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Andreev spin qubit protected by Franck-Condon blockade

P. D. Kurilovich1, T. Vakhtel2, T. Connolly1, C. G. L. Bøttcher1, and B. van Heck3

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Phys. Rev. B 113, 115429 – Published 27 March, 2026

DOI: https://doi.org/10.1103/l478-yp15

Abstract

Andreev levels localized in a weak link between two superconductors can trap a superconducting quasiparticle. If there is a spin-orbit coupling in the link, the spin of the quasiparticle couples to the Josephson current. This effect can be leveraged to control and read out the spin of the quasiparticle, thus using it as a qubit. One of the factors limiting the performance of such an Andreev spin qubit is spin relaxation. Here, we theoretically demonstrate that the relaxation lifetime can be enhanced by utilizing the coupling between the Andreev spin and the supercurrent in a transmon circuit. The coupling ensures that the flip of the quasiparticle spin can only happen if it is accompanied by the excitation of multiple plasmons, as dictated by the Franck-Condon principle. This blocks spin relaxation at temperatures small compared to plasmon energy.

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Outline

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Phys. Rev. B 113, 115429– Published 27 March, 2026
  • Received 13 June 2025
  • Revised 16 January 2026
  • Accepted 12 March 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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