Hilbert space fragmentation at the origin of disorder-free localization in the lattice Schwinger model

TL;DR

This paper investigates disorder-free localization in the lattice Schwinger model, revealing that Hilbert space fragmentation and dynamical constraints lead to ergodicity breaking and ultraslow entanglement growth.

Contribution

It identifies Hilbert space fragmentation as the origin of disorder-free localization and characterizes its effects on thermalization and entanglement dynamics in the Schwinger model.

Findings

Hilbert space fragmentation causes ergodicity breaking.

Ultraslow entanglement growth is due to dynamical constraints.

Charge sector averaging suggests single-logarithmic or weak power law entanglement growth.

Abstract

Lattice gauge theories, discretized cousins of continuum gauge theories arising in the Standard Model, have become important platforms for exploring non-equilibrium quantum phenomena. Recent works have reported the possibility of disorder-free localization in the lattice Schwinger model. Using degenerate perturbation theory and numerical simulations based on exact diagonalization and matrix product states, we perform a detailed characterization of thermalization breakdown in the Schwinger model, including its spectral properties, structure of eigenstates, and out-of-equilibrium quench dynamics. We scrutinize the strong-coupling limit of the model, in which an intriguing double-logarithmic-in-time growth of entanglement was previously proposed from the initial vacuum state. We identify the origin of this ultraslow growth of entanglement as due to approximate Hilbert space fragmentation and the emergence of a dynamical constraint on particle hopping, which gives rise to sharp jumps in the entanglement entropy dynamics within individual background charge sectors. Based on the statistics of jump times, we argue that entanglement growth, averaged over charge sectors, is more naturally explained as either single-logarithmic or a weak power law in time. Our results suggest the existence of a single ergodicity-breaking regime due to Hilbert space fragmentation, whose properties are reminiscent of conventional many-body localization within numerically accessible system sizes.