Random permutation circuits beyond qubits are quantum chaotic

Random permutation circuits were recently introduced as minimal models for local many-body dynamics that can be interpreted both as classical and quantum. Standard dynamical complexity indicators such as damage spreading and out-of-time-order correlators (OTOCs), show that these systems exhibit sensitivity to initial conditions in the classical setting and operator scrambling in the quantum setting. Here, we address their quantum chaoticity—a stricter property—by studying the time evolution of local operator entanglement (LOE). We show that the behavior of LOE in random permutation circuits depends on the dimension of the local configuration space <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>q</a:mi> </a:math> . When <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mi>q</b:mi> <b:mo>=</b:mo> <b:mn>2</b:mn> </b:mrow> </b:math> , i.e., the circuits act on qubits, random permutations are Clifford and the LOE of any local operator is bounded by a constant, indicating that they are not truly chaotic. On the other hand, when the dimension of the local configuration space exceeds two, the LOE grows linearly in time. We prove this in the limit of large <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mi>q</c:mi> </c:math> and present numerical evidence that a three-dimensional local configuration space is sufficient for a linear growth of LOE. Our findings highlight that quantum chaos can be produced by essentially classical dynamics. Moreover, we show that LOE can be defined also in the classical realm and put it forward as a universal indicator chaos, both quantum and classical.