Session K06: Out-of-Equilibrium Trapped Gases

Phase diagram and dynamics of magnetic Bose polarons

We investigate the nonequilibrium dynamics of magnetic polarons by considering an impurity atom dressed by spin-wave excitations in a one-dimensional spinor Bose gas. The phase diagram which contains self-bound and repulsive magnetic polarons as well as phase-separated regime is obtained as a function of Rabi-coupling and spin-spin interactions. A waveform of the magnetic polaron is extracted through an effective model. The residue is found to drop for strong impurity-spin interactions towards the polaron catastrophe. We infer the dynamical persistence of the respective repulsive and attractive branches associated with the generation of strongly correlated spin-waves. Strikingly, a dynamical decay at early stages of the dynamics is observed for strong impurity-medium interactions followed by a diffusive polaron behavior. The impurity can be utilized to probe and manipulate spin-demixing by suppressing ferromagnetic spin-spin correlations. Controllable spin-demixing and correlated magnetic polarons are experimentally realizable with current technologies.   

Spontaneous Persistent Current Formation in a Fermionic Superfluid Ring

The Kibble-Zurek mechanism (KZM) is a paradigmatic prediction of universal critical dynamics as a system is quenched across a continuous phase transition. In narrow annular geometries, quench-induced coarsening gives rise todefects which merge to form spontaneous supercurrents, whose winding numbers are predicted to scale universally with the quench time via the KZM. We report on the statistics of spontaneous persistent current formation in a ring of superfluid 6Li driven across its critical point. Namely, we measure scalings of post-quench winding number cumulants over several orders of magnitude of quench times, and with varying amounts of initial angular momentum bias imparted to the normal fluid above the transition. We compare our findings with novel analytic predictions obtained from a linearized stochastic time-dependent Landau-Ginzburg model.   

The Generation of Turbulence within Shaken Bose-Einstein Condensates

Following recent experimental studies, we model the generation of three-dimensional quantum turbulence by shaking harmonically trapped atomic Bose-Einstein condensates. We compare the experimentally observable quantities (e.g. two-dimensional density images and density spectra) with the quantities which are either measured in related experiments with superfluid helium or are relevant in turbulence theory. We conclude that the quantum turbulence generated, a mixture of strong fragmented density fluctuations and small vortex loops,  is unlike any other forms of quantum turbulence which has been theoretically investigated, thus posing a new challenge the theoretical understanding of quantum turbulence.   

Oscillations of a Bose-Fermi mixture

Mixtures of quantum fluids lie at the forefront of research into strongly-correlated quantum matter. In our dual-species ultracold atomic gas experiment, we study the dynamics of impurities interacting with a BEC bath. In equilibrium, impurities can form well-defined quasiparticles, Bose polarons, by interacting with the BEC [1]. We study their dynamics by inducing center-of-mass oscillations in both species at varying impurity-bath interaction strength. Our observations offer a promising starting point in the experimental study of the dynamics of Bose-Fermi mixtures in the impurity limit.

 

[1] Z. Z. Yan, Y. Ni, C. Robens, and M. W. Zwierlein, Science 368, 190 (2020).   

Compressible quantum turbulence with ultra-cold atoms.

Ultracold atoms provide an experimental platform for studying compressible quantum turbulence with broad applications as quantum simulators of other systems including neutron stars. As first suggested by Feynman, quantized vortices play a key role in quantum turbulence, providing a microscopic mechanism for energy transfer across different scales.  Theoretical descriptions of quantum turbulence tend to focus on properties of these vortices, but this description is complicated when the fluid is compressible as there are multiple ways to partition the energy between rotational and compressional flow.  In this talk I will discuss some aspects of compressible quantum turbulence, and showcase results of large-scale simulations including turbulence in rotating and non-rotating systems.

    

Sound Propagation in Degenerate 133Cs-6Li Bose-Fermi Mixtures

We investigate sound propagation in Bose-Einstein condensates of ¹³³Cs atoms immersed in degenerate Fermi gases of ⁶Li atoms. Deep in quantum degeneracy, fermionic excitations near the Fermi surface mediate interactions between bosons, modifying the sound speed in the condensates.

With a high-resolution microscope and a digital micromirror device, we create and image sound excitations in the BEC. The sound speed is measured at varying interspecies interaction strengths, utilizing a Li-Cs Feshbach resonance. At negative interspecies interactions, we observe heavy reduction of the sound speed while no effect is measured at positive interspecies interactions.Our measurements further reveal the nature of fermion-mediated interactions between bosons in the strong coupling regime.   

Out of equilibrium superfluid density evolution of dipolar Bose-Einstein condensate

We investigate theoretically the condensate depletion and the superfluid density evolution of an ultracold Bose gas in a time dependent weak random potential. The bosons are interacting both through an isotropic short-range contact interaction and an anisotropic long-range dipole-dipole interaction. The temporal evolution of the superfluid density and the condensate depletion is followed throughout switching on/off the random potential and compared with previous literature. We identify the equilibrium and the dynamical contributions to the two aforementioned quantities. A disorder switch on/off protocol is considered afterwards and it clarifies the role of the two respective contributions.   

Generating Different Types of Quantum Turbulence.

Cold atomic systems provide a platform for studying quantum turbulence that can be used as a quantum simulator for macroscopic systems like neutron stars. In this talk, I will discuss how experiments can be designed by tuning parameters such as particle number, trapping geometry, and coupling strength, to generate different amounts and types of turbulence for both bosonic and fermionic superfluids across the BEC-BCS crossover. Although superfluids have no fundamental dissipation, these differences can be qualitatively explained, in terms of effective dissipation, after coarse graining. Control over the amount of generated turbulence is crucial for tuning experiments to model superfluids with large turbulent structures.   

Probing scale-invariant prethermal regime in systems quenched to criticality

An experimentally feasible way to probe quantum phase transitions is through dynamical measurements where one quenches to the close vicinity of a quantum critical point (QCP). Upon quenching to a QCP, quantum many-body systems exhibit a prethermal temporal regime in one-point observables. Here we present an analytical framework for the transverse field Ising chain (TFIC) that leads us to a scale-invariant and universal scaling function of this critically prethermal regime both in edge and bulk observables. Our work provides a quantitative definition of critical slowing down of system observables, due to the quantum phase transition, in far from equilibrium setting. It further explains the robustness behind the nonequilibrium exponents that are numerically extracted for the spatially minimal and dynamical order parameters of TFIC.   

Nonequilibrium metastability of the attractive one-dimensional Bose gas

Many-body quantum systems out-of-equilibrium host phases of matter that simply do not exist in equilibrium scenarios: the one-dimensional Bose Gas (1dBG) with contact attractive interactions is an outstanding example of this dichotomy. The 1dBG is a ubiquitous effective description of many cold atoms experiments, where the presence of Feshbach resonances allows for a dynamical exploration of the whole interactions' range. On the theory side, a hallmark of the 1dBG is its integrability, which hinders thermalization and allows for analytical and exact insight. Within the attractive phase, the 1dBG forms bound states with arbitrary large negative binding energy, critically harming the stability of the gas at equilibrium. On the other hand, integrability prevents thermalization and enhances the stability of the gas.

In this talk, I discuss how, by slowly changing the interactions from positive to negative, it is possible to engineer a stable nonequilibrium phase. I build on new developments in the hydrodynamic theory of integrable models and provide an exact analytical solution of the protocol directly at the many-body level, showing the controlled formation of bound states and creating a far-richer scenario than the well-known Super Tonks-Girardeau phase. Experimental signatures in realistic settings of this new nonequilibrium phase are discussed.