On the integrability structure of the deformed rule-54 reversible cellular automaton

TL;DR

This paper explores the integrability of deformations of the rule-54 reversible cellular automaton, revealing conserved charges in quantum cases and constructing steady states in stochastic cases, along with a new criterion for integrability.

Contribution

It introduces the integrability structures of quantum and stochastic deformations of RCA54, including conserved charges and steady state constructions, and proposes a new empirical integrability criterion.

Findings

Existence of a range-6 conserved charge in the quantum deformation.

Construction of the non-equilibrium steady state using a hybrid matrix ansatz.

Introduction of digit complexity as a criterion for integrability.

Abstract

We study quantum and stochastic deformations of the rule-54 reversible cellular automaton (RCA54) on a 1+1-dimensional spatiotemporal lattice, focusing on their integrability structures in two distinct settings. First, for the quantum deformation, which turns the model into an interaction-round-a-face brickwork quantum circuit (either on an infinite lattice or with periodic boundary conditions), we show that the shortest-range nontrivial conserved charge commuting with the discrete-time evolution operator has a density supported on six consecutive sites. By constructing the corresponding range-6 Lax operator, we prove that this charge belongs to an infinite tower of mutually commuting conserved charges generated by higher-order logarithmic derivatives of the transfer matrix. With the aid of an intertwining operator, we further prove that the transfer matrix commutes with the discrete-time evolution operator. Second, for the stochastic deformation, which renders the model into a Markov-chain circuit, we investigate open boundary conditions that couple the system at its edges to stochastic reservoirs. In this setting, we explicitly construct the non-equilibrium steady state (NESS) by means of a staggered patch matrix ansatz, a hybrid construction combining the previously used commutative patch-state ansatz for the undeformed RCA54 with the matrix-product ansatz. Finally, we propose a simple empirical criterion for detecting integrability or exact solvability in a given model setup, introducing the notion of digit complexity.