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Buried Dirac Points in Quantum Spin Hall Insulators: Implications for Majorana Kramers Pair-Based Quantum Computing

J. J. Cuozzo1,2,*, W. Yu3, X. Shi4, A. J. Muhowski3, S. D. Hawkins3, J. F. Klem3, E. Rossi5, and W. Pan2,†

  • *Contact author: jjcuozz@sandia.gov
  • †Contact author: wpan@sandia.gov
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PRX Quantum 7, 010313 – Published 20 January, 2026

DOI: https://doi.org/10.1103/tw65-82r9

Abstract

Quantum spin Hall insulators (QSHIs) host helical electronic edge states that are protected from backscattering due to time-reversal symmetry (TRS). Despite considerable work investigating QSHI edge states, there is still an open question about their unexpected resilience to large magnetic fields where TRS is undoubtedly broken. In this work, we investigate the transport properties of helical edge states in a QSHI-superconductor (QSHI-SC) junction formed by a InAs(15 nm)/GaSb(5 nm) double quantum well and a superconducting tantalum (Ta) constriction. We observe a robust conductance plateau up to 2 T, signaling resilient edge-state transport. Using a modified Landauer-Büttiker analysis, we find that the zero-field conductance is consistent with 98% Andreev-reflection probability owing to the high transparency of the (InAs/GaSb)-Ta interface. Such resilience is consistent with the Dirac point for the edge states being buried in the bulk valence band. We further theoretically show that a buried Dirac point does not affect the robustness of the quasi-one-dimensional topological superconducting phase. We find that a buried Dirac point favors the hybridization of Majorana Kramers pairs (MKPs)—predicted to exist in a QSHI-SC constriction—and fermionic modes in the QSHI vacuum edge resulting in extended MKP states, highlighting the subtle role of buried Dirac points in probing MKPs.

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Popular Summary

For many years, there has been intense interest and research on topological superconductivity for the prospect of identifying exotic topological modes (e.g. Majorana zero modes) and using them for fault-tolerant topological quantum computing (TQC). Most of the current approaches to TQC, however, require applying a magnetic field which presents challenges to scalability and operation of TQC. Therefore, realizing Majorana zero modes without a magnetic field is highly desired. In this paper, we report the study of a structure where we can realize two pairs of Majorana zero modes—so-called Majorana Kramer pairs (MKPs)—without applying magnetic fields or breaking time-reversal symmetry.

In our study, we examine in an InAs/GaSb double quantum well the nonlocal electrical conductance across a superconducting Ta constriction. We demonstrate helical edge transport in our device and measure a nearly quantized conductance that is anomalously robust to a large external magnetic field. Detailed theoretical simulations show that the quantized conductance is due to a nearly perfect Andreev-reflection probability at the superconducting Ta constriction, signaling a clean (InAs/GaSb)-Ta interface. Our detailed numerical simulations further show that the anomalously robust conductance is indicative of a buried Dirac point in the edge-state band structure that can, surprisingly, help stabilize MKPs in the presence of a weak external magnetic field.

Our work is a key step toward demonstrating the feasibility of MKPs in a quantum spin Hall–superconductor heterostructure and highlights the importance of detailed theoretical modeling to capture key observable features necessary for future studies of MKPs and TQC.

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PRX Quantum 7, 010313– Published 20 January, 2026
  • Received 24 March 2025
  • Revised 7 July 2025
  • Accepted 3 October 2025
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