamic and vortex features of shell-shaped Bose-Einstein condensates (BECs). Hollow BECs are being investigated in the Cold Atom Laboratory (CAL) aboard the International Space Station in bubble traps and can also occur in terrestrial optical lattice settings or interiors of neutron stars. In close keeping with the CAL data obtained so far, we employ multiple techniques to compute the BEC critical temperature for a bubble trapped gas focusing on signatures of the topological transition from a filled-to-hollow spherical geometry as the gas adiabatically expands. We find that the critical temperature decreases near linearly with trap detuning frequency and that the presence of a topological transition may be inferred from the gas temperature. Looking towards physics of hollow BEC shells after the transition, we study nucleation of vortices in such systems when rotated. We show that vortex nucleation is energetically favorable in hollow shells at rotation rates smaller than for filled sphere BECs. This critical rotation rate increases with shell thickness in an almost linear fashion as well. Such distinctions between filled and hollow condensates and the topological transition between the two are poised to inform future CAL investigations.   

Transport measurements of the unitary Fermi gas

Understanding transport in strongly interacting Fermi gases is one of the most significant challenges of many-body physics. Here, we present recent measurements of the transport properties of a homogeneous, strongly interacting Fermi gas of $^6$Li atoms in the unitarity limit. We study the coupled transport of momentum and heat through the attenuation of the first-(density wave) and second-sound (entropy wave). Using a novel local thermometer, we observe the local temperature variations of second sound and measure the thermal diffusivity as well as the superfluid fraction. These results exclude a Fermi liquid description for the unitary Fermi gas and instead reveal a diffusivity attaining the Heisenberg limit, given by $\hbar/m$.   

Tricritical physics in two-dimensional $p$-wave superfluids

We study effects of quantum fluctuations on two-dimensional $p+ip$ superfluids near resonance. In the standard paradigm, phase transitions between superfluids and zero density vacuum are continuous. When strong quantum fluctuations near resonance are present, the line of continuous phase transitions terminates at two tricritical points near resonance, between which the transitions are expected to be first-order ones. The size of the window where first-order phase transitions occur is shown to be substantial when the coupling is strong. Near first-order transitions, superfluids self-contract due to phase separations between superfluids and vacuum.   

Experimental observation of non-ergodic behavior in the tilted Fermi-Hubbard model

Using ultracold fermionic $^{\mathrm{40}}$K atoms in an optical lattice we study the dynamics of the one-dimensional Fermi-Hubbard model subject to an external linear potential (``tilt''), which has recently attracted considerable theoretical and experimental interest in the context of ergodicity-breaking and constrained dynamics. Starting from a charge-density wave initial state (quarter filling) we measure the spin-resolved time evolution of the occupation imbalance between even and odd lattice sites as a local probe of localization. We identify two fundamentally different regimes: At short times we measure parity-projected real-space Bloch oscillations which, depending on the strength of the tilt, exhibit interaction induced damping and frequency modulation. At long times the dynamics reveal a robust steady state imbalance up to about 300 tunneling times, whose value depends on the interaction strength. We compare our experimental results to numerical calculations employing tDMRG on short time scales and exact diagonalization on long timescales and find excellent agreement throughout. Finally, we couple adjacent 1D systems to probe the crossover from a non-ergodic 1D to an ergodic 2D system and find a spin-dependent decay of the imbalance depending on the transverse coupling strength.   

Nonrigidity effects in the He(1s2s)+H$_2$ collision pair

Low-energy collisions exhibit the quantum nature of matter and physical phenomena. In our work, we show a crucial role of the flexibility of molecule in anisotropic atom--diatom collisions in sub-kelvin regime. We study Penning ionization reactions between excited helium atoms and ground state hydrogen molecules. Our results from state-of-the-art {\it ab initio} calculations are in excellent agreement with the recent experimental findings [Klein {\it et al.} Nature Phys. 13, 35--38 (2017)]. We reveal that the nonrigidity effect of H$_2$ on the reaction rate structure, not recognized in the previous study, is indispensable to correctly describe the observed resonances without a need for any empirical adjustment. We demonstrate that the approach beyond the widely used rigid-rotor approximation is required even when rigorous computations are carried out at the FCI level of theory.   

Bose-Einstein condensate with widely tunable atom number

The diverse array of ultracold atomic experiments naturally require Bose-Einstein condensates (BECs) and degenerate Fermi gases with atom number differing by orders of magnitude. Some experiments benefit from monstrous BECs to sympathetically cool Fermi gases, while 3d lattice experiments often require tiny BECs. We deterministically and stably produce both large or small BECs using a tunable mixture of magnetically sensitive and insensitive states. By using magnetic field gradients to drive evaporation, we eject all of the magnetically sensitive atoms leaving behind pure BECs in the magnetically insensitive state. Here we experimentally demonstrate and analyze our system’s capabilities in tuning the atom number and stability of the condensate.   

Robust gray molasses cooling of YO to 4 $\mu $K for optical dipole trap

We report robust sub-Doppler cooling of YO and progress towards loading into an optical dipole trap. YO molecules are first trapped in a DC magneto-optical trap (MOT), followed with cooling in gray molasses (GM). One of the hyperfine ground states has a vanishing Land\'{e} g-factor, which makes cooling of YO at 4 $\mu $K robust over a wide range of magnetic field, laser intensity, and detunings (one and two-photon). The magnetic insensitivity enables further spatial compression of the molecular cloud by alternating GM and MOT under continuous operation of the quadrupole magnetic field. This scheme creates the highest phase space density of 3.3*10\textasciicircum -8 for laser cooled molecules in free space.   

Control of molecular ultracold plasma relaxation dynamics in mm-wave and radio-frequency fields

Resonant mm-wave fields in the range from 50 to 100 GHz drive f$\longrightarrow $g transitions in a state-selected nf(2) Rydberg gas of NO. This transformation dramatically increases the early time intensity of high-Rydberg resonances in the selected field ionization (SFI) spectrum as well as in an enhanced long-time plasma signal. We associate these enhanced features with a decrease in the rate of predissociation owing to an increase in Rydberg orbital angular momentum. A 250 ns 60 MHz radio frequency pulse with a peak-to-peak amplitude as low as 400mV/cm, applied with zero delay similarly increases the signal of associated with a residue of lower-n Rydberg molecules detected microseconds later. Applied later to a plasma in a state of arrested relaxation, however, the same radiofrequency field depletes the residual Rydberg signal. We associate both effects with Rydberg electronic orbital angular momentum mixing. At early times the applied field mixes the photoselected nf state mixes with longer-lived states of high angular momentum. Later, electrons released by the radio frequency field collide with Rydberg molecules trapped in states of high angular momentum driving a predissociative flux through channels of low l.   

Measuring Coatings Brownian Noise for LIGO Mirrors

The Laser Interferometer Gravitational-Wave Observatory (LIGO) uses precision laser interferometry to probe minute gravitational wave signals originating from distant astrophysical events. LIGO uses dielectric coatings on the large test mass mirrors to achieve low loss, high reflectivity ($>99.9994\%$) and cancellation of thermo-optic noise. This enables the measurement of displacement due to gravitational waves (GW) to the order of $10^{-19}m/\sqrt{\mathrm{Hz}}$. Present noise floor limitations in the interferometer, in part, come from the mirror coatings Brownian noise. This experiment directly measures estimate for this noise due to crystalline coatings made with alternate layers of $Al_{0.92}Ga_{0.08}As$ and GaAs. The noise is measured by beating light from two identical high finesse cavities, locked via high-performance feedback with two independent lasers, and measuring the fluctuations in the beat note frequency. This approach complements the indirect measurement methods and helps in developing the theory of the origin of this noise. This work contributes to the ongoing efforts on increasing the sensitivity for future GW detectors as well as other high precision optical experiments. We present our latest estimate of this noise contribution and details of the experiment.   

Not Participating

Experimental investigations of photoionization cross sections from the 6s5d $^{\mathrm{1}}$D$_{\mathrm{2\thinspace }}$excited state are reported for atomic Ytterbium. A heat pipe-cum-linear thermionic diode ion detector employing saturation technique and working in space charge limited mode has been used for generating the atomic vapors of Yb. A Nd:YAG pumped narrow bandwidth (\textasciitilde 0.2 cm$^{\mathrm{-1}})$ Hanna-type dye laser charged with LDS-698 dye and tuned at 722.6 nm is used for the two-photon resonance transition 6s$^{\mathrm{2}} \quad^{\mathrm{1}}$S$_{\mathrm{0}} \quad \to $ 6s5d $^{\mathrm{1}}$D$_{\mathrm{2}}$. The excited state population is then promoted to the ionization threshold at 439.2 nm and above threshold at 375 nm, 355 nm, and 300 nm. The intensity of the exciting laser (722.6 nm) is kept fixed while that of the ionizing laser is varied using neutral density filters. The data is plotted between ionizing laser energy and photo-ion signal. The experimental data points are fitted using the least square fit algorithm which yields photoionization cross sections at the ionization threshold and in the continuum.   

Not Participating

A two-color pump (TCP) scheme for extreme ultraviolet harmonics generation in gases is a route to generate even and odd harmonics. Here we analyze the high-order harmonics generation (HHG) in atomic and molecular gases using 1 mm long gas jet. The dynamics of the odd and even harmonic yields was studied for orthogonally polarized fields of fundamental radiation and second harmonic of 1030 nm, 35 fs 50 kHz pulses. We analyzed the variations of the Cooper minima in the HHG spectra in the case of the single color pump (SCP) of argon gas jet. The control of depth and width of Cooper minima in argon gas can be realized by changing the jet position with regard to the focal plane of focusing lens. In the meantime, the two-color pump scheme of HHG in gases can also be considered as a method to control the Cooper minima in HHG spectra. We compared SCP and TCP HHG in the case of Ar, O$_{\mathrm{2}}$, and N$_{\mathrm{2}}$. The relative efficiencies of HHG were analyzed depending on the thickness of the barium borate crystal used for second harmonic generation.   

Not Participating

Spins of noble gases are a unique system that exhibits hours-long coherence at room temperature and above. These spins, unfortunately, are not coupled to any optical transition and therefore have not been employed in any quantum optics application to date. We present a new approach to this challenge, forming a quantum interface between noble-gas spins and light, utilizing alkali atoms as mediators. First, we study the coupling of light to the spin orientation of alkali vapor and demonstrate a 400-millisecond storage lifetime, a record for an optical memory at room temperature. We then show that the spin orientation can mediate the coupling between light and noble-gas spins via random, thermal, spin-exchange collisions. We provide a full quantum model of this interface and show, theoretically and experimentally, that it is coherent and externally controllable. We study the optimal strategies for realizing hour-long quantum memories and entanglement of remote ensembles. Finally, we experimentally reach the strong-coupling regime with helium-3 spins and demonstrate optical spectroscopy of line as narrow as 0.1 Hz and light storage with extremely long lifetimes.   

Not Participating

The spins of alkali atoms in a warm vapor are in thermal equilibrium, disordered and unpolarized. Most often, optical pumping with circularly-polarized light is used for driving the them into a particular orientation. Here we study unique pumping conditions that lead to bifurcation of the total spin orientation, i.e., to alignment of all spins (randomly) either parallel or anti-parallel to a defined axis. The bifurcation mechanism relies inter-atomic spin-exchange coupling that settles only when all spins point to the same direction. We show theoretically and experimentally that this collective mechanism is associated with a non-equilibrium phase transition. We identify the critical exponents and observe critical slowing down of the spin buildup time, which reaches several seconds, 2-3 orders of magnitude larger than the single-atom lifetime. Moreover, we observe similar substantial increase in the 'life-time' of the symmetry-broken spin when approaching the critical point. This system can be used to study critical phenomena in out-of-equilibrium scenarios. In particular, regarding a single ensemble as one 'collective' Ising spin, an array of such spins, coupled using light, can form an Ising machine or other condensed-matter spin models.   

Not Participating

Ultracold atoms confined to a ring-shaped trap can have effects of the non-trivial topology manifest through the periodic boundary condition, creating the analog of de Broglie’s model of stationary electronic orbits in a potentially macroscopic quantum state. Introduction of an azimuthal lattice structure serves to couple the allowed modes just as a laser field couples the electronic states. Thus cold atoms in a ring lattice can serve as a comprehensive quantum optics simulator with some advantages such as, strong nonlinearity can be naturally induced, and the modes in a ring exist as extended states in real space allowing new opportunities for manipulation and visualization. We examine the behavior of such a system as function of relevant parameters, comparing and contrasting with counterparts in electronic states within atoms interacting with light fields.   

Not Participating

Ultracold atoms offer a versatile platform for the experimental study of synthetic topological matter. Owing to maneuverability in a cold atomic system, topological properties have recently been investigated from in-equilibrium quench dynamics. Here, we present our implementation of dynamical detection of band topology in 2D. In the previous work~\cite{soc2019}, the spin dynamics has been monitored in an optical Raman lattice after the quench between topologically trivial and nontrivial regimes, which indirectly reflects the band topology. We extend the detection technique by dynamically controlling the phase of the Raman potential that induces spin-orbit couplings in the lattice. We demonstrate that topological charges can be obtained from time-averaged spin textures after a series of sequential quench processes. This method can be generalized to all dimensions. \begin{thebibliography}{2} \bibitem {soc2019} B. Song, C. He, S. Niu, L. Zhang, Z. Ren, X.-J. Liu, and G.-B. Jo, Nature Physics 15, 911 (2019). \end{thebibliography}   

Not Participating

Plasma expansion is an important process in solar and astrophysical plasmas and plasmas created from laser-matter interactions. It can also present complex fundamental phenomena that reveal both kinetic and hydrodynamic effects. This poster examines the expansion of an ultracold neutral plasma (UNP) with an initially exponentially decaying density distribution. This density distribution arises for UNPs created by ionizing atoms trapped in a quadrupole magnetic field. We compare the dynamics of an exponential plasma to the well-studied expansion of a plasma with a Gaussian density distribution, focusing on the velocity field and ion temperature evolution. For the same initial electron temperature and characteristic size, the expansion velocity is faster for an exponential plasma than a Gaussian plasma. As the exponential plasma expands, the ions heat then cool, whereas the ions only cool as a Gaussian plasma expands.   

Not Participating

State-mixing interactions can compromise the effectiveness of the Rydberg excitation blockade when ultracold atoms are excited to high-lying states near Forster resonance. Up to $\sim $50{\%} of atoms can be found in dipole coupled product states within tens of ns after excitation. We use state-selective field ionization spectroscopy to measure, on a shot-by-shot basis, the distribution of Rydberg states populated during narrowband laser excitation of ultracold rubidium atoms. Our method allows us to quantify both the number of additional Rydberg excitations added by each mixing event, as well as the extent to which state-mixing ``breaks'' the blockade. We use a Monte Carlo method to model the effect of experimental noise sources on our data. We find good agreement when we assume that state-mixing is a three-body process, except near exact Forster resonance.   

Not Participating

We are developing a pulsed $^{87}$Rb-$^{21}$Ne Co-magnetometer in order to reduce high 1/f noise that limits performance of previous generations of continuously pumped (CW) alkali-metal-noble-gas co-magnetometers, while retaining their key benefits, namely suppressed sensitivity to magnetic fields, and unsuppressed sensitivity to non-magnetic spin couplings. In this arrangement, we polarize $^{87}$Rb with $\sigma_+$ laser pulses and probe spin dynamics with off-resonant linearly polarized light in the dark time following each pumping pulse. Parameters of interest are extracted by fitting each dark time signal to a functional form characterized by an exponentially damped sinusoid and an exponential decay. Compared to the CW arrangement, where couplings of interest and systematic effects all contribute to one DC signal, the pulsed co-magnetometer's non-DC signal allows separation of various systematic effects from real signal, thus eliminating sources of 1/f noise. The pulsed co-magnetometer has also demonstrated additional advantages including dual-axis sensitivity, and capability for suppressing response to pump beam deflections, which is a major contribution to 1/f noise in the CW case.   

Not Participating

Linear polyatomic molecules are highly sensitive probes of nuclear spin-dependent parity violation (NSDPV). Measurements in these systems will enable experimental determination of poorly known electroweak coupling parameters. To date, measurements have focused on heavy nuclei where the NSDPV effect is enhanced by relativistic and collective nuclear effects. However, cold trapped polyatomic molecules should allow for the NSDPV effect to be measured to 10{\%} uncertainty in nuclei as light as Be. We focus on four light species: Be and Mg cyanide and isocyanide. Importantly, molecular and nuclear calculations are highly accurate for these light systems, allowing experiment to directly test Standard Model predictions.   

Not Participating

h $-abstract-$\backslash $pard Seven-times ionized cerium ion has Sb I-like structure, and 5s$^{\mathrm{2}}$5p$^{\mathrm{3}}$ $^{\mathrm{4}}$S$_{\mathrm{3/2}}$ as the ground state. The 5s$^{\mathrm{2}}$5p$^{\mathrm{3}}$- 5s5p$^{\mathrm{4}}$ transitions transition array was studied using observed cerium spectrum. This spectrum was recorded on a 3-m normal incidence vacuum spectrograph at Antigonish laboratory (Canada). In this study all possible energy levels of the ground configuration were established using the Visual Line-and-Level Identification Program (IDEN2). This analysis was theoretically supported by relativistic (multiconfiguration Dirac-Hartree-Fock) and pseudo relativistic (Hartree-Fock with relativistic correction) method followed by configuration interaction. Along with the energy levels, transition probabilities, oscillator strength and lifetimes were also calculated. The energy levels were then optimized using observed electronic transitions using the level optimisation computer code ``LOPT''. $\backslash $pard-/abstract-$\backslash $\tex   

Not Participating

Beryllium has been long considered too difficult to laser cool and trap due in part to the large transition energy and single-photon ionization from the excited state. With the advent of new laser systems in the UV [1], the large transition energy is now addressed. Previous calculations [2] show a rather small ionization cross section from the excited ${}^1P_1$ manifold into the ionization channel. We present estimates for laser cooling and trapping neutral beryllium using the singlet manifold, and find a set of detuning and saturation parameters to reduce photo-ionization over the course of a laser-trapping and cooling experiment. The results show promise for future experiments in laser trapping and cooling with neutral beryllium. [1] Eismann, Ulrich, et al. "Short, shorter, shortest: Diode lasers in the deep ultraviolet." Laser Focus World 52.6 (2016): 39-44. [2] Kim, Dae-Soung, et al. "Photoionization of the excited $1s^2 2s2p\,{}^{1,3}P^o$ states of atomic beryllium." Physical Review A 64.4 (2001): 042713.