Minimal noise subsystems

TL;DR

This paper develops a numerical method to identify subspaces or subsystems with minimal noise in quantum systems, especially when symmetry-breaking perturbations affect decoherence-free subspaces, revealing new minimal noise subsystems under certain conditions.

Contribution

The paper introduces a numerical approach to find minimal noise subsystems in perturbed quantum noise models, extending the concept of decoherence-free subspaces.

Findings

No better subspace than the original DFS under local noise perturbations.

Perturbed collective noise models can have minimal noise subsystems different from the original DFS.

The minimal noise subsystem can outperform the original DFS when symmetry is broken by perturbations.

Abstract

The existence of a decoherence-free subspace/subsystem (DFS) requires that the noise possesses a symmetry. In this work we consider noise models in which perturbations break this symmetry, so that the DFS for the unperturbed model experiences noise. We ask whether in this case there exist subspaces/subsystems that have less noise than the original DFS. We develop a numerical method to search for such minimal noise subsystems and apply it to a number of examples. For the examples we examine, we find that if the perturbation is local noise then there is no better subspace/subsystem than the original DFS. We also show that if the noise model remains collective, but is perturbed in a way that breaks the symmetry, then the minimal noise subsystem is distinct from the original DFS, and improves upon it.