Characterization of Radiation-Induced Errors in Superconducting Qubits Protected with Various Gap-Engineering Strategies

  1. H. Douglas Pinckney,
  2. Thomas McJunkin,
  3. Alan W. Hunt,
  4. Patrick M. Harrington,
  5. Hannah P. Binney,
  6. Max Hays,
  7. Yenuel Jones-Alberty,
  8. Kate Azar,
  9. Felipe Contipelli,
  10. Renée DePencier Piñero,
  11. Jeffrey M. Gertler,
  12. Michael Gingras,
  13. Aranya Goswami,
  14. Cyrus F. Hirjibehedin,
  15. Mingyu Li,
  16. Mathis Moes,
  17. Bethany M. Niedzielski,
  18. Mallika T. Randeria,
  19. Ryan Sitler,
  20. Matthew K. Spear,
  21. Hannah Stickler,
  22. Jiatong Yang,
  23. Wouter Van De Pontseele,
  24. Mollie E. Schwartz,
  25. Jeffrey A. Grover,
  26. Kevin Schultz,
  27. Kyle Serniak,
  28. Joseph A. Formaggio,
  29. and William D. Oliver
Impacts from high-energy particles cause correlated errors in superconducting qubits by increasing the quasiparticle density in the vicinity of the Josephson junctions (JJs). Such errors
are particularly harmful as they cannot be easily remedied via conventional error correcting codes. Recent experiments reduced correlated errors by making the difference in superconducting gap energy across the JJ larger than the qubit energy. In this work, we assess gap engineering near the JJ (δΔJJ) and the capacitor/ground-plane (δΔM1) by exposing arrays of transmon qubits to two sources of radiation. For α-particles from an 241Am source, we observe T1 errors correlated in space and time, supporting a hypothesis that hadronic cosmic rays are a major contributor to the 10−10 error floor observed in Ref. 1. For electrons from a pulsed linear accelerator, we observe temporally correlated T1 and T2 errors, this measurement is insensitive to spatial correlations. We observe that the severity of correlated T1 errors is reduced for qubit arrays with a greater degree of gap engineering at the JJ. For both T1 and T2 errors, the recovery time is hastened by an increased δΔM1, which we attribute to the trapping of quasiparticles into the capacitor/ground-plane. We construct a model of quasiparticle dynamics that qualitatively agrees with our observations. This work reinforces the multifaceted influence of radiation on superconducting qubits and provides strategies for improving radiation resilience.

Dephasing in Fluxonium Qubits from Coherent Quantum Phase Slips

  1. Mallika T. Randeria,
  2. Thomas M. Hazard,
  3. Agustin Di Paolo,
  4. Kate Azar,
  5. Max Hays,
  6. Leon Ding,
  7. Junyoung An,
  8. Michael Gingras,
  9. Bethany M. Niedzielski,
  10. Hannah Stickler,
  11. Jeffrey A. Grover,
  12. Jonilyn L. Yoder,
  13. Mollie E. Schwartz,
  14. William D. Oliver,
  15. and Kyle Serniak
Phase slips occur across all Josephson junctions (JJs) at a rate that increases with the impedance of the junction. In superconducting qubits composed of JJ-array superinductors —
such as fluxonium — phase slips in the array can lead to decoherence. In particular, phase-slip processes at the individual array junctions can coherently interfere, each with an Aharonov–Casher phase that depends on the offset charges of the array islands. These coherent quantum phase slips (CQPS) perturbatively modify the qubit frequency, and therefore charge noise on the array islands will lead to dephasing. By varying the impedance of the array junctions, we design a set of fluxonium qubits in which the expected phase-slip rate within the JJ-array changes by several orders of magnitude. We characterize the coherence times of these qubits and demonstrate that the scaling of CQPS-induced dephasing rates agrees with our theoretical model. Furthermore, we perform noise spectroscopy of two qubits in regimes dominated by either CQPS or flux noise. We find the noise power spectrum associated with CQPS dephasing appears to be featureless at low frequencies and not 1/f. Numerical simulations indicate this behavior is consistent with charge noise generated by charge-parity fluctuations within the array. Our findings broadly inform JJ-array-design tradeoffs, relevant for the numerous superconducting qubit designs employing JJ-array superinductors.