Slow growth of out-of-time-order correlators and entanglement in integrable disordered systems

TL;DR

This paper studies how information and entanglement spread slowly in disordered integrable systems, revealing nonlocal effects and logarithmic entanglement growth despite the absence of interactions.

Contribution

It demonstrates slow information spreading and entanglement growth in disordered integrable models due to nonlocal excitations and zero modes, expanding understanding of quantum dynamics in such systems.

Findings

Out-of-time-order correlators spread slowly beyond localization length.

Entanglement entropy grows logarithmically in time after a quench.

Strong zero modes lead to an exponential hierarchy of time scales.

Abstract

We investigate how information spreads in three paradigmatic one-dimensional models with spatial disorder. The models we consider are unitarily related to a system of free fermions and are thus manifestly integrable. We demonstrate that out-of-time-order correlators can spread slowly beyond the single-particle localization length, despite the absence of many-body interactions. This phenomenon is shown to be due to the nonlocal relationship between elementary excitations and the physical degrees of freedom. We argue that this non-locality becomes relevant for time-dependent correlation functions. In addition, a slow logarithmic-in-time growth of the entanglement entropy is observed following a quench from an unentangled initial state. We attribute this growth to the presence of strong zero modes, which gives rise to an exponential hierarchy of time scales upon ensemble averaging. Our work on disordered integrable systems complements the rich phenomenology of information spreading and we discuss broader implications for general systems with non-local correlations.