Rigorous results on topological superconductivity with particle number conservation

TL;DR

This paper rigorously demonstrates that a number-conserving model of topological superconductivity retains key properties of mean-field models, bridging the gap between idealized theories and realistic physical systems.

Contribution

It provides the first rigorous proof that number-conserving models can exhibit topological superconductivity features traditionally shown by mean-field models.

Findings

Existence of a finite energy gap in fixed particle number sectors

Presence of long-range Majorana-like correlations

Ground state fermion parity changes with boundary conditions

Abstract

Most theoretical studies of topological superconductors and Majorana-based quantum computation rely on a mean-field approach to describe superconductivity. A potential problem with this approach is that real superconductors are described by number-conserving Hamiltonians with long-range interactions, so their topological properties may not be correctly captured by mean-field models that violate number conservation and have short-range interactions. To resolve this issue, reliable results on number-conserving models of superconductivity are essential. As a first step in this direction, we use rigorous methods to study a number-conserving toy model of a topological superconducting wire. We prove that this model exhibits many of the desired properties of the mean-field models, including a finite energy gap in a sector of fixed total particle number, the existence of long range "Majorana-like" correlations between the ends of an open wire, and a change in the ground state fermion parity for periodic vs. anti-periodic boundary conditions. These results show that many of the remarkable properties of mean-field models of topological superconductivity persist in more realistic models with number-conserving dynamics.