Colm A. Ryan

Self-Consistent Quantum Process Tomography

  1. Seth T. Merkel,
  2. Jay M. Gambetta,
  3. John A. Smolin,
  4. S. Poletto,
  5. A. D. Córcoles,
  6. B. R. Johnson,
  7. Colm A. Ryan,
  8. and M. Steffen
Quantum process tomography is a necessary tool for verifying quantum gates and diagnosing faults in architectures and gate design. We show that the standard approach of process tomography
is grossly inaccurate in the case where the states and measurement operators used to interrogate the system are generated by gates that have some systematic error, a situation all but unavoidable in any practical setting. These errors in tomography can not be fully corrected through oversampling or by performing a larger set of experiments. We present an alternative method for tomography to reconstruct an entire library of gates in a self-consistent manner. The essential ingredient is to define a likelihood function that assumes nothing about the gates used for preparation and measurement. In order to make the resulting optimization tractable we linearize about the target, a reasonable approximation when benchmarking a quantum computer as opposed to probing a black-box function.
arxiv pdf

Characterization of addressability by simultaneous randomized benchmarking

  1. Jay M. Gambetta,
  2. A. D. Corcoles,
  3. S. T. Merkel,
  4. B. R. Johnson,
  5. John A. Smolin,
  6. Jerry M. Chow,
  7. Colm A. Ryan,
  8. Chad Rigetti,
  9. S. Poletto,
  10. Thomas A. Ohki,
  11. Mark B. Ketchen,
  12. and M. Steffen
The control and handling of errors arising from cross-talk and unwanted interactions in multi-qubit systems is an important issue in quantum information processing architectures. We
introduce a benchmarking protocol that provides information about the amount of addressability present in the system and implement it on coupled superconducting qubits. The protocol consists of randomized benchmarking each qubit individually and then simultaneously, and the amount of addressability is related to the difference of the average gate fidelities of those experiments. We present the results on two similar samples with different amounts of cross-talk and unwanted interactions, which agree with predictions based on simple models for the amount of residual coupling.
arxiv pdf