Out-of-time-order correlations and the fine structure of eigenstate thermalisation

TL;DR

This paper demonstrates that out-of-time-order correlators (OTOCs) are precise tools for probing the detailed structure of eigenstate thermalisation, revealing correlations and energy scales associated with quantum thermalisation in many-body systems.

Contribution

The study explicitly connects OTOC dynamics with the fine structure of ETH, introducing an operator-dependent energy scale and analyzing its finite-size scaling.

Findings

OTOCs reveal correlations beyond standard ETH.

Identification of an operator-dependent energy scale $oldsymbol{\omega_{ extrm{GOE}}}$.

Linear finite-size scaling of $oldsymbol{\omega_{ extrm{GOE}}}$ in studied models.

Abstract

Out-of-time-order correlators (OTOCs) have become established as a tool to characterise quantum information dynamics and thermalisation in interacting quantum many-body systems. It was recently argued that the expected exponential growth of the OTOC is connected to the existence of correlations beyond those encoded in the standard Eigenstate Thermalisation Hypothesis (ETH). We show explicitly, by an extensive numerical analysis of the statistics of operator matrix elements in conjunction with a detailed study of OTOC dynamics, that the OTOC is indeed a precise tool to explore the fine details of the ETH. In particular, while short-time dynamics is dominated by correlations, the long-time saturation behaviour gives clear indications of an operator-dependent energy scale $\omega_{\textrm{GOE}}$ associated to the emergence of an effective Gaussian random matrix theory. We provide an estimation of the finite-size scaling of $\omega_{\textrm{GOE}}$ for the general class of observables composed of sums of local operators in the infinite-temperature regime and found linear behaviour for the models considered.