Session B1: Atom Interferometry and New Techniques with Ultracold Atoms

Low-temperature, high-density magneto-optical trapping of potassium using the open 4S $\rightarrow$ 5P transition at 405\,nm

We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic $^{40}$K is captured using a magneto-optical trap (MOT) on the closed $\mathrm{4S_{1/2}} \rightarrow \mathrm{4P_{3/2}} $ transition at 767\,nm and then transferred, with unit efficiency, to a MOT on the open $\mathrm{4S_{1/2}} \rightarrow \mathrm{5P_{3/2} }$ transition at 405\,nm. Because the $\mathrm{5P_{3/2} }$ state has a smaller line width than the $\mathrm{4P_{3/2}}$ state, the Doppler limit is reduced. We observe temperatures as low as 63(6)\,$\mu$K, the coldest potassium MOT reported to date. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of $\beta = 2 \times 10^{-10}$\,cm$^3\,$s$^{-1}$, and estimate an upper bound of $8 \times 10^{-18}$\,cm$^2$ for the ionization cross section of the 5P state at 405\,nm. The combined temperature and density improvement in the 405\,nm MOT is a twenty-fold increase in phase space density over our 767\,nm MOT, showing enhanced pre-cooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405\,nm MOT of bosonic $^{41}$K.   

Four techniques to achieve deeper Fermi degeneracy in Fermi-Bose mixtures

The study of exotic superfluid phases of ultracold atoms requires the achievement of deeper Fermi degeneracy with respect to the one already available. I will describe four techniques for efficient sympathetic cooling of Fermi gases with a different species Bose gas: bichromatic optical dipole [1] and light-assisted magnetic trapping [2], quasi one-dimensional Fermi-Bose mixtures [3], and fast adiabatic cooling [4]. \\[4pt] [1] R. Onofrio and C. Presilla, Phys. Rev. Lett. 89, 100401 (2002);\\[0pt] [2] M. Brown-Hayes and R. Onofrio, Phys. Rev. A 70, 063614 (2004); [3] M. Brown-Hayes et al., Phys. Rev. A 78, 013617 (2008);\\[0pt] [4] S. Choi, R. Onofrio, and B. Sundaram, arXiv 1109.4908.   

Optimized sympathetic cooling of atomic mixtures via fast adiabatic strategies

The talk will explore the extent to which frictionless cooling techniques may be useful in sympathetic cooling of Fermi gases. It is argued that optimal cooling of an atomic species may be obtained by means of sympathetic cooling with another species whose trapping frequency is dynamically changed to maintain constancy of the Lewis-Riesenfeld adiabatic invariant, which in turn determines the temporal-profile of the changing frequency. An important motivating factor is that an usually undesired feature of these techniques, i.e., the fact that the atomic cloud does not increase its phase-space density and therefore its degeneracy, turns into a crucial asset when viewed from the perspective of maintaining the gas in the nondegenerate regime, thus making it an optimal coolant. Advantages and limitations of this cooling strategy are discussed, with particular regard to the possibility of cooling Fermi gases to a deeper degenerate regime. We also show that the links between the suggested method and quantum squeezing.   

Quantum degenerate Bose-Fermi mixture of chemically different atomic species with widely tunable interactions

We have created a quantum degenerate Bose-Fermi mixture of 23Na and 40K with widely tunable interactions via broad interspecies Feshbach resonances. Twenty Feshbach resonances between 23Na and 40K were identified. The large and negative triplet background scattering length between 23Na and 40K causes a sharp enhancement of the fermion density in the presence of a Bose condensate. As explained via the asymptotic bound-state model (ABM), this strong background scattering leads to a series of wide Feshbach resonances observed at low magnetic fields. Our work opens up the prospect to create chemically stable, fermionic ground state molecules of 23Na-40K where strong, long-range dipolar interactions will set the dominant energy scale.   

Condensate Fraction in a BEC Dimer

Recent experiments studying a Bose Einstein Condensate (BEC) in a two-mode system, equivalent to a ``dimer,'' have shown that many qualitative dynamical features of the BEC can be understood from motions in the underlying classical (two-dimensional) phase space (phi, z). Using a Bose-Hubbard model for the dimer, we focus on quantum deviations from motions in the classical phase space. We introduce a ``quantum'' phase space (QPS), which we define as the minimum condensate fraction c(tau;phi,z) of initial coherent states (phi,z) in the time interval [0,tau]. We find that lines of equal condensate fraction in the QPS do mimic the classical trajectories of constant energy in many respects, such that the QPS clearly reflects Josephson oscillations and self-trapping. However, novel quantum features beyond the classical description appear at finite time tau. These include symmetry breaking and enhanced c(tau; phi, z) near the classical hyperbolic fixed point and along a ridge near the classical separatrix. These features of the QPS can be readily studied in current experiments.   

Resonant Hawking radiation in Bose-Einstein condensates

We study double-barrier interfaces separating regions of asymptotically subsonic and supersonic flow of Bose-condensed atoms [1]. These setups contain at least one black hole sonic horizon from which the analogue of Hawking radiation should be generated and emitted against the flow in the subsonic region. Multiple coherent scattering by the double-barrier structure strongly modulates the transmission probability of phonons, rendering it very sensitive to their frequency. As a result, resonant tunneling occurs with high probability within a few narrow frequency intervals. This gives rise to highly non-thermal spectra with sharp peaks. We find that these peaks are mostly associated with decaying resonances and only occasionally with dynamical instabilities. Even at achievable non-zero temperatures, the radiation peaks can be dominated by spontaneous emission, i.e. enhanced zero-point fluctuations, and not, as is often the case in analogue models, by stimulated emission.\\[4pt] [1] I. Zapata, M. Albert, R. Parentani, F. Sols, New J. Phys. 13, 063048 (2011).   

Momentum Resolved Optical Lattice Modulation Spectroscopy for Bosons in Optical Lattice

We propose a new method of optical lattice modulation spectroscopy for studying the spectral function of ultracold bosons in an optical lattice. We show that different features of the single particle spectral function in different quantum phases can be obtained by measuring the change in momentum distribution after the modulation. In the Mott phase, this gives information about the momentum dependent gap to particle-hole excitations as well as their spectral weight. In the superfluid phase, one can obtain the spectrum of the gapless Bogoliubov quasiparticles as well as the gapped amolitude fluctuations. The distinct evolution of the response with modulation frequency in the two phases can be used to identify these phases and the quantum phase transition separating them.   

Population of atoms in the output of an atom Michelson interferometer

A cloud of Bose-Einstein condensate (BEC) sitting at the center of a weakly confining trap in an atom Michelson interferometer is split into two clouds that travel along different paths. The two clouds evolve and accumulate relative phase between them due to field of interest, confining potential and interatomic interactions. At the end of the interferometric cycle, the two clouds are recombined and the population of atoms found in the cloud at rest and the moving clouds depends on the relative phase. We derive an expression for the probability of counting any number of atoms within each cloud after recombination, study the dependence of the probability on the relative phase and relate our findings with experiments.   

Prototyping method for Bragg-type atom interferometers

We present a method for rapid modeling of new Bragg ultracold atom-interferometer (AI) designs useful for assessing the performance of such interferometers. The method simulates the overall effect on the condensate wave function in a given AI design using two separate elements. These are (1) modeling the effect of a Bragg pulse on the wave function and (2) approximating its evolution during the intervals between the pulses. The actual sequence of these pulses and intervals is then followed to determine the approximate final wave function from which the interference pattern can be calculated. The exact evolution between pulses is assumed to be governed by the Gross-Pitaevskii equation (GPE). We have developed both 1D and 3D versions of this method and have determined their validity by comparing their predicted interference patterns with those obtained by numerical integration of the 1D GPE and with the results of an experiment performed at NIST. We find good agreement between the 1D interference patterns predicted by this method and those found by the GPE. We show that we can reproduce the results of the NIST experiment and that this method provides estimates of 1D interference patterns 10,000 times faster than direct integration of the GPE.   

Applications of chirped Raman adiabatic rapid passage to atom interferometry

We present robust atom optics, based on chirped Raman adiabatic rapid passage (ARP), in the context of atom interferometry. Such ARP light pulses drive coherent population transfer between two hyperfine ground states by sweeping the frequency difference of two fixed-intensity optical fields with large single photon detunings. Since adiabatic transfer is less sensitive to atom temperature and non-uniform Raman beam intensity than standard Raman pulses, this approach should improve the stability of atom interferometers operating in dynamic environments. In such applications, chirped Raman ARP may also provide advantages over the previously demonstrated stimulated Raman adiabatic passage (STIRAP) technique, which requires precise modulation of beam intensity and zeroing of the single photon detuning. We demonstrate a clock interferometer with chirped Raman ARP pulses, and compare its stability to that of a conventional Raman pulse interferometer. We also discuss potential improvements to inertially sensitive atom interferometers. Copyright {\copyright} 2011 by The Charles Stark Draper Laboratory, Inc. All rights reserved.   

Berry-gauge tuned Bose-Einstein condensate gyroscope

If stable, the many-body ground state of a dilute gas of ultra-cold, bosonic atoms occupying a superposition of two internal (hyperfine) states is a Bose-Einstein condensate (BEC) of effective spin 1/2 bosons. The superfluid BEC dynamics admits long-lived quantized vortex states in which the complex phase of the superfluid order parameter, which we call the charge phase, undergoes an integer number of $2\pi$ windings along a multiply connected path - a closed trajectory that encloses a region in which the superfluid density vanishes. In response to an overall rotation of the ring, a quantization event can occur that can be used to sense rotation. Unfortunately, the sensitivity of the ring BEC gyroscope would be limited as the quantization event sets in at a rotation frequency that is not as low as the frequencies measured by other devices such as ring laser gyroscopes. We show that the recently realized synthetic magnetic fields, in which the controlled position dependence of the spin results in an effective gauge field, can tune the BEC ring gyroscope to trigger a quantization event at much smaller rotation frequency. In addition, the effective gauge field can undergo its own quantization events in which the spin vector undergoes an integer number of $2\pi$ or $4\pi$ windings.   

Frustration and glassiness in spin models with cavity-mediated interactions

Optical cavity photons can mediate interatomic interactions that are both long-ranged and sign-changing, thus enabling the realization of models and phases (such as supersolids [1]) that are inaccessible in conventional optical lattices. Here, we introduce a general scheme for realizing frustrated magnetic models possessing spin-glass states, using three-level atoms trapped at fixed positions inside a transversely pumped multimode cavity. We show that the effective Hamiltonian for such a system is analogous to the Hopfield associative-memory model [2], and, like the Hopfield model, possesses a spin glass phase for a sufficiently large ratio of the number of cavity modes to atoms. We argue that the spin glass phase should be accessible with realistic experimental parameters, discuss experimental signatures of the spin glass phase, and address the impact of dissipative processes such as cavity photon decay on the realizable phases and phase transitions. \\[4pt] [1] K. Baumann et al., \textit{Nature} 464, 1301 (2010); S. Gopalakrishnan et al., \textit{Nat. Phys.} 5, 845 (2009). \\[0pt] [2] J.J. Hopfield, \textit{Proc. Nat. Acad. Sci} 79, 2554 (1982).   

Manipulation of ultracold rubidium atoms using a single linearly chirped laser pulse

At ultracold temperatures, atoms are free from thermal motion, which makes them ideal objects of investigations in the fields of high precision spectroscopy, metrology, quantum computation, producing Bose condensates, etc. The quantum state of ultracold atoms may be created and manipulated by optical pulses of rather low intensity and by making use of quantum control methods. Here, we discuss how the field phase variation, pulse duration and intensity, when perform in conformity, may yield a desired population distribution within the hyperfine structure of alkali atoms. We theoretically investigate ultracold Rb vapor using picosecond chirped pulses of $kW/cm^2$ beam intensity and show a possibility of controllable population transfer between hyperfine levels of $5^{2}S_{1/2}$ state through Raman resonances. Satisfying the one-photon resonance condition with the $5^{2}P_{1/2}$ or $5^{2}P_{3/2}$ state allows us to enter the adiabatic region of population transfer at very low field intensities, such that corresponding Rabi frequencies are less or equal to the hyperfine split. This methodology provides with a robust way to create a specifically designed superposition state in Rb in the basis of the hyperfine levels and perform state manipulation.   

Optical response of dark exciton Bose-Einstein condensate

We study the optical response of Bose-Einstein condensate (BEC) formed by dark excitons in semiconductors. We consider the example of GaAs, where dark excitons are formed by heavy-holes and electrons with opposite signs of projections of their spins resulting in natural fragmentation of the condensate. Direct coupling of such excitons is dipole prohibited and, therefore, the optical response of semiconductor with dark condensate is provided through interaction of light with bright exciton states. We show that the Coulomb interaction of optically excited bright excitons with dark excitons residing in the BEC leads to effective renormalization of characteristics of bright excitons --- e.g. their mass decreases --- dependening on the density of the condensate. As a result the exciton resonances experiences red-shift with decreasing amplitude. This provides an opportunity for indirect spectroscopy of dark exciton BEC.   

Electron field noise sensing with a scanning ion

Controlled shuttling of ions in a planar ion trap can serve as highly sensitive method for probing the electric field.\footnote{G. Huber et al. A trapped-ion local field probe. \textit{Appl Phys B} \textbf{100}, 725-730 (2010).} We apply this idea to the measurement of field noise effects on a microfabricated planar trap as a function of ion position. In particular, by shuttling an ion between different trapping zones, the field noise can be determined near different material surfaces, in particular dielectric materials. Surface effects on these materials are suspected to cause field fluctuations that decohere the motional state of trapped ions. By shuttling the ion to a determined location and measuring the electric field noise, a measure of the electric field noise can be determined as a function of proximity to dielectrics. A systematic examination of field noise due to dielectric versus metallic surfaces is valuable in, for example, a trapped-ion cavity QED system, where the high-reflectivity mirrors used in the optical cavities are large, exposed dielectric regions.\footnote{P. Herskind, S. Wang, M. Shi, Y. Ge, M. Cetina and I. Chuang, \textit{Optics Letters}, Vol. 36, Issue 16, 3045-3047 (2011).}