Nicola Casali

Evaluating radiation impact on transmon qubits in above and underground facilities

  1. Francesco De Dominicis,
  2. Tanay Roy,
  3. Ambra Mariani,
  4. Mustafa Bal,
  5. Nicola Casali,
  6. Ivan Colantoni,
  7. Francesco Crisa,
  8. Angelo Cruciani,
  9. Fernando Ferroni,
  10. Dounia L Helis,
  11. Lorenzo Pagnanini,
  12. Valerio Pettinacci,
  13. Roman M Pilipenko,
  14. Stefano Pirro,
  15. Andrei Puiu,
  16. Alexander Romanenko,
  17. David v Zanten,
  18. Shaojiang Zhu,
  19. Anna Grassellino,
  20. and Laura Cardani
Superconducting qubits can be sensitive to abrupt energy deposits caused by cosmic rays and ambient radioactivity. Previous studies have focused on understanding possible correlated
effects over time and distance due to cosmic rays. In this study, for the first time, we directly compare the response of a transmon qubit measured initially at the Fermilab SQMS above-ground facilities and then at the deep underground Gran Sasso Laboratory (INFN-LNGS, Italy). We observe same average qubit lifetime T1 of roughly 80 microseconds at above and underground facilities. We then apply a fast decay detection protocol and investigate the time structure, sensitivity and relative rates of triggered events due to radiation versus intrinsic noise, comparing above and underground performance of several high-coherence qubits. Using gamma sources of variable activity we calibrate the response of the qubit to different levels of radiation in an environment with minimal background radiation. Results indicate that qubits respond to a strong gamma source and it is possible to detect particle impacts. However, when comparing above and underground results, we do not observe a difference in radiation induced-like events for these sapphire and niobium-based transmon qubits. We conclude that the majority of these events are not radiation related and to be attributed to other noise sources which by far dominate single qubit errors in modern transmon qubits.
arxiv pdf

Operating in a deep underground facility improves the locking of gradiometric fluxonium qubits at the sweet spots

  1. Daria Gusenkova,
  2. Francesco Valenti,
  3. Martin Spiecker,
  4. Simon Günzler,
  5. Patrick Paluch,
  6. Dennis Rieger,
  7. Larisa-Milena Pioraş-Ţimbolmaş,
  8. Liviu P. Zârbo,
  9. Nicola Casali,
  10. Ivan Colantoni,
  11. Angelo Cruciani,
  12. Stefano Pirro,
  13. Laura Cardani,
  14. Alexandru Petrescu,
  15. Wolfgang Wernsdorfer,
  16. Patrick Winkel,
  17. and Ioan M. Pop
We demonstrate flux-bias locking and operation of a gradiometric fluxonium artificial atom using two symmetric granular aluminum (grAl) loops to implement the superinductor. The gradiometric
fluxonium shows two orders of magnitude suppression of sensitivity to homogeneous magnetic fields, which can be an asset for hybrid quantum systems requiring strong magnetic field biasing. By cooling down the device in an external magnetic field while crossing the metal-to-superconductor transition, the gradiometric fluxonium can be locked either at 0 or Φ0/2 effective flux bias, corresponding to an even or odd number of trapped fluxons, respectively. At mK temperatures, the fluxon parity prepared during initialization survives to magnetic field bias exceeding 100Φ0. However, even for states biased in the vicinity of 1Φ0, we observe unexpectedly short fluxon lifetimes of a few hours, which cannot be explained by thermal or quantum phase slips. When operating in a deep-underground cryostat of the Gran Sasso laboratory, the fluxon lifetimes increase to days, indicating that ionizing events activate phase slips in the grAl superinductor.
arxiv pdf

Reducing the impact of radioactivity on quantum circuits in a deep-underground facility

  1. Laura Cardani,
  2. Francesco Valenti,
  3. Nicola Casali,
  4. Gianluigi Catelani,
  5. Thibault Charpentier,
  6. Massimiliano Clemenza,
  7. Ivan Colantoni,
  8. Angelo Cruciani,
  9. Luca Gironi,
  10. Lukas Grünhaupt,
  11. Daria Gusenkova,
  12. Fabio Henriques,
  13. Marc Lagoin,
  14. Maria Martinez,
  15. Giorgio Pettinari,
  16. Claudia Rusconi,
  17. Oliver Sander,
  18. Alexey V. Ustinov,
  19. Marc Weber,
  20. Wolfgang Wernsdorfer,
  21. Marco Vignati,
  22. Stefano Pirro,
  23. and Ioan M. Pop
As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information
processing. However, coherence needs to further improve by orders of magnitude to reduce the prohibitive hardware overhead of current error correction schemes. Reaching this goal hinges on reducing the density of broken Cooper pairs, so-called quasiparticles. Here, we show that environmental radioactivity is a significant source of nonequilibrium quasiparticles. Moreover, ionizing radiation introduces time-correlated quasiparticle bursts in resonators on the same chip, further complicating quantum error correction. Operating in a deep-underground lead-shielded cryostat decreases the quasiparticle burst rate by a factor fifty and reduces dissipation up to a factor four, showcasing the importance of radiation abatement in future solid-state quantum hardware.
arxiv pdf