Joseph A. Formaggio

Synchronous Detection of Cosmic Rays and Correlated Errors in Superconducting Qubit Arrays

  1. Patrick M. Harrington,
  2. Mingyu Li,
  3. Max Hays,
  4. Wouter Van De Pontseele,
  5. Daniel Mayer,
  6. H. Douglas Pinckney,
  7. Felipe Contipelli,
  8. Michael Gingras,
  9. Bethany M. Niedzielski,
  10. Hannah Stickler,
  11. Jonilyn L. Yoder,
  12. Mollie E. Schwartz,
  13. Jeffrey A. Grover,
  14. Kyle Serniak,
  15. William D. Oliver,
  16. and Joseph A. Formaggio
Quantum information processing at scale will require sufficiently stable and long-lived qubits, likely enabled by error-correction codes. Several recent superconducting-qubit experiments,
however, reported observing intermittent spatiotemporally correlated errors that would be problematic for conventional codes, with ionizing radiation being a likely cause. Here, we directly measured the cosmic-ray contribution to spatiotemporally correlated qubit errors. We accomplished this by synchronously monitoring cosmic-ray detectors and qubit energy-relaxation dynamics of 10 transmon qubits distributed across a 5x5x0.35 mm3 silicon chip. Cosmic rays caused correlated errors at a rate of 1/(10 min), accounting for 17±1% of all such events. Our qubits responded to essentially all of the cosmic rays and their secondary particles incident on the chip, consistent with the independently measured arrival flux. Moreover, we observed that the landscape of the superconducting gap in proximity to the Josephson junctions dramatically impacts the qubit response to cosmic rays. Given the practical difficulties associated with shielding cosmic rays, our results indicate the importance of radiation hardening — for example, superconducting gap engineering — to the realization of robust quantum error correction.
arxiv pdf

Impact of ionizing radiation on superconducting qubit coherence

  1. Antti Vepsäläinen,
  2. Amir H. Karamlou,
  3. John L. Orrell,
  4. Akshunna S. Dogra,
  5. Ben Loer,
  6. Francisca Vasconcelos,
  7. David K. Kim,
  8. Alexander J. Melville,
  9. Bethany M. Niedzielski,
  10. Jonilyn L. Yoder,
  11. Simon Gustavsson,
  12. Joseph A. Formaggio,
  13. Brent A. VanDevender,
  14. and William D. Oliver
The practical viability of any qubit technology stands on long coherence times and high-fidelity operations, with the superconducting qubit modality being a leading example. However,
superconducting qubit coherence is impacted by broken Cooper pairs, referred to as quasiparticles, with a density that is empirically observed to be orders of magnitude greater than the value predicted for thermal equilibrium by the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. Previous work has shown that infrared photons significantly increase the quasiparticle density, yet even in the best isolated systems, it still remains higher than expected, suggesting that another generation mechanism exists. In this Letter, we provide evidence that ionizing radiation from environmental radioactive materials and cosmic rays contributes to this observed difference, leading to an elevated quasiparticle density that would ultimately limit superconducting qubits of the type measured here to coherence times in the millisecond regime. We further demonstrate that introducing radiation shielding reduces the flux of ionizing radiation and positively correlates with increased coherence time. Albeit a small effect for today’s qubits, reducing or otherwise mitigating the impact of ionizing radiation will be critical for realizing fault-tolerant superconducting quantum computers.
arxiv pdf