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

We study Floquet systems with translationally invariant nearest-neighbor two-site gates. Depending on the order in which the gates are applied on an <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>N</a:mi></a:math>-site system with periodic boundary conditions, there are factorially many different circuit configurations. We prove that there are only <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>N</b:mi><b:mo>−</b:mo><b:mn>1</b:mn></b:mrow></b:math> 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.