Many-Body Delocalization in the Presence of a Quantum Bath

Abstract

Closed generic quantum many-body systems may fail to thermalize under certain conditions even after long times, a phenomenon called many-body localization (MBL). Numerous studies support the stability of the MBL phase in strongly disordered one-dimensional systems. However, the situation is much less clear when a small part of the system is ergodic, a scenario which also has important implications for the existence of many-body localization in higher dimensions. Here we address this question experimentally using a large-scale quantum simulator of ultracold bosons in a two-dimensional optical lattice. We prepare two-component mixtures of varying relative population and implement a disorder potential which is experienced only by one of the components. The second nondisordered “clean” component plays the role of a bath of adjustable size that is collisionally coupled to the “dirty” component. Our experiments show how the dynamics of the dirty component, which, when on its own, show strong evidence of localization, become affected by the coupling to the clean component. For a high clean population, the clean component appears to behave as an effective bath for the system which leads to its delocalization, while for a smaller clean population, the ability of the bath to destabilize the system becomes strongly reduced. Our results reveal how a finite-sized quantum system can bring another one towards thermalization, in a regime of complex interplay between disorder, tunneling, and intercomponent interactions. They provide a new benchmark for effective theories aiming to capture the complex physics of MBL in the weakly localized regime.

Alternate abstract:

Plain Language Summary

Some isolated quantum systems with disordered potentials fail to ever reach thermal equilibrium, a phenomenon known as many-body localization (MBL). An outstanding question concerns the stability of this phenomenon. While it is well known that exposing the quantum system to the outside world destroys localization, it is not clear what happens when the MBL system is connected to another small quantum system known as a quantum bath. Does a quantum bath destroy the localization, and if so, how small can that bath be? We experimentally address these questions by observing a mixture of cold interacting atoms, some of which are in a laser-induced disordered (MBL) state and some acting as a quantum bath.

We use a quantum-gas microscope to prepare an atomic cloud with an initial density pattern and track its dynamics over a very long time. When preparing a purely disordered system, we observe a persistence of the initial pattern, consistent with MBL. But by increasing the population in the homogeneous bath component, the initial pattern eventually vanishes, which signals an efficient delocalization due to the bath.

Our work provides the first insight into the thermalizing dynamics caused by a quantum bath in a complex regime of interactions and disorder, a problem that is hard to simulate with existing numerical methods.