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Readout failures in superconducting qubits due to two-level system defects in tunnel junctions

Jürgen Lisenfeld*, Alexander K. Händel, Alexander Bilmes, and Alexey V. Ustinov

  • *Contact author: juergen.lisenfeld@kit.edu
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Phys. Rev. Research 8, 033382 – Published 30 September, 2026

DOI: https://doi.org/10.1103/ypm5-lh8d

Abstract

Material defects give rise to parasitic two-level systems (TLS) that present a major source of decoherence in superconducting qubits. Here, we study a strongly coupled TLS that resides in the tunnel barrier of transmon qubit. We use multiphoton spectroscopy and TLS strain tuning to explore the rich spectrum of the interacting three-partite system consisting of TLS, qubit, and its readout resonator. This reveals a strong effective resonant coupling between the TLS and the qubit's readout resonator, which dresses the resonator states and results in a resonance frequency shift that spoils the readout signal. Our finding presents yet another way how material defects can interfere with qubit operation and hinder the realization of solid-state quantum processors.

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Phys. Rev. Research 8, 033382– Published 30 September, 2026
  • Received 5 May 2026
  • Accepted 2 September 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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