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Localized Josephson hot spots due to two-level systems

Joshuah T. Heath1,2,*, Alexander C. Tyner1,2, Thue Christian Thann3, Vincent P. Michal3, Peter Krogstrup3, Mark Kamper Svendsen3, and Alexander V. Balatsky1,2,†

  • 1Nordita, Stockholm University, and KTH Royal Institute of Technology, Hannes Alfvéns väg 12, SE-106 91 Stockholm, Sweden
  • 2Department of Physics, University of Connecticut, Storrs, Connecticut 06269, USA
  • 3NNF Quantum Computing Programme, Niels Bohr Institute, University of Copenhagen, Denmark

  • *Contact author: joshuah.t.heath@su.se
  • †Contact author: balatsky@kth.se
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Phys. Rev. Applied 25, 014022 – Published 9 January, 2026

DOI: https://doi.org/10.1103/ncn4-m48z

Abstract

Superconducting qubits are often adversely affected by two-level systems (TLSs) within the Josephson junction, which contribute to decoherence and subsequently limit the performance of the qubit. By treating the TLS as a soft (i.e., low-frequency) bosonic mode localized in real space, we find that a single TLS in either the amorphous-oxide surface or the superconducting bulk may result in a localized “hot spot” of amplified Josephson energy. Such amplification is shown to have a non-negligible effect on the T1 time of a simple phase qubit, regardless of whether or not the TLS is on resonance with the qubit frequency. With this study, we identify unique fingerprints of TLS defects in the Josephson current and qubit decoherence time.

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Phys. Rev. Applied 25, 014022– Published 9 January, 2026
  • Received 10 April 2025
  • Revised 24 October 2025
  • Accepted 20 November 2025
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