Charge $2e/3$ superconductivity and topological degeneracies without localized zero modes in bilayer fractional quantum Hall states

TL;DR

This paper proposes a novel method to realize topologically protected degeneracies and fractional Josephson effects in bilayer fractional quantum Hall systems coupled to superconductors, without relying on localized zero modes.

Contribution

It introduces a new approach to create and measure topological degeneracies in bilayer FQH states via charge $2e/3$ quasiparticle pairing, avoiding localized parafermion zero modes.

Findings

Charge $2e/3$ quasiparticle pairing induces topological degeneracies.

Observation of fractional $2e/3$ AC Josephson effect.

Analog of non-Abelian braiding without localized zero modes.

Abstract

It has been recently shown that non-Abelian defects with localized parafermion zero modes can arise in conventional Abelian fractional quantum Hall (FQH) states. Here we propose an alternate route to creating, manipulating, and measuring topologically protected degeneracies in bilayer FQH states coupled to superconductors, without the creation of localized parafermion zero modes. We focus mainly on electron-hole bilayers, with a $\pm 1/3$ Laughlin FQH state in each layer, with boundaries that are proximity-coupled to a superconductor. We show that the superconductor induces charge $2e/3$ quasiparticle-pair condensation at each boundary of the FQH state, and that this leads to (1) topologically protected degeneracies that can be measured through charge sensing experiments and (2) a fractional charge $2e/3$ AC Josephson effect. We demonstrate that an analog of non-Abelian braiding is possible, despite the absence of a localized zero mode. We discuss several practical advantages of this proposal over previous work, and also extensions to electron-electron bilayers at $\nu = 2/3 + 1/3$ coupled to superconductivity, and to electron-hole bilayers with only interlayer tunneling.