Mixed-state phase structure of gauge-Higgs subsystem codes under logical-preserving decoherence

TL;DR

This paper investigates how local-gauge-symmetric decoherence affects the phase structure of gauge-Higgs subsystem codes, revealing an unconventional mixed state criticality where logical information remains intact.

Contribution

It introduces a detailed analysis of the phase diagram of gauge-Higgs subsystem codes under decoherence, including the concept of mixed state criticality and robustness of logical information.

Findings

Decoherence induces an unconventional mixed state criticality.

Logical information is preserved despite mixed state criticality.

Robustness depends on the initial mixed state of gauge qubits.

Abstract

Some of lattice-gauge-theory models, in particular gauge-Higgs model (GHM), can be regarded and work as a subsystem code. This work studies the effect of local-gauge-symmetric decoherence on the GHM from the perspective of the subsystem code. We clarify the global phase diagram of the subsystem code. In particular, the decoherence induces an unconventional critical mixed state, where the logical information is preserved but the rest of the system exhibits mixed state criticality. For a fixed point, the decohered subsystem code is understood by the ``gauging out" prescription. By mapping the GHM to the toric code subject to decoherence, we can understand the properties of the subsystem code. We further discuss and investigate the robustness of the logical space of the subsystem code. Although this kind of subsystem code can be produced by using any bulk mixed state in the GHM, its robustness is a subtle problem due to the mixed critical gauge qubits. We consider some specific unitary for examining the robustness of the stored quantum information. For dynamical unitary perturbations described by interactions between the logical qubit and gauge qubits, the deformation of the subsystem code drastically depends on the initial mixed state of the gauge qubits.