I.-C. Hoi

State preservation by repetitive error detection in a superconducting quantum circuit

  1. J. Kelly,
  2. R. Barends,
  3. A. G. Fowler,
  4. A. Megrant,
  5. E. Jeffrey,
  6. T. C. White,
  7. D. Sank,
  8. J. Y. Mutus,
  9. B. Campbell,
  10. Yu Chen,
  11. Z. Chen,
  12. B. Chiaro,
  13. A. Dunsworth,
  14. I.-C. Hoi,
  15. C. Neill,
  16. P. J. J. O'Malley,
  17. C. Quintana,
  18. P. Roushan,
  19. A. Vainsencher,
  20. J. Wenner,
  21. A. N. Cleland,
  22. and John M. Martinis
Quantum computing becomes viable when a quantum state can be preserved from environmentally-induced error. If quantum bits (qubits) are sufficiently reliable, errors are sparse and
quantum error correction (QEC) is capable of identifying and correcting them. Adding more qubits improves the preservation by guaranteeing increasingly larger clusters of errors will not cause logical failure – a key requirement for large-scale systems. Using QEC to extend the qubit lifetime remains one of the outstanding experimental challenges in quantum computing. Here, we report the protection of classical states from environmental bit-flip errors and demonstrate the suppression of these errors with increasing system size. We use a linear array of nine qubits, which is a natural precursor of the two-dimensional surface code QEC scheme, and track errors as they occur by repeatedly performing projective quantum non-demolition (QND) parity measurements. Relative to a single physical qubit, we reduce the failure rate in retrieving an input state by a factor of 2.7 for five qubits and a factor of 8.5 for nine qubits after eight cycles. Additionally, we tomographically verify preservation of the non-classical Greenberger-Horne-Zeilinger (GHZ) state. The successful suppression of environmentally-induced errors strongly motivates further research into the many exciting challenges associated with building a large-scale superconducting quantum computer.
arxiv pdf

Probing the quantum vacuum with an artificial atom in front of a mirror

  1. I.-C. Hoi,
  2. A. F. Kockum,
  3. L. Tornberg,
  4. A. Pourkabirian,
  5. G. Johansson,
  6. P. Delsing,
  7. and C. M. Wilson
Quantum fluctuations of the vacuum are both a surprising and fundamental phenomenon of nature. Understood as virtual photons flitting in and out of existence, they still have a very
real impact, \emph{e.g.}, in the Casimir effects and the lifetimes of atoms. Engineering vacuum fluctuations is therefore becoming increasingly important to emerging technologies. Here, we shape vacuum fluctuations using a „mirror“, creating regions in space where they are suppressed. As we then effectively move an artificial atom in and out of these regions, measuring the atomic lifetime tells us the strength of the fluctuations. The weakest fluctuation strength we observe is 0.02 quanta, a factor of 50 below what would be expected without the mirror, demonstrating that we can hide the atom from the vacuum.
arxiv pdf

Characterization and reduction of microfabrication-induced decoherence in superconducting quantum circuits

  1. C. M. Quintana,
  2. A. Megrant,
  3. Z. Chen,
  4. A. Dunsworth,
  5. B. Chiaro,
  6. R. Barends,
  7. B. Campbell,
  8. Yu Chen,
  9. I.-C. Hoi,
  10. E. Jeffrey,
  11. J. Kelly,
  12. J. Y. Mutus,
  13. P. J. J. O'Malley,
  14. C. Neill,
  15. P. Roushan,
  16. D. Sank,
  17. A. Vainsencher,
  18. J. Wenner,
  19. T. C. White,
  20. A. N. Cleland,
  21. and John M. Martinis
Many superconducting qubits are highly sensitive to dielectric loss, making the fabrication of coherent quantum circuits challenging. To elucidate this issue, we characterize the interfaces
and surfaces of superconducting coplanar waveguide resonators and study the associated microwave loss. We show that contamination induced by traditional qubit lift-off processing is particularly detrimental to quality factors without proper substrate cleaning, while roughness plays at most a small role. Aggressive surface treatment is shown to damage the crystalline substrate and degrade resonator quality. We also introduce methods to characterize and remove ultra-thin resist residue, providing a way to quantify and minimize remnant sources of loss on device surfaces.
arxiv pdf

Optimal quantum control using randomized benchmarking

  1. J. Kelly,
  2. R. Barends,
  3. B. Campbell,
  4. Y. Chen,
  5. Z. Chen,
  6. B. Chiaro,
  7. A. Dunsworth,
  8. A. G. Fowler,
  9. I.-C. Hoi,
  10. E. Jeffrey,
  11. A. Megrant,
  12. J. Mutus,
  13. C. Neill,
  14. P. J. J. O'Malley,
  15. C. Quintana,
  16. P. Roushan,
  17. D. Sank,
  18. A. Vainsencher,
  19. J. Wenner,
  20. T. C. White,
  21. A. N. Cleland,
  22. and John M. Martinis
We present a method for optimizing quantum control in experimental systems, using a subset of randomized benchmarking measurements to rapidly infer error. This is demonstrated to improve
single- and two-qubit gates, minimize gate bleedthrough, where a gate mechanism can cause errors on subsequent gates, and identify control crosstalk in superconducting qubits. This method is able to correct parameters to where control errors no longer dominate, and is suitable for automated and closed-loop optimization of experimental systems
arxiv pdf

Scattering of coherent states on a single artificial atom

  1. B. Peropadre,
  2. J. Lindkvist,
  3. I.-C. Hoi,
  4. C.M. Wilson,
  5. J.J. Garcia-Ripoll,
  6. P. Delsing,
  7. and G. Johansson
In this work we theoretically analyze a circuit QED design where propagating quantum microwaves interact with a single artificial atom, a single Cooper pair box. In particular, we derive
a master equation in the so-called transmon regime, including coherent drives. Inspired by recent experiments, we then apply the master equation to describe the dynamics in both a two-level and a three-level approximation of the atom. In the two-level case, we also discuss how to measure photon antibunching in the reflected field and how it is affected by finite temperature and finite detection bandwidth.
arxiv pdf