Topological Modes in Monitored Quantum Dynamics

TL;DR

This paper explores topological phenomena in monitored quantum dynamics of free-fermion systems, revealing protected topological modes and their manipulation, including braiding of Majorana modes, in 1+1D symmetry classes DIII and A.

Contribution

It identifies topological area-law phases in monitored free-fermion systems and demonstrates how to manipulate and braid topological modes within these phases.

Findings

Topological domain walls host protected dynamical modes.

Topological modes can be manipulated via domain-wall programming.

Braiding of Majorana modes generates entanglement in monitored dynamics.

Abstract

Dynamical quantum systems both driven by unitary evolutions and monitored through measurements have proved to be fertile ground for exploring new dynamical quantum matters. While the entanglement structure and symmetry properties of monitored systems have been intensively studied, the role of topology in monitored dynamics is much less explored. In this work, we investigate novel topological phenomena in the monitored dynamics through the lens of free-fermion systems. Free-fermion monitored dynamics were previously shown to be unified with the Anderson localization problem under the Altland-Zirnbauer symmetry classification. Guided by this unification, we identify the topological area-law-entangled phases in the former setting through the topological classification of disordered insulators and superconductors in the latter. As examples, we focus on 1+1D free-fermion monitored dynamics in two symmetry classes, DIII and A. We construct quantum circuit models to study different topological area-law phases and their domain walls in the respective symmetry classes. We find that the domain wall between topologically distinct area-law phases hosts dynamical topological modes whose entanglement is protected from being quenched by the measurements in the monitored dynamics. We demonstrate how to manipulate these topological modes by programming the domain-wall dynamics. In particular, for topological modes in class DIII, which behave as unmeasured Majorana modes, we devise a protocol to braid them and study the entanglement generated in the braiding process.