Weak ergodicity breaking in Josephson-junction arrays

TL;DR

This paper investigates weak ergodicity breaking in Josephson-junction arrays, revealing persistent low-entanglement eigenstates that prevent thermalization and resemble quantum scars, with ergodic behavior increasing with system size.

Contribution

It uncovers weak ergodicity breaking and quantum scar-like eigenstates in Josephson-junction arrays, showing their impact on thermalization and how system geometry and flux influence ergodicity.

Findings

Persistent low-entanglement eigenstates exist even with strong Josephson interactions.

Charge-density-wave states do not thermalize and maintain order over long times.

Ergodicity increases with system size, especially with added magnetic flux in ladder geometries.

Abstract

We study the quantum dynamics of Josephson junction arrays. We find isolated groups of low-entanglement eigenstates, that persist even when the Josephson interaction is strong enough to destroy the organization of the spectrum in multiplets, and a perturbative description is no longer possible. These eigenstates provide a weak ergodicity breaking, and are reminiscent of the quantum scars. Due to the presence of these eigenstates, initializing with a charge-density-wave state, the system does not thermalize and the charge-density-wave order persists for long times. Considering global ergodicity probes, we find that the system tends towards more ergodicity for increasing system size: The parameter range where the bulk of the eigenstates look nonergodic shrinks for increasing system size. We study two geometries, a one-dimensional chain and a two-leg ladder. In the latter case, adding a magnetic flux makes the system more ergodic.