Session B5: Rydberg Atoms and Molecules in Cold Gases and Ultracold Plasmas

Rydberg Excitation in a Sr BEC

Despite its apparent simplicity, the system of Rydberg atoms immersed in a Bose Einstein Condensate (BEC) is surprisingly rich and is the subject of numerous theoretical proposals. We present experimental studies of this system in which atoms from a BEC of neutral Sr are excited to the $5sns^3S_1$ Rydberg series via two photon excitation through the metastable $^3P_1$ state. We will describe studies of the interactions between Rydberg and background BEC atoms and our progress towards experiments with Rydberg dressing. Alkaline-earth metal atoms offer many new opportunities for these studies, such as an optically active core electron which can be used to manipulate and detect Rydberg atoms. In comparison to two photon excitation schemes using only dipole allowed transitions, the narrow linewidth of the $^1S_0-^3P_1$ transition allows for the attainment of larger Rabi frequencies for the same loss rate due to spontaneous emission.   

Interactions of Rydberg atoms in a Bose-Einstein condensate

Recent developments in ultracold Rydberg gases led to the prediction and discovery of ultra-long-range Rydberg molecules. A Rydberg atom can interact with a ground state atom and form a bound state, the so-called ``trilobite state,'' via an oscillatory Born-Oppenheimer potential arising from Rydberg electron-ground state atom low-energy scattering. Two Rydberg atoms can also be bound via a dispersion potential, and form a ``macrodimer.'' Here we consider two Rydberg atoms immersed in a dilute Bose-Einstein condensate (BEC). The interaction between the two Rydberg atoms is studied, and the formation of a possible ultra-long-range Rydberg molecule is shown.   

Ultra-long range Red-Shifted Cs Trilobite Molecules in a 1064 nm Crossed Dipole Trap

We present results on our Cs ultracold Rydberg atom experiments involving trilobite molecules. A novel binding mechanism arises from the attractive, low-energy scattering of a Rydberg electron from a neighboring ground state atom. The states formed by this binding mechanism are referred to as trilobite states. The states we observe are to the red of the $nS$ atomic resonance and, due to a large mixing with high angular momentum states, have large permanent dipole moments on the order of $1 $kD. To verify these dipole moments, it is necessary to observe the response to the molecules to an electric field. We present measurements of the Stark shifts of the trilobite states in Cs due to the application of a constant external electric field.   

Photoassociation of long-range $nD$ Rydberg molecules

A recently discovered class of long-range Rydberg molecules has generated a significant amount of theoretical and experimental interest [1,2]. We present on the observation of long-range homonuclear diatomic $nD$ Rydberg molecules photoassociated out of an ultracold gas of $^{87}$Rb atoms for 34$\le n \le$40 [3]. We measure the ground-state binding energies of $^{87}$Rb$(nD-5S_{1/2})$ molecular states to be larger than those of their $^{87}$Rb$(nS-5S_{1/2})$ counterparts, showing the dependence of the molecular bond on the angular momentum of the Rydberg atom. We probe the transition of $^{87}$Rb$(nD-5S_{1/2})$ molecules from the molecular-binding-dominant regime at low $n$ [Hund's cases (a)] to a fine-structure-dominant regime at high $n$ [Hund's case (c)]. A Fermi model that includes the fine structure of the $nD$ Rydberg atom and hyperfine structure of the $5S_{1/2}$ pertuber is presented that describes this transition. The resulting molecular potentials and bound states are in good agreement with the experimental data.\\[4pt] [1] C. H. Greene, A. S. Dickinson, and H. R. Sadeghpour, PRL, \textbf{85}, 2458-2461 (2000).\\[0pt] [2] V. Bendkowsky et al., Nature, \textbf{458}, 1005-1008 (2009).\\[0pt] [3] D. A. Anderson, S. A. Miller, and G. Raithel, arXiv:1401.2477.   

Bistability in off-resonantly driven ultracold Rydberg gases

When exciting dense, ultracold gases to Rydberg states on resonance, the number of excitations within a finite volume is limited due to the long range van-der Waals interaction. Away from resonance initial excitation is suppressed, however an excited atom shifts surrounding atoms into resonance and facilitates further excitations. We discuss whether this mechanism launches an avalanche of excitations and what ultimately limits its growth in a many body system. A simplified mean field model for the driven, dissipative system shows in certain parameter regimes the emergence of bistability between a weakly and a strongly excited many body state. Within mean field theory we derive steady-state excitation rates and statistics as well as tunneling times between the two fixed points. Monte Carlo wavefunction simulations of the full dynamics in small systems and classical rate equation calculations show how this bistability is visible in stationary and dynamic observables in current experiments on small Rydberg ensembles.   

A Low-Inductance Ioffe Trap for Antihydrogen Spectroscopy

Experiments at CERN's antiproton decelerator facility have demonstrated the production and trapping of small numbers of cold antihydrogen atoms with confinement times up to 1000 s. The ATRAP collaboration aims to increase the quantity of anti-atoms involved to accommodate both Lyman-alpha laser cooling and 1S-2S spectroscopy of the antihydrogen, so we have undertaken an upgrade to our apparatus centered around an improved neutral-particle confining Ioffe trap that significantly increases the trap depth and decreases the turn-off time while still retaining three-axis laser access. This trap has been tested and is performing close to the design specifications, making us optimistic that we will soon achieve significantly higher numbers of trapped antihydrogen atoms per trial. We will then be in a better position to measure the antihydrogen 1S-2S transition frequency as a precision test of CPT symmetry.   

Plasma Oscillation Damping in an Ultracold Neutral Plasma

In sufficiently low-density ultracold plasmas, free (i.e. non-driven) plasma oscillations can be induced that persist for timescales on the order of the oscillation period. These oscillations are initiated by a short electric field pulse. The oscillation amplitude and frequency can be probed by applying a second short electric field pulse at a chosen delay time after the first. The fraction of electrons that leave the ultracold plasma in response to this second pulse exhibits a damped sine wave character, allowing for the determination of the damping time constant of the free plasma oscillation. We expect that this damping time should be related to both collision-based (e.g. electron-ion collisions) and non-collision-based factors. We present our measurements of the oscillation damping time as a function of ultracold plasma density and temperature.   

Adiabatic expansion cooling of ions in ultracold neutral plasmas

Ultracold neutral plasmas (UNPs), created by photoionizing laser-cooled atoms have ions which inherit very low temperatures. However, a process known as disorder induced heating (correlation heating) quickly heats the ions, limiting the equilibrium Coulomb coupling parameter to approximately two, regardless of initial conditions. This places UNPs just barely into the strongly coupled (non-ideal) regime. It has been predicted that the subsequent expansion of the plasma into the surrounding vacuum results in adiabatic cooling as well as correlation cooling and should result in more strongly coupled ions. Using laser induced fluorescence spectroscopy and taking care to minimize other heating processes like electron-ion thermalization and heating from ion-acoustic-wave excitations, we have measured the ion temperature evolution of UNPs and observed adiabatic cooling of the ions, by up to an order of magnitude. These measurements will be presented along with efforts to model the evolution and the effect on the Coulomb coupling parameter.   

Early time dynamics of strongly coupled ultracold neutral Ca$^+$ and Ca$^{2+}$ plasmas

Ultracold neutral plasmas are generated by photoionizing laser-cooled atoms. Due to their extremely low temperatures, ultracold plasmas fall into the ``strongly coupled'' regime, where strong coupling is characterized by the parameter $\Gamma$. This dimensionless parameter, given by the ratio of the Coulomb potential energy to the average kinetic energy of the ions, describes the complete thermodynamic state of a strongly coupled system. This makes it possible to study the fundamental behavior of strongly coupled systems as manifested in high energy-density plasmas using low energy table-top experiments. We report progress on an experiment in laser-cooled calcium designed to increase the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. Measurements of the effect that the Ca$^{2+}$ ions have on the temperature of the Ca$^+$ ions as a function of the second ionization fraction are discussed.   

Self-sustained Trojan Wave Packets on the Honeycomb Lattice

We have recently showed that counterintuitively to Earnshaw's theorem of electrostatics the stable configurations of quantum charges may exist in rotating frame forming so called Rutherford atom without any external field support [1] when the single localized semi-classical electron is orbiting tiny core consisting of the nucleus and the second electron which is in elliptical charge state. Here we show the existence of dynamic ferroelectric Rydberg matter when all hydrogen atoms are placed symmetrically on the honeycomb lattice and because of the nonvanishing local field from the nearest neighbors they sustain each other without the external field. This is in the analogy to ferromagnetic phase transition within Bethe-Peierls-Weiss theory. Because the local field due to the nearest dipole rotation is none stationary also in strength in the selected lattice center the whole structure is shape-breathing around the static dipole average. We present both the time-averaged mean-field theory as well as we conduct the numerical simulations using the time-dependent Hartree equation with the effective self interaction.\\[4pt] [1] M. Kalinski, J. H. Eberly, J. A. West, and C. R. Stroud, Jr., ``Rutherford atom in quantum theory,'' Phys. Rev. A 67, 032503 (2005).