Quantum Hall valley ferromagnets as a platform for topologically protected quantum memory

Abstract

Materials hosting topologically protected non-Abelian zero modes offer the exciting possibility of storing and manipulating quantum information in a manner that is protected from decoherence at the hardware level. In this paper, we study the possibility of realizing such excitations along line defects in certain fractional quantum Hall states in multivalley systems. Such line defects have been recently observed experimentally between valley polarized Hall states on the surface of Bi(111), and excitations near these defects appear to be gapped (gapless) depending on the presence (absence) of interaction-induced gapping perturbations constrained by momentum selection rules, while the position of defects is determined by strain. In this paper, we use these selection rules to show that a hybrid structure involving a superlattice imposed on such a multivalley quantum Hall surface realizes non-Abelian anyons, which can then be braided by modulating strain locally to move line defects. Specifically, we explore such defects in Abelian fractional quantum Hall states of the form $\ensuremath{\nu}=2/m$ using a K-matrix approach, and identify relevant gapping perturbations. Charged modes on these line defects remain gapped, while charge neutral valley pseudospin modes may be gapped with the aid of two (mutually orthogonal) superlattices, which pin noncommuting fields. When these superlattices are alternated along the line defect, non-Abelian zero modes result at points where the gapping perturbation changes. Given that these pseudospin modes carry no net physical charge or spin, the setup eschews utilizing superconducting and magnetic elements to engineer gapping perturbations. We provide a scheme to braid these modes using strain modulation, and confirm that the resulting unitaries satisfy a representation of the braid group.