FOR 5522: Quantum thermalization, localization, and constrained dynamics with interacting ultracold atoms

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A long-standing and fascinating question from statistical physics aims at understanding how equilibrium physics —based on macroscopic parameters— emerges from the microscopic dynamics of classical or quantum systems. For closed quantum systems, this question acquires an additional twist since the formalism to describe quantum chaos and irreversibility is still subject to ongoing research. The advent of quantum simulators, most notably, interacting ultracold atoms, has led to an exceptional increase in research activity and groundbreaking experiments in this context. On the theory side, there are strong evidence and criteria for ergodic dynamics in interacting many-body systems, while many-body localization has emerged as a potential exception from thermalization. As an intermediate behavior, systems with constrained dynamics are of central interest. The goal of our initiative is to connect these three pillars of nonequilibrium systems—ergodicity, many-body localization, and constrained dynamics. In particular, we will devise and carry out ultracold atom experiments that qualitatively and quantitatively serve to address and clarify open key questions concerning thermalization time scales, the emergence of hydrodynamics, the existence of many-body localized phases and the characterization of the corresponding transition, and most notably, the realization and study of several instances of constrained dynamics. Concerning constrained dynamics, we will investigate Hilbert-space fragmentation, fractonic systems, kinetically constrained models, and nonequilibrium dynamics in lattice-gauge theories. We will identify experimental setups and suitable observables to study these instances of slow dynamics in detail using the capabilities of ultracold atomic gases. Our experiments include bosonic quantum-gas microsocopes, a fermionic Yb experiment that will realize a lattice-gauge theory, and a heavy-light mixture with an extreme mass imbalance. Our theoretical approaches combine state-of-the-art computational methods, the theory of systems with constrained dynamics and many-body localization, analytical methods, quantum optics, and statistical physics. We expect that the very close experiment theory collaborations in this research unit will lead to novel results that are expected to substantially advance the understanding of nonequilibrium dynamics. As a long-term perspective, our research will have an impact on concepts for controlling thermalization and non-ergodicity to design artificial quantum matter with new properties and functionalities.