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Quasiparticle Poisoning of Superconducting Qubits with Active Gamma Irradiation

C.P. Larson1, E. Yelton1, K. Dodge2, K. Okubo1, J. Batarekh1,3, V. Iaia1, N.A. Kurinsky4, and B.L.T. Plourde1,3,*

  • *Contact author: bplourde@wisc.edu
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PRX Quantum 6, 030339 – Published 2 September, 2025

DOI: https://doi.org/10.1103/2lyd-8swv

Abstract

When a high-energy particle, such as a γ ray or muon, impacts the substrate of a superconducting qubit chip, large numbers of electron-hole pairs and phonons are created. The ensuing dynamics of the electrons and holes changes the local offset-charge environment for qubits near the impact site. The phonons that are produced have energy above the superconducting gap in the films that compose the qubits, leading to quasiparticle excitations above the superconducting ground state when the phonons impinge on the qubit electrodes. An elevated density of quasiparticles degrades qubit coherence, leading to errors in qubit arrays. Because these pair-breaking phonons spread throughout much of the chip, the errors can be correlated across a large portion of the array, posing a significant challenge for quantum error correction. In order to study the dynamics of γ-ray impacts on superconducting qubit arrays, we use a γ-ray source outside the dilution refrigerator to controllably irradiate our devices. By using charge-sensitive transmon qubits, we can measure both the offset-charge shifts and quasiparticle poisoning due to the γ irradiation at different doses. We study correlations between offset-charge shifts and quasiparticle poisoning for different qubits in the array and compare these with numerical modeling of charge and phonon dynamics following a γ-ray impact. We thus characterize the poisoning footprint of these impacts and quantify the performance of structures for mitigating phonon-mediated quasiparticle poisoning.

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Popular Summary

Superconducting qubits are a leading platform for implementing fault-tolerant quantum computers. However, impacts of high-energy particles, such as γ rays from background radioactivity, are detrimental to superconducting devices because they lead to enhanced qubit errors that are correlated throughout the array. Current quantum error-correction schemes cannot recover from such correlated errors. We perform experiments using a calibrated γ ray source to irradiate superconducting qubit arrays. We characterize the spatial footprint of the poisoning process following a γ ray impact that leads to correlated errors, and use this to assess the performance of different hardware strategies for mitigating the poisoning.

We use some of the qubits in our arrays to sense the charge environment, which is modified following an impact of an ionizing γ ray that generates many electron-hole pairs. The impact event also produces numerous energetic phonons that can spread throughout the entire chip, leading to temporary poisoning of the qubits, and hence enhanced errors, when the phonons impinge on the qubit device layer. We use this charge-sensing mode, combined with detailed numerical modeling of the charge dynamics, as a trigger for an impact, then study correlations in the phonon-mediated poisoning levels on all qubits in the array.

Quantum error-correction schemes that can identify and manage impacts during the run-time of a quantum processor have been developed recently, and knowledge of the poisoning footprint is vital. Further studies using active radiation sources to characterize strategies for mitigating correlated errors will aid the development of radiation-resilient quantum processors.

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PRX Quantum 6, 030339– Published 2 September, 2025
  • Received 13 March 2025
  • Accepted 9 July 2025
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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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