N. A. Kurinsky

Quasiparticle poisoning of superconducting qubits with active gamma irradiation

  1. C. P. Larson,
  2. E. Yelton,
  3. K. Dodge,
  4. K. Okubo,
  5. J. Batarekh,
  6. V. Iaia,
  7. N. A. Kurinsky,
  8. and B. L. T. Plourde
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 this 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.
arxiv pdf

Correlated Charge Noise and Relaxation Errors in Superconducting Qubits

  1. C. D. Wilen,
  2. S. Abdullah,
  3. N. A. Kurinsky,
  4. C. Stanford,
  5. L. Cardani,
  6. G. D'Imperio,
  7. C. Tomei,
  8. L. Faoro,
  9. L.B. Ioffe,
  10. C. H. Liu,
  11. A. Opremcak,
  12. B. G. Christensen,
  13. J. L. DuBois,
  14. and R. McDermott
The central challenge in building a quantum computer is error correction. Unlike classical bits, which are susceptible to only one type of error, quantum bits („qubits“)
are susceptible to two types of error, corresponding to flips of the qubit state about the X- and Z-directions. While the Heisenberg Uncertainty Principle precludes simultaneous monitoring of X- and Z-flips on a single qubit, it is possible to encode quantum information in large arrays of entangled qubits that enable accurate monitoring of all errors in the system, provided the error rate is low. Another crucial requirement is that errors cannot be correlated. Here, we characterize a superconducting multiqubit circuit and find that charge fluctuations are highly correlated on a length scale over 600~μm; moreover, discrete charge jumps are accompanied by a strong transient suppression of qubit energy relaxation time across the millimeter-scale chip. The resulting correlated errors are explained in terms of the charging event and phonon-mediated quasiparticle poisoning associated with absorption of gamma rays and cosmic-ray muons in the qubit substrate. Robust quantum error correction will require the development of mitigation strategies to protect multiqubit arrays from correlated errors due to particle impacts.
arxiv pdf