Free-Fermion Dynamics with Measurements: Topological Classification and Adaptive Preparation of Topological States

TL;DR

This paper introduces a topological classification framework for fermionic dynamical systems with measurements, linking symmetry, topology, and adaptive circuits to prepare and analyze topological states in various dimensions.

Contribution

It develops a unified classification scheme for free and interacting fermionic dynamics, establishing a bulk-boundary correspondence and demonstrating topological state preparation via adaptive circuits.

Findings

Two classification schemes based on symmetry and topology are established.

Finite-range adaptive circuits can prepare topological steady states in any dimension.

Numerical simulations confirm the convergence to Chern insulator ensembles and robustness to noise.

Abstract

We develop a general framework for classifying fermionic dynamical systems with measurements using symmetry and topology. We discuss two complementary classification schemes based on the Altland-Zirnbauer tenfold way: (1) the many-body evolution operator (mEO) symmetry class, which classifies fermionic dynamics at the many-body level and generalizes to interacting dynamics, and (2) the single-particle transfer matrix (sTM) symmetry class, which classifies free-fermion dynamics at the single-particle level and connects to Anderson localization physics. In the free-fermion limit, these two frameworks are in one-to-one correspondence and yield equivalent topological classifications of area-law entangled dynamical phases. This leads to a novel dynamical bulk-boundary correspondence: the topology of the dynamical system's spacetime \textit{bulk} determines the topology of the area-law entangled steady-state ensemble living on its temporal \textit{boundary}. Building on this correspondence, we provide a general realization of topological dynamical phases using Gaussian adaptive circuits. They are designed to prepare and stabilize free-fermion topological states as their steady states in \textit{any} spatial dimension. While circuits with exponentially local operations can stabilize a single topological steady state, those with finite-range operations can reach a topological steady-state ensemble. As a demonstration, we explicitly construct and simulate 2+1d adaptive circuits that realize mEO-class-A topological dynamics. We show that the finite-range versions converge to an ensemble of Chern insulators in ${\cal O}(1)$ circuit depth. We numerically study the topological phase transitions and dynamical domain-wall modes between different topological dynamical phases in this symmetry class. We also analyze the robustness of our adaptive circuit protocol to coherent noise.