Session J5: Unitary Quantum Gases

Coherent evolution of a BEC quenched to unitarity

Following recent experimental results [1], we theoretically study the coherent time evolution of a zero-temperature BEC whose scattering length is quenched suddenly towards unitarity. Despite the resonant atom-atom interactions, the condensate does not deplete instantaneously, and this allows us to describe the short-time behavior with a mean-field-like, many-body variational wavefunction. We compute the dynamics of various observables such as the momentum distribution, Tan's contact, density-density correlations, and the final Feshbach molecule fraction (after quenching back to finite scattering length for imaging). The nonequilibrium behavior of the momentum distribution suggests the presence of unexpected subleading terms that are absent in equilibrium.\\[4pt] [1] P. Makotyn, et al, Nat. Phys. {\bf 10}, 116 (2014).   

Three-body physics in quenched unitary Bose gases

A degenerate Bose gas, quenched to unitarity, displays rapid losses that are attributed to three-body recombination. The rate at which this occurs is an item of keen interest in producing and probing a unitary Bose gas. In this work we explore the three-body physics relevant for unitary Bose gases using the hyperspherical adiabatic representation and determine the population of Efimov states formed during the quench and their subsequent decay rate by assuming a local interaction model in which a harmonic potential mimics the finite density of other particles. Our findings [2], consistent with experiments at JILA [1], indicate that the three-body loss time scales are generally longer than the system's equilibration time, therefore bolstering this scheme as an efficient route to create and explore the dynamics of unitary Bose gases. Supported by National Science Foundation, AFOSR-MURI, ARO-MURI, NDSEG and NRC.\\[4pt] [1] P. Makotyn, C. E. Klauss, D. L. Goldberger, E. A. Cornell, and D. S. Jin, Nat. Phys. 10, 116119 (2014), arXiv:1308.3696;\\[0pt] [2] A. G. Sykes, J. P. Corson, J. P. D'Incao, A. P. Koller, C. H. Greene, A. M. Rey, K. R. A. Hazzard, J. L. Bohn, arXiv:1308.0828.   

Three-body loss rate of unitary Bose gas

Quantum gases at unitarity can exhibit interesting features, for instance their universal thermodynamics. In the past, unitary Fermi gases in degenerate limit have been studied extensively. As recent experiments [1, 2] show unitary Bose gases can be stabilized at relatively high temperatures, we would like to ask an important question whether a Bose gas can persist in a well defined thermodynamic state at lower temperatures, even to the degenerate limit [3] where the medium affects the three-body loss rate crucially. By calculating the three-body recombination rate while taking into account the scattering with the medium, we have an estimate of the temperature (scale) above which thermodynamic quantities of a metastable branch can be studied in a unitary Bose gas. \\[4pt] [1] Rem, B. S., et al. ``Lifetime of the Bose Gas with Resonant Interactions.'' Physical review letters 110.16 (2013): 163202. \\[0pt] [2] Fletcher, Richard J., et al. ``Stability of a unitary Bose gas.'' Physical review letters 111.12 (2013): 125303. \\[0pt] [3] Makotyn, Philip, et al. ``Universal dynamics of a degenerate unitary Bose gas.'' arXiv preprint arXiv:1308.3696 (2013).   

Tuning Interactions in Two and Three Dimensions

We present recent work on a Bose gas with tuneable interactions in two very different geometries. In three dimensions, we study the stability of a thermal 39K Bose gas across a broad Feshbach resonance, focusing on the unitary regime. We measure the general scaling laws relating the particle-loss and heating rates to the temperature, scattering length, and atom number. As a consequence of species-specific Efimov physics, we find 39K to be particularly promising for studies of many-body physics in a unitary Bose gas. We also present more recent work on a two-dimensional trapping configuration. Such a setup permits investigation of the subtle interplay between interactions and condensation in this regime, in particular the crossover between BEC and BKT physics.   

Dilute Bose gases with large scattering length using Bijl-Jastrow wavefunctions

The Bijl-Jastrow wavefunction, which explicitly includes two-body correlations, has been reasonably successful in explaining macroscopic properties of liquid Helium at low temperatures. We apply the same techniques to understand dilute Bose gases with an effective zero-range interaction (employing the Bethe-Peierls boundary condition rather than including an explicit two-body potential). We discuss the renormalisation issues which arise as a result of this diverging zero-range boundary condition. We calculate observables such as the ground state energy, the condensate fraction, and Tan's contact in the system, with particularly interest in the regime where the gas parameter $na^3$ is appreciable ($n$ being the number density and $a$ the scattering length). Finally, we will discuss possible extensions and avenues for further research.   

The Degenerate Unitary Bose Gas

The degenerate unitary Bose gas has generally been deemed experimentally inaccessible because of three-body loss rates that increase dramatically with increasing scattering length, a. Starting with a $^{85}$Rb BEC, we investigate dynamics of a unitary Bose gas for timescales that are short compared to the loss. We find that the momentum distribution of the unitary Bose gas evolves on timescales fast compared to losses, and that both the timescale for this evolution and the limiting shape of the momentum distribution are consistent with universal scaling with density. This work demonstrates that a unitary Bose gas can be created and probed dynamically, and thus opens the door for further exploration of this novel strongly interacting quantum liquid.   

Solitonic Vortex in a Strongly-Interacting Fermi Gas

We investigate the solitonic excitation observed in our previous experiments [Yefsah \textit{et al}., Nature \textbf{499}, 426 (2013)] for a unitary Fermi gas with tomographic imaging. In this work, we directly access the local density of our 3D clouds by imaging a thin layer of atoms, which we achieve with a masked pumping beam that transfers atoms outside of the selected layer into an undetected state. Using the tomographic imaging, which circumvents the density integration along the probing axis, we identify unambiguously this excitation as a solitonic vortex. In particular, we rule out the vortex ring scenario predicted by several theory groups. Our measurements provide a quantitative benchmark for the theories of non-equilibrium dynamics of strongly-interacting superfluids.   

Observation of the Leggett-Rice effect in a unitary Fermi gas

Currents can reveal essential qualities of a system that are not evident from equilibrium measurements. In a trapped cloud, spin currents are natural to study because they can exist without net mass transport. Spin diffusivity, like conductivity, is a measure of the scattering rate. Precession of spin current, also called the Leggett-Rice effect [1], is a measure of the coherent interactions between excitations. In a degenerate Fermi gas of potassium tuned to a Feshbach resonance, we measure the dynamics of a superposition of two hyperfine states. Using a spin-echo sequence, we probe both the phase and amplitude of magnetization dynamics due to transverse spin currents. Transport coefficients are measured as a function of temperature and of scattering length, at and near unitarity.\\[4pt] [1] A.\ J.\ Leggett and M.\ Rice, {\it PRL} {\bf 20}, 586 (1968); A.\ J.\ Leggett, {\em J.\ Phys.\ C} {\bf 3}, 448 (1970).   

The Transience of Defects in an Equilibrating Unitary Fermi Gas

We present numerical simulations of phase imprinting experiments in trapped Fermi gases. Here we investigate the behavior of the defects formed as the fluid returns to equilibrium, and relate this to the recent observation of oscillating defects (claimed to be solitonic) in a Fermi gas [1]. We consider dynamics associated with the time-dependent Ginzburg-Landau equation, which contains dissipation and stochastic noise [2]. All defects we find are surprisingly short-lived, and we explain the origin of their transience. Solitons and vortex rings generally rapidly decay to more stable precessing line vortices. We speculate that these line depletions may be relevant to experimental observations [1]. We address how the trap, system size, and starting conditions affect the type of defect, its lifetime, and its oscillation period. \\[4pt] [1] T. Yefsah et al., Nature 499 426 (2013).\\[0pt] [2] A. Glatz, H. Roberts, I. Aranson, and K. Levin, PRB 84 180501 (2011).