Quantum thermalization through entanglement in an isolated many-body system

TL;DR

This paper experimentally demonstrates how quantum entanglement enables thermalization in an isolated many-body system, showing local thermal behavior despite the global state remaining pure, thus bridging quantum mechanics and statistical physics.

Contribution

It provides direct experimental evidence of entanglement-driven thermalization in a quantum system, confirming the Eigenstate Thermalization Hypothesis with local measurements.

Findings

Thermalization occurs locally while the global state remains pure.

Entanglement entropy acts as thermal entropy during thermalization.

Measurements align with statistical ensembles and ETH predictions.

Abstract

The concept of entropy is fundamental to thermalization, yet appears at odds with basic principles in quantum mechanics. Statistical mechanics relies on the maximization of entropy for a system at thermal equilibrium. However, an isolated many-body system initialized in a pure state will remain pure during Schr\"{o}dinger evolution, and in this sense has static, zero entropy. The underlying role of quantum mechanics in many-body physics is then seemingly antithetical to the success of statistical mechanics in a large variety of systems. Here we experimentally study the emergence of statistical mechanics in a quantum state, and observe the fundamental role of quantum entanglement in facilitating this emergence. We perform microscopy on an evolving quantum system, and we see thermalization occur on a local scale, while we measure that the full quantum state remains pure. We directly measure entanglement entropy and observe how it assumes the role of the thermal entropy in thermalization. Although the full state remains measurably pure, entanglement creates local entropy that validates the use of statistical physics for local observables. In combination with number-resolved, single-site imaging, we demonstrate how our measurements of a pure quantum state agree with the Eigenstate Thermalization Hypothesis and thermal ensembles in the presence of a near-volume law in the entanglement entropy.