Session U08: Out-of-Equilibrium Trapped Gases

Exploring bidirectional dynamic scaling in condensation dynamics far from equilibrium

The closed-system condensation dynamics of quench-cooled Bose gases has been shown to feature bidirectional dynamic scaling in momentum space, with particle- and energy-conserving transport towards the IR and UV, respectively [1]. Such behavior has been predicted in the context of nonthermal fixed points, and the scaling exponents have been postulated to delineate far-from-equilibrium universality classes. However, the robustness of such behavior with respect to the initial far-from-equilibrium conditions, which is a key ingredient in the theoretical proposals, has remained largely unexplored experimentally. Here we explore the far-from-equilibrium condensation dynamics starting from a novel quantum-chaotic state, which features an isotropic and essentially uniform occupation of low-momentum states with a well-defined high-momentum cutoff. We generate this state by violently driving a noninteracting box-trapped Bose-Einstein condensate in the presence of weak disorder, and then tune the interparticle interactions using a Feshbach resonance to initiate the relaxation dynamics. We exploit our ability to engineer these textbook initial states to study the robustness of the scaling exponents, including how they depend on the energy of the system, the total particle number, and the strength of the interparticle interactions.

[1] J. A. P. Glidden et al., Nat. Phys. 17, 457 (2021)   

The Emergence of Classical Hydrodynamics from Quantum Turbulence

Quantum turbulence is characterized by quantized vortices, which tangle and interact in a "turbulent" manner. This may hold the key to poorly understood phenomena such as pulsar glitches, where the rate of rotation of a spinning neutron star suddenly and inexplicably increases. In this work, we investigate how classical turbulence emerges from the quantum hydrodynamics of superfluids, and consider the role that chaos may play.   

Fluctuations in out-of-equilibrium Bose gases

Fluctuations in thermodynamic quantities are well understood for systems in thermal equilibrium and reveal important macroscopic properties of the system via fluctuation-dissipation relations. Extending such descriptions to out-of-equilibrium systems is an open problem. Ultracold atomic systems provide an ideal setting for testing theories of out-of-equilibrium physics owing to their isolation from the environment and the ability to control equilibration timescales via Feshbach resonances. Using both non-interacting and weakly interacting ultracold 39K gases trapped in a cylindrical box, we study how driving the system far from equilibrium increases momentum-space fluctuations compared to equilibrium Bose gases.   

Manipulating Fermi Polarons by Strong Driving

The nature of quasiparticles determines many thermodynamic and transport properties of quantum systems. We report on the control of the Fermi polaron - a paradigmatic quasiparticle emerging from contact interactions between impurities and a Fermi sea - by driving the impurity between two internal states using a radio-frequency (RF) field of tunable strength and detuning. Using the effective magnetization of the impurity as our observable, we find that the polaron energy approaches zero as the drive strength increases, and the decay rate increases by an order of magnitude, indicating the polaron's properties are widely tunable by the RF drive. The behavior is qualitatively well captured by the T-matrix theory.   

Nonlinear Acoustics in Ultracold Fermi Fluids

Near- and far-from-equilibrium dynamics are among the most interesting phenomena in quantum many-body systems and pose major challenges to theoretical descriptions. Here we use a degenerate Fermi gas in a box trap as a platform to investigate nonlinear sound waves in ultracold Fermi fluids. The uniform density and sharp boundary condition of the setup allow for clean excitation and observation of waves. We benchmark our measurements in the linear response regime by measuring the density-density response in ideal and weakly interacting Fermi gases, which in the T=0 limit, is the celebrated Lindhard function. In the nonlinear response regime, we encounter several surprises, including the emergence of a local minimum in the response versus drive frequency of a weakly interacting gas, and the unexpectedly high nonlinear sensitivity of the unitary gas.   

Kibble-Zurek scaling in a homogeneous unitary Fermi gas

The classification of nonequilibrium physics into a universal framework, regardless of the microscopic constituents of the excitations, has served as a benchmark for understanding its sophisticated dynamics. The Kibble-Zurek (KZ) mechanism, which underlies the formation of topological defects after a system's control parameter is quenched through a continuous phase transition, predicts a power-law dependence of the average number of defects on the quench duration. Here, we report our experiment of a linear temperature quench on a spatially homogeneous unitary Fermi gas across a thermal-to-superfluid transition. The spatially homogeneous trap allows us to analyze the defect formation and order parameter coarsening dynamics without having to rely on the local density approximation. Over a wide range of quench duration, we observe a KZ scaling behavior and show a close agreement with theoretical values depicted in the 3D XY model. For sufficiently fast quench rates, we suggest that the condensate growth dynamics after the end of the quench plays a pivotal role on the generation of topological defects.   

Subdiffusive dynamic scaling in a chaotic noninteracting Bose gas

Investigating the properties of quantum-chaotic states is of fundamental interest, but experimental realizations of these systems are few (a famous example being the quantum kicked rotor). Here, we realize a three-dimensional quantum-chaotic system by strongly driving a noninteracting Bose-Einstein condensate trapped in a cylindrical box trap with disorder. The full momentum distribution exhibits sub-diffusive dynamic scaling, which we reproduce in Schrödinger equation simulations. The timescale of the dynamics depends on the disorder strength and parameters of the drive, but the shape of momentum-space profiles and their scaling exponents are robust. Finally, we show that increasing the strength of interparticle interactions smoothly connects this single-particle behavior with that of a wave-turbulent cascade.   

The lattice melting transition in a two-dimensional BEC

The ground state of a rapidly rotating superfluid is an Abrikosov lattice of quantised vortices, which is akin to a crystalline solid and defines the zero-temperature state of a two-dimensional system of single sign vortices. At sufficiently higher energies, the lattice melts and can be approximated as a strongly correlated vortex liquid. These states of vortex matter have gained prominence in the theory of the fractional quantum hall effect, where the 2D electron gas moves analogous to vortices in an incompressible fluid, and the vortex density maps to the density of the quantum hall droplet.

The development of quasi-2D BEC superfluids in optical box potentials has resulted in new experimental systems for exploring the dynamics of point-like vortices [1, 2]. In this work, we experimentally observe the low energy phases of chiral vortex matter, including the predicted excess edge density at the vortex cluster edge [3]. The experiment initialises a vortex lattice in a disc-shaped BEC by rearranging 19 optically-pinned vortices to realise a vortex crystal of arbitrary size. The vortices are then released from the pins, and the vortex lattice melts as it gains energy from small-scale vibrations of the optical trapping potential. After several seconds of hold time, a robust signature of the edge density overshoot is observed. At longer hold times, the crystal melts fully. While facilitating the exploration of low-energy vortex configurations, this work also refines experimental techniques for creating arbitrary arrangements of vortices, paving the way for experiments in more complex trap geometries.

[1] G. Gauthier, M. T. Reeves, X. Yu, A. S. Bradley, M. A. Baker, T. A. Bell, H. Rubinsztein-Dunlop, M. J. Davis, T. W. Neely, Science 364, 1264-1267 (2019).

[2] M. T. Reeves, K. Goddard-Lee, G. Gauthier, O. R. Stockdale, H. Salman, T. Edmonds, X. Yu, A. S. Bradley, M. Baker, H. Rubinsztein-Dunlop, M. J. Davis, T. W. Neely, Physical Review X, 12, 011031 (2022).

[3] A. Bogatskiy and P. Wiegmann. Edge Wave and Boundary Layer of Vortex Matter, Physical Review Letters 122, 214505 (2019).   

Quantifying Turbulence in Fermionic Superfluids

We analyze the largest fermionic cold atom simulations where turbulence is imprinted externally in the form of a vortex lattice along all 3 dimensions. Although numerically the vortex length can be calculated, experimentally measuring it is extremely difficult. To address this, we use a shock wave as a probe and extract the superfluid transport properties. The goal being that extracting a hydrodynamic effective viscosity would give a good quantitative model for turbulence. A few methods are explored for identifying turbulence as well.