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Breakdown of the Kerr-Cat Qubit Tunneling Time Induced by Auxiliary Circuit Modes

Othmane Benhayoune-Khadraoui1,*, Cristóbal Lledó1,2, and Alexandre Blais1,3

  • 1Institut Quantique and Département de Physique, Université de Sherbrooke, Sherbrooke J1K 2R1 Quebec, Canada
  • 2Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago 837.0415, Chile
  • 3CIFAR, Toronto, Ontario M5G 1M1, Canada

  • *Contact author: othmane.benhayoune.khadraoui@usherbrooke.ca
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Phys. Rev. X 16, 041012 – Published 9 October, 2026

DOI: https://doi.org/10.1103/z8jm-q289

Abstract

Kerr-cat qubits have been experimentally shown to exhibit a large noise bias, with one decay channel suppressed by several orders of magnitude. In superconducting implementations, increasing the microwave drive on the nonlinear oscillator that hosts the Kerr-cat qubit should, in principle, further enhance this bias. Instead, experiments reveal that above a critical drive amplitude the tunneling time, which is the less dominant decay channel, ceases to increase and even decreases. Here, we show that this breakdown arises from the multimode nature of the circuit implementation. Specifically, additional modes, including the buffer mode used to deliver the stabilizing drive and higher modes of the Josephson junction array, can induce multiphoton resonances that sharply degrade Kerr-cat coherence. We uncover this mechanism by retaining the full circuit nonlinearities and treating the strong drive exactly within a Floquet-Markov framework that accounts for quasidegeneracies of the Kerr-cat spectrum. Our results not only provide an explanation for the sudden reduction of the tunneling time, but also demonstrate that the Kerr-cat qubit can be very robust in the presence of a carefully engineered electromagnetic environment. Beyond the Kerr-cat qubit, the tools developed here open the door to study a broad range of drive-induced phenomena in dissipative superconducting circuits, in both resonant and off-resonant driving regimes. Examples include inelastic scattering in circuits with arbitrary anharmonicity and drive strength as well as subharmonically driven devices such as parametric amplifiers and couplers and other protected qubits whose coherence is governed by quasidegeneracies.

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Popular Summary

Cat qubits are promising because they can exhibit a strong noise bias, with phase-flip errors largely suppressed relative to bit-flip errors, potentially reducing hardware overhead for quantum error correction. In Kerr-cat qubits, this suppression arises from interference that inhibits tunneling through the effective barrier separating two wells. Here, we show that this protection can break down under strong driving, when additional modes enable multiphoton resonances that reduce the tunneling time. Using an open-system model that captures the interplay of strong driving, nonlinearity, and dissipation in a multimode setting, we identify when these resonances occur and how they affect the tunneling time. We also suggest practical design strategies, including careful placement of auxiliary-mode frequencies and filtering, to suppress these processes. More broadly, our work shows that auxiliary modes can impose important limits on strongly driven quantum circuits and should be considered an integral part of circuit design.

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Phys. Rev. X 16, 041012– Published 9 October, 2026
  • Received 31 October 2025
  • Revised 30 July 2026
  • Accepted 9 September 2026
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