Operator Entanglement in Local Quantum Circuits II: Solitons in Chains of Qubits

TL;DR

This paper provides exact analytical results on the dynamics of local-operator entanglement in quantum circuits with ultralocal solitons, classifies such circuits, and explores how soliton movement affects entanglement growth.

Contribution

It classifies all circuits with ultralocal solitons, proves entanglement saturation in certain cases, and derives explicit formulas for entanglement entropies in these systems.

Findings

Ultralocal solitons only occur in dual-unitary circuits.

Entanglement saturates for operators on even/odd sites with solitons moving in one direction.

Operators on the odd sublattice can have unbounded entanglement in chiral circuits.

Abstract

We provide exact results for the dynamics of local-operator entanglement in quantum circuits with two-dimensional wires featuring ultralocal solitons, i.e. single-site operators which, up to a phase, are simply shifted by the time evolution. We classify all circuits allowing for ultralocal solitons and show that only dual-unitary circuits can feature moving ultralocal solitons. Then, we rigorously prove that if a circuit has an ultralocal soliton moving to the left (right), the entanglement of local operators initially supported on even (odd) sites saturates to a constant value and its dynamics can be computed exactly. Importantly, this does not bound the growth of complexity in chiral circuits, where solitons move only in one direction, say to the left. Indeed, in this case we observe numerically that operators on the odd sublattice have unbounded entanglement. Finally, we present a closed-form expression for the local-operator entanglement entropies in circuits with ultralocal solitons moving in both directions. Our results hold irrespectively of integrability.