Symmetry-protected sign problem and magic in quantum phases of matter

TL;DR

This paper introduces the concepts of symmetry-protected sign problem and magic to analyze the complexity of symmetry-protected topological phases, showing that certain SPT states inherently possess these properties due to their boundary symmetry actions.

Contribution

It defines symmetry-protected sign problem and magic, proves their presence in specific SPT phases, and explores their implications for quantum state complexity and boundary phenomena.

Findings

1D Z2 x Z2 SPT states have a symmetry-protected sign problem.

2D Z2 SPT states exhibit symmetry-protected magic.

Explicit decorated domain wall models of SPT phases are provided.

Abstract

We introduce the concepts of a symmetry-protected sign problem and symmetry-protected magic to study the complexity of symmetry-protected topological (SPT) phases of matter. In particular, we say a state has a symmetry-protected sign problem or symmetry-protected magic, if finite-depth quantum circuits composed of symmetric gates are unable to transform the state into a non-negative real wave function or stabilizer state, respectively. We prove that states belonging to certain SPT phases have these properties, as a result of their anomalous symmetry action at a boundary. For example, we find that one-dimensional $\mathbb{Z}_2 \times \mathbb{Z}_2$ SPT states (e.g. cluster state) have a symmetry-protected sign problem, and two-dimensional $\mathbb{Z}_2$ SPT states (e.g. Levin-Gu state) have symmetry-protected magic. Furthermore, we comment on the relation between a symmetry-protected sign problem and the computational wire property of one-dimensional SPT states. In an appendix, we also introduce explicit decorated domain wall models of SPT phases, which may be of independent interest.