Session X05: Dynamics of Cold Atoms in Optical Lattices

Observation of Many-body Dynamical Delocalization in a Kicked 1D Ultracold Gas

In the long-time limit, the classical kicked rotor system displays typical chaos phenomena and diffusive energy growth above a critical kick strength, while its quantum counterpart can exhibit dynamical localization that features energy saturation. However, the behavior of the dynamically localized state in the presence of many-body interactions has long been an open question in the theory realm. Here we report the first experimental study of the many-body effect in dynamically localized quantum kicked rotor (QKR), and our observations of an interaction-driven dynamical delocalization in a 1D ultracold gas of Yb-174 kicked by a pulsed optical lattice. The many-body quantum chaos manifests as a sub-diffusive energy growth in the long-time limit, and our observations shed light on an area that’s difficult to approach from a theoretical perspective.   

Quantum many-body chaos in driven and lattice trapped Bose mixtures

We propose a Floquet protocol for Rabi coupled Bose mixtures to study the dynamical emergence of quantum chaos. The chaotic dynamics emerge independently of the miscibility character of the mixture for both repulsive or attractive interactions, whereas the characteristic time of chaos- many-body Thouless time- is affected. We characterize the Thouless time by determining how it changes with respect to the physical system parameters, e.g., the driving frequency, tunneling and interaction strengths. In addition to determining Thouless time scaling with respect to the system size as usually employed in random quantum circuit models, here we also study the effect of atom number. Strikingly, we exemplify that the Thouless time features a power-law increase for varying atom number. As expected in local quantum systems as a sign of quantum many-body chaos, we observe a sharp rise in the spectral form factor before the prediction of random matrix theory (RMT). In this pre-RMT region, which is observed to be suppressed at unit filling, we demonstrate the universality of systems with different atom numbers.    

Observation of interaction-driven dynamical delocalization of the 3D Anderson insulator

The quantum kicked rotor (QKR) is a paradigmatic system to study the Anderson model. The 3D Anderson model is known to display a metal-insulator transition. However, the addition of interaction to the 3D Anderson model has remained unexplored experimentally. We report on our observation of interaction-driven dynamical delocalization of the 3D Anderson insulator. We load a ¹⁷⁴Yb BEC into 1D tubes formed by a two-dimensional optical lattice. We then pulse on a one-dimensional lattice with a fixed kick period along the axial direction of the tubes. The pulse amplitude is modulated by two incommensurate frequencies to simulate the 3D Anderson model. We measure the energy of the system by analyzing the momentum profile of the atom cloud after some number kicks. We study the delocalization of the 3D Anderson insulator for different interaction strengths, kick strengths, and modulation amplitudes, and measure the power-law exponent. Our experiment sheds light on the transport dynamics in a disordered medium in the presence of interaction.   

Observation of the quantum boomerang effect

A particle in an Anderson-localized system, if launched in any direction, should on average return to its starting point and stay there. Despite the central role played by Anderson localization in the modern understanding of condensed matter, this "quantum boomerang" effect, an essential feature of the localized state, was only recently theoretically predicted. We report the first experimental observation of the quantum boomerang effect. Using a degenerate Bose gas and a phase-shifted pair of optical lattices, we not only confirm the predicted dependence of the boomerang effect on Floquet gauge, but also elucidate the crucial role of initial state symmetries.  Highlighting the key role of localization, we observe that as stochastic kicking destroys dynamical localization, the quantum boomerang effect also disappears. Measured dynamics are in agreement with numerical models and with predictions of an analytical theory we present, which clarifies the connection between time-reversal symmetry and boomerang dynamics. These results showcase a unique experimental probe of the underlying quantum nature of Anderson localized matter.   

Spatial Dynamics in Three Dimensional Lattice Confined Spinor Gases

We present an experimental study of spatial distributions after nonequilibrium quantum quenches across superfluid to Mott insulator phase transitions in three-dimensional ultracold gases using standard imaging techniques.  Using spin mixing oscillations we are able to indirectly determine the spatial distributions of our system.  This indirect method can be applied to other atomic species and may be helpful to study spatial dynamics of three dimensional lattice systems as lattice site-resolved imaging in such systems is still difficult to implement.  Our results indicate that during these quench sequences atoms undergo complex spatial dynamics while redistributing within a harmonic trap and suggest spatial distributions reach an equilibrium value when the lattice quench speed is sufficiently slow to ensure the atoms initially located in the trap center have enough time to move towards the trap boundaries and equilibrate.  The extracted spatial distributions therefore have a strong dependence on the lattice quench speed.  Our data also confirm that number distributions can be manipulated by properly designing quantum quench sequences, which may have important applications in attaining different many-body quantum phases.    

Collective Effects in Matter-Wave Emission

Ultracold atoms in state-selective optical lattices open a new window for the study of quantum-optical phenomena [1], allowing for the experimental observation of Lamb shifts, non-Markovian decay, bound-state beats, and polaritons. In this talk, we discuss the connection between matter-wave polariton formation and collective radiative phenomena, including super/subradiance, timed Dicke states, and directional emission; and we give an outlook on possible experiments.

    

Transport in a kicked Aubry-André Harper model

We report the realization of the kicked Aubry-André Harper model using ultracold strontium in a pulsed 1D bichromatic optical lattice. Rich transport dynamics are predicted over a phase diagram of varying disorder and kick periodicity, but the breakdown of the tight-binding approximation for large kick periods and disorder strengths limits investigation. By engineering apodized kicking waveforms, we are able to circumvent the breakdown of the tight-binding approximation and suppress heating. Use of such waveforms extends the range of this quantum simulator by 5 orders of magnitude in parameter space and enables access to regions of anomalous transport. In the revealed regime we both theoretically and experimentally observe signatures of wavefunction multifractality across an extended region away from the critical point.   

Probing the extended Fermi-Hubbard Model with ultracold ytterbium

The Fermi-Hubbard Model (FHM) in condensed-matter physics describes the interplay between the kinetic energy and the on-site interaction of electrons in a lattice. Ultracold atoms in optical lattices have allowed to investigate this model with an unprecedented level of control and tunability. While the original focus has been on the FHM of spin 1/2 particles and SU(2) symmetry, extended Hubbard models, such as multi-band and SU(N>2)-symmetric models, can now increasingly be addressed both in experiment and theory. In our experiment, we implement Hamiltonians such as the SU(N)-FHM with tunable N≤6 with ultracold ytterbium. We prepare ¹⁷³Yb atoms in a two-dimensional square optical lattice, and locally probe the FHM for variable symmetry and interactions, allowing for direct comparisons of different systems and benchmarking of theoretical models.   

On the relation between ultracold atoms in optical lattices and a rigid rotor in external fields

In the present work, we study the band structure of ultra-cold atoms in an optical superlattice and establish a relation between this system and a two-dimensional quantum rigid rotor interacting with external orienting and aligning fields [1,2]. Using this analogy, we present analytic expressions of band edge energies and wavefunctions and provide insight into the semi-finite band structure of the superlattice. Furthermore, we show that it is possible to translate essential concepts of the rigid rotor, such as orientation and alignment, into the localization of ultracold atoms along a superlattice. We believe our results bring some advances in the field of coherent control of rotational degrees into the realm of atoms in optical lattices.

[1] S. Becker, M. Mirahmadi, B. Schmidt, K. Schatz and B. Friedrich, Eur. Phys. J. D 71 149 (2017).

[2] M. Mirahmadi, B. Schmidt and B. Friedrich, New J. Phys. 23 063040 (2021).

    

Numerical simulation of microscopic thermalization in a one-dimensional harmonically trapped gas coupled to a chain of masses

Many physical systems can be modeled as a large number of small, mobile particles coupled to a dissipative thermal bath. Ubiquitous phenomena such as Brownian motion, friction, and quantum back action can be analyzed in this framework. In this work, we construct a minimal experimentally-realizable model in which mobile particles are confined to a harmonic potential and coupled to a minimal thermal bath composed of a chain of coupled masses. Using a classical formulation, we show that, even when the mobile particles interact only with a single mass of the chain, they experience thermalization and drag. We derive general integro-differential equations of motion and apply them to several model systems. We study the scaling of model parameters, such as the relative masses of mobile and chain particles, the range, and the strength of the interaction. We also explore the effect of many mobile particles, as well as the timescale for the memory kernel in the interaction. This minimal model successfully captures thermal equilibration, where the energy distribution of the mobile particles approaches the expected Boltzmann form. The mobile particles' motion exhibits minimal cross-correlations, validating the independent particle approximation. Finally, we discuss several proposals for controllable experimental realizations of this model, including trapped ion chains, hybrid ion-atom systems, neutral atom bright solitons, and neutral atoms in an optical lattice.