Quantifying and Controlling Prethermal Nonergodicity in Interacting Floquet Matter

Abstract

The use of periodic driving for synthesizing many-body quantum states depends crucially on the existence of a prethermal regime, which exhibits drive-tunable properties while forestalling the effects of heating. This dependence motivates the search for direct experimental probes of the underlying localized nonergodic nature of the wave function in this metastable regime. We report experiments on a many-body Floquet system consisting of atoms in an optical lattice subjected to ultrastrong sign-changing amplitude modulation. Using a double-quench protocol, we measure an inverse participation ratio quantifying the degree of prethermal localization as a function of tunable drive parameters and interactions. We obtain a complete prethermal map of the drive-dependent properties of Floquet matter spanning four square decades of parameter space. Following the full time evolution, we observe sequential formation of two prethermal plateaux, interaction-driven ergodicity, and strongly frequency-dependent dynamics of long-time thermalization. The quantitative characterization of the prethermal Floquet matter realized in these experiments, along with the demonstration of control of its properties by variation of drive parameters and interactions, opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering.

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Plain Language Summary

The ability to create and control quantum systems that are far out of equilibrium opens up intriguing possibilities for creating new states of matter whose properties are tuned via an external drive. However, this requires researchers to abandon the existing powerful tool set of equilibrium statistical mechanics, leaving them without effective theoretical and experimental techniques to characterize these quantum states. To overcome this challenge, we studied a “prethermal” ensemble of atoms, in which the initial state information is preserved against the effects of heating for macroscopic times. Our experiments allowed us to characterize some properties of the ensemble as well as to control other properties.

In our experiments, we work with a quantum gas of lithium atoms in an optical lattice (a trap built from counterpropagating laser beams). By modulating the amplitude of laser light in the lattice, we prepare a state of matter with properties that could be tuned by adjusting the drive amplitude and frequency. Images of the ensemble reveal a detailed map of state occupations, in good agreement with theoretical predictions. As the system evolves during the modulation, we observe the emergence and destruction of prethermal states, interaction-driven heating, and an unexpected dependence between the frequency of the drive and the dynamics of eventual thermalization.

The complete experimental control and quantitative characterization of prethermal-driven matter demonstrated here opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering.