Resolving catastrophic error bursts from cosmic rays in large arrays of superconducting qubits

  1. Matt McEwen,
  2. Lara Faoro,
  3. Kunal Arya,
  4. Andrew Dunsworth,
  5. Trent Huang,
  6. Seon Kim,
  7. Brian Burkett,
  8. Austin Fowler,
  9. Frank Arute,
  10. Joseph C Bardin,
  11. Andreas Bengtsson,
  12. Alexander Bilmes,
  13. Bob B. Buckley,
  14. Nicholas Bushnell,
  15. Zijun Chen,
  16. Roberto Collins,
  17. Sean Demura,
  18. Alan R. Derk,
  19. Catherine Erickson,
  20. Marissa Giustina,
  21. Sean D. Harrington,
  22. Sabrina Hong,
  23. Evan Jeffrey,
  24. Julian Kelly,
  25. Paul V. Klimov,
  26. Fedor Kostritsa,
  27. Pavel Laptev,
  28. Aditya Locharla,
  29. Xiao Mi,
  30. Kevin C. Miao,
  31. Shirin Montazeri,
  32. Josh Mutus,
  33. Ofer Naaman,
  34. Matthew Neeley,
  35. Charles Neill,
  36. Alex Opremcak,
  37. Chris Quintana,
  38. Nicholas Redd,
  39. Pedram Roushan,
  40. Daniel Sank,
  41. Kevin J. Satzinger,
  42. Vladimir Shvarts,
  43. Theodore White,
  44. Z. Jamie Yao,
  45. Ping Yeh,
  46. Juhwan Yoo,
  47. Yu Chen,
  48. Vadim Smelyanskiy,
  49. John M. Martinis,
  50. Hartmut Neven,
  51. Anthony Megrant,
  52. Lev Ioffe,
  53. and Rami Barends
Scalable quantum computing can become a reality with error correction, provided coherent qubits can be constructed in large arrays. The key premise is that physical errors can remain
both small and sufficiently uncorrelated as devices scale, so that logical error rates can be exponentially suppressed. However, energetic impacts from cosmic rays and latent radioactivity violate both of these assumptions. An impinging particle ionizes the substrate, radiating high energy phonons that induce a burst of quasiparticles, destroying qubit coherence throughout the device. High-energy radiation has been identified as a source of error in pilot superconducting quantum devices, but lacking a measurement technique able to resolve a single event in detail, the effect on large scale algorithms and error correction in particular remains an open question. Elucidating the physics involved requires operating large numbers of qubits at the same rapid timescales as in error correction, exposing the event’s evolution in time and spread in space. Here, we directly observe high-energy rays impacting a large-scale quantum processor. We introduce a rapid space and time-multiplexed measurement method and identify large bursts of quasiparticles that simultaneously and severely limit the energy coherence of all qubits, causing chip-wide failure. We track the events from their initial localised impact to high error rates across the chip. Our results provide direct insights into the scale and dynamics of these damaging error bursts in large-scale devices, and highlight the necessity of mitigation to enable quantum computing to scale.

Optimal design of a superconducting transmon qubit with tapered wiring

  1. John M. Martinis
Analytical formulas are presented for simplified but useful qubit geometries that predict surface dielectric loss when its thickness is much less than the metal thickness, the limiting
case needed for real devices. These formulas can thus be used to precisely predict loss and optimize the qubit layout. Surprisingly, a significant fraction of surface loss comes from the small wire that connects the Josephson junction to the qubit capacitor. Tapering this wire is shown to significantly lower its loss. Also predicted are the size and density of the two-level state (TLS) spectrum from individual surface dissipation sites.

Removing leakage-induced correlated errors in superconducting quantum error correction

  1. M. McEwen,
  2. D. Kafri,
  3. Z. Chen,
  4. J. Atalaya,
  5. K. J. Satzinger,
  6. C. Quintana,
  7. P. V. Klimov,
  8. D. Sank,
  9. C. Gidney,
  10. A. G. Fowler,
  11. F. Arute,
  12. K. Arya,
  13. B. Buckley,
  14. B. Burkett,
  15. N. Bushnell,
  16. B. Chiaro,
  17. R. Collins,
  18. S.Demura,
  19. A. Dunsworth,
  20. C. Erickson,
  21. B. Foxen,
  22. M. Giustina,
  23. T. Huang,
  24. S. Hong,
  25. E. Jeffrey,
  26. S. Kim,
  27. K. Kechedzhi,
  28. F. Kostritsa,
  29. P. Laptev,
  30. A. Megrant,
  31. X. Mi,
  32. J. Mutus,
  33. O. Naaman,
  34. M. Neeley,
  35. C. Neill,
  36. M.Niu,
  37. A. Paler,
  38. N. Redd,
  39. P. Roushan,
  40. T. C. White,
  41. J. Yao,
  42. P. Yeh,
  43. A. Zalcman,
  44. Yu Chen,
  45. V. N. Smelyanskiy,
  46. John M. Martinis,
  47. H. Neven,
  48. J. Kelly,
  49. A. N. Korotkov,
  50. A. G. Petukhov,
  51. and R. Barends
Quantum computing can become scalable through error correction, but logical error rates only decrease with system size when physical errors are sufficiently uncorrelated. During computation,
unused high energy levels of the qubits can become excited, creating leakage states that are long-lived and mobile. Particularly for superconducting transmon qubits, this leakage opens a path to errors that are correlated in space and time. Here, we report a reset protocol that returns a qubit to the ground state from all relevant higher level states. We test its performance with the bit-flip stabilizer code, a simplified version of the surface code for quantum error correction. We investigate the accumulation and dynamics of leakage during error correction. Using this protocol, we find lower rates of logical errors and an improved scaling and stability of error suppression with increasing qubit number. This demonstration provides a key step on the path towards scalable quantum computing.

Information Constraints for Scalable Control in a Quantum Computer

  1. John M. Martinis
When working to understand quantum systems engineering, there are many constraints to building a scalable quantum computer. Here I discuss a constraint on the qubit control system from
an information point of view, showing that the large amount of information needed for the control system will put significant constraints on the control system. The size the qubits is conjectured to be an important systems parameter.

Saving superconducting quantum processors from qubit decay and correlated errors generated by gamma and cosmic rays

  1. John M. Martinis
Error-corrected quantum computers can only work if errors are small and uncorrelated. Here I show how cosmic rays or stray background radiation affects superconducting qubits by modeling
the phonon to electron/quasiparticle down-conversion physics. For present designs, the model predicts about 57\% of the radiation energy breaks Cooper pairs into quasiparticles, which then vigorously suppress the qubit energy relaxation time (T1∼ 160 ns) over a large area (cm) and for a long time (ms). Such large and correlated decay kills error correction. Using this quantitative model, I show how this energy can be channeled away from the qubit so that this error mechanism can be reduced by many orders of magnitude. I also comment on how this affects other solid-state qubits.

Learning Non-Markovian Quantum Noise from Moiré-Enhanced Swap Spectroscopy with Deep Evolutionary Algorithm

  1. Murphy Yuezhen Niu,
  2. Vadim Smelyanskyi,
  3. Paul Klimov,
  4. Sergio Boixo,
  5. Rami Barends,
  6. Julian Kelly,
  7. Yu Chen,
  8. Kunal Arya,
  9. Brian Burkett,
  10. Dave Bacon,
  11. Zijun Chen,
  12. Ben Chiaro,
  13. Roberto Collins,
  14. Andrew Dunsworth,
  15. Brooks Foxen,
  16. Austin Fowler,
  17. Craig Gidney,
  18. Marissa Giustina,
  19. Rob Graff,
  20. Trent Huang,
  21. Evan Jeffrey,
  22. David Landhuis,
  23. Erik Lucero,
  24. Anthony Megrant,
  25. Josh Mutus,
  26. Xiao Mi,
  27. Ofer Naaman,
  28. Matthew Neeley,
  29. Charles Neill,
  30. Chris Quintana,
  31. Pedram Roushan,
  32. John M. Martinis,
  33. and Hartmut Neven
Two-level-system (TLS) defects in amorphous dielectrics are a major source of noise and decoherence in solid-state qubits. Gate-dependent non-Markovian errors caused by TLS-qubit coupling
are detrimental to fault-tolerant quantum computation and have not been rigorously treated in the existing literature. In this work, we derive the non-Markovian dynamics between TLS and qubits during a SWAP-like two-qubit gate and the associated average gate fidelity for frequency-tunable Transmon qubits. This gate dependent error model facilitates using qubits as sensors to simultaneously learn practical imperfections in both the qubit’s environment and control waveforms. We combine the-state-of-art machine learning algorithm with Moiré-enhanced swap spectroscopy to achieve robust learning using noisy experimental data. Deep neural networks are used to represent the functional map from experimental data to TLS parameters and are trained through an evolutionary algorithm. Our method achieves the highest learning efficiency and robustness against experimental imperfections to-date, representing an important step towards in-situ quantum control optimization over environmental and control defects.

Resolving the positions of defects in superconducting quantum bits

  1. Alexander Bilmes,
  2. Anthony Megrant,
  3. Paul Klimov,
  4. Georg Weiss,
  5. John M. Martinis,
  6. Alexey V. Ustinov,
  7. and Jürgen Lisenfeld
Solid-state quantum coherent devices are quickly progressing. Superconducting circuits, for instance, have already been used to demonstrate prototype quantum processors comprising a
few tens of quantum bits. This development also revealed that a major part of decoherence and energy loss in such devices originates from a bath of parasitic material defects. However, neither the microscopic structure of defects nor the mechanisms by which they emerge during sample fabrication are understood. Here, we present a technique to obtain information on locations of defects relative to the thin film edge of the qubit circuit. Resonance frequencies of defects are tuned by exposing the qubit sample to electric fields generated by electrodes surrounding the chip. By determining the defect’s coupling strength to each electrode and comparing it to a simulation of the field distribution, we obtain the probability at which location and at which interface the defect resides. This method is applicable to already existing samples of various qubit types, without further on-chip design changes. It provides a valuable tool for improving the material quality and nano-fabrication procedures towards more coherent quantum circuits.

Electric field spectroscopy of material defects in transmon qubits

  1. Jürgen Lisenfeld,
  2. Alexander Bilmes,
  3. Anthony Megrant,
  4. Rami Barends,
  5. Julian Kelly,
  6. Paul Klimov,
  7. Georg Weiss,
  8. John M. Martinis,
  9. and Alexey V. Ustinov
Superconducting integrated circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the downside of the solid-state approach is
that superconducting qubits suffer strongly from energy dissipation and environmental fluctuations caused by atomic-scale defects in device materials. Further progress towards upscaled quantum processors will require improvements in device fabrication techniques which need to be guided by novel analysis methods to understand and prevent mechanisms of defect formation. Here, we present a new technique to analyse individual defects in superconducting qubits by tuning them with applied electric fields. This provides a new spectroscopy method to extract the defects‘ energy distribution, electric dipole moments, and coherence times. Moreover, it enables one to distinguish defects residing in Josephson junction tunnel barriers from those at circuit interfaces. We find that defects at circuit interfaces are responsible for about 60% of the dielectric loss in the investigated transmon qubit sample. About 40% of all detected defects are contained in the tunnel barriers of the large-area parasitic Josephson junctions that occur collaterally in shadow evaporation, and only about 3% are identified as strongly coupled defects which presumably reside in the small-area qubit tunnel junctions. The demonstrated technique provides a valuable tool to assess the decoherence sources related to circuit interfaces and to tunnel junctions that is readily applicable to standard qubit samples.

Diabatic gates for frequency-tunable superconducting qubits

  1. R. Barends,
  2. C. M. Quintana,
  3. A. G. Petukhov,
  4. Yu Chen,
  5. D. Kafri,
  6. K. Kechedzhi,
  7. R. Collins,
  8. O. Naaman,
  9. S. Boixo,
  10. F. Arute,
  11. K. Arya,
  12. D. Buell,
  13. B. Burkett,
  14. Z. Chen,
  15. B. Chiaro,
  16. A. Dunsworth,
  17. B. Foxen,
  18. A. Fowler,
  19. C. Gidney,
  20. M. Giustina,
  21. R. Graff,
  22. T. Huang,
  23. E. Jeffrey,
  24. J. Kelly,
  25. P. V. Klimov,
  26. F. Kostritsa,
  27. D. Landhuis,
  28. E. Lucero,
  29. M. McEwen,
  30. A. Megrant,
  31. X. Mi,
  32. J. Mutus,
  33. M. Neeley,
  34. C. Neill,
  35. E. Ostby,
  36. P. Roushan,
  37. D. Sank,
  38. K. J. Satzinger,
  39. A. Vainsencher,
  40. T. White,
  41. J. Yao,
  42. P. Yeh,
  43. A. Zalcman,
  44. H. Neven,
  45. V. N. Smelyanskiy,
  46. and John M. Martinis
We demonstrate diabatic two-qubit gates with Pauli error rates down to 4.3(2)⋅10−3 in as fast as 18 ns using frequency-tunable superconducting qubits. This is achieved by synchronizing
the entangling parameters with minima in the leakage channel. The synchronization shows a landscape in gate parameter space that agrees with model predictions and facilitates robust tune-up. We test both iSWAP-like and CPHASE gates with cross-entropy benchmarking. The presented approach can be extended to multibody operations as well.

Fluctuations of Energy-Relaxation Times in Superconducting Qubits

  1. P. V. Klimov,
  2. J. Kelly,
  3. Z. Chen,
  4. M. Neeley,
  5. A. Megrant,
  6. B. Burkett,
  7. R. Barends,
  8. K. Arya,
  9. B. Chiaro,
  10. Yu Chen,
  11. A. Dunsworth,
  12. A. Fowler,
  13. B. Foxen,
  14. C. Gidney,
  15. M. Giustina,
  16. R. Graff,
  17. T. Huang,
  18. E. Jeffrey,
  19. Erik Lucero,
  20. J. Y. Mutus,
  21. O. Naaman,
  22. C. Neill,
  23. C. Quintana,
  24. P. Roushan,
  25. Daniel Sank,
  26. A. Vainsencher,
  27. J. Wenner,
  28. T. C. White,
  29. S. Boixo,
  30. R. Babbush,
  31. V. N. Smelyanskiy,
  32. H. Neven,
  33. and John M. Martinis
Superconducting qubits are an attractive platform for quantum computing since they have demonstrated high-fidelity quantum gates and extensibility to modest system sizes. Nonetheless,
an outstanding challenge is stabilizing their energy-relaxation times, which can fluctuate unpredictably in frequency and time. Here, we use qubits as spectral and temporal probes of individual two-level-system defects to provide direct evidence that they are responsible for the largest fluctuations. This research lays the foundation for stabilizing qubit performance through calibration, design, and fabrication.