Classification of same-gate quantum circuits and their space-time symmetries with application to the level-spacing distribution

TL;DR

This paper classifies various configurations of Floquet quantum circuits with translational symmetry, revealing that spectral equivalence classes are characterized by space-time symmetries, which influence quantum chaos indicators like level-spacing distributions.

Contribution

It proves the existence of N-1 spectral equivalence classes for different gate arrangements and links their symmetries to quantum chaos diagnostics.

Findings

Only N-1 spectral classes exist for different gate orderings.

Each class has a nontrivial space-time symmetry affecting spectral properties.

Level-spacing distribution analysis requires considering roots of the Floquet operator.

Abstract

We study Floquet systems with translationally invariant nearest-neighbor 2-site gates. Depending on the order in which the gates are applied on an N-site system with periodic boundary conditions, there are factorially many different circuit configurations. We prove that there are only N-1 different spectrally equivalent classes which can be viewed either as a generalization of the brick-wall or of the staircase configuration. Every class, characterized by two integers, has a nontrivial space-time symmetry with important implications for the level-spacing distribution -- a standard indicator of quantum chaos. Namely, in order to study chaoticity one should not look at eigenphases of the Floquet propagator itself, but rather at the spectrum of an appropriate root of the propagator.