Stability of topological superconducting qubits with number conservation

TL;DR

This paper rigorously demonstrates that key properties of topological superconducting qubits, such as a protected qubit subspace and exponentially small energy splitting, persist in a number-conserving model, supporting their robustness for quantum computing.

Contribution

It proves that the essential features of topological qubits hold in a realistic number-conserving model, extending previous mean-field results.

Findings

Existence of low-energy qubit states separated by a finite gap.

Exponential suppression of energy splitting with system size.

Robustness of topological properties in number-conserving models.

Abstract

The study of topological superconductivity is largely based on the analysis of simple mean-field models that do not conserve particle number. A major open question in the field is whether the remarkable properties of these mean-field models persist in more realistic models with a conserved total particle number and long-range interactions. For applications to quantum computation, two key properties that one would like to verify in more realistic models are (i) the existence of a set of low-energy states (the qubit states) that are separated from the rest of the spectrum by a finite energy gap, and (ii) an exponentially small (in system size) bound on the splitting of the energies of the qubit states. It is well known that these properties hold for mean-field models, but so far only property (i) has been verified in a number-conserving model. In this work we fill this gap by rigorously establishing both properties (i) and (ii) for a number-conserving toy model of two topological superconducting wires coupled to a single bulk superconductor. Our result holds in a broad region of the parameter space of this model, suggesting that properties (i) and (ii) are robust properties of number-conserving models, and not just artifacts of the mean-field approximation.