Error mitigation via stabilizer measurement emulation

  1. A. Greene,
  2. M. Kjaergaard,
  3. M. E. Schwartz,
  4. G. O. Samach,
  5. A. Bengtsson,
  6. M. O'Keeffe,
  7. D. K. Kim,
  8. M. Marvian,
  9. A. Melville,
  10. B. M. Niedzielski,
  11. A. Vepsalainen,
  12. R. Winik,
  13. J. Yoder,
  14. D. Rosenberg,
  15. S. Lloyd,
  16. T. P. Orlando,
  17. I. Marvian,
  18. S. Gustavsson,
  19. and W. D. Oliver
Dynamical decoupling (DD) is a widely-used quantum control technique that takes advantage of temporal symmetries in order to partially suppress quantum errors without the need resource-intensive
error detection and correction protocols. This and other open-loop error mitigation techniques are critical for quantum information processing in the era of Noisy Intermediate-Scale Quantum technology. However, despite its utility, dynamical decoupling does not address errors which occur at unstructured times during a circuit, including certain commonly-encountered noise mechanisms such as cross-talk and imperfectly calibrated control pulses. Here, we introduce and demonstrate an alternative technique – `quantum measurement emulation‘ (QME) – that effectively emulates the measurement of stabilizer operators via stochastic gate application, leading to a first-order insensitivity to coherent errors. The QME protocol enables error suppression based on the stabilizer code formalism without the need for costly measurements and feedback, and it is particularly well-suited to discrete coherent errors that are challenging for DD to address.

A Quantum Instruction Set Implemented on a Superconducting Quantum Processor

  1. M. Kjaergaard,
  2. M. E. Schwartz,
  3. A. Greene,
  4. G. O. Samach,
  5. A. Bengtsson,
  6. M. O'Keeffe,
  7. C. M. McNally,
  8. J. Braumüller,
  9. D. K. Kim,
  10. P. Krantz,
  11. M. Marvian,
  12. A. Melville,
  13. B. M. Niedzielski,
  14. Y. Sung,
  15. R. Winik,
  16. J. Yoder,
  17. D. Rosenberg,
  18. K. Obenland,
  19. S. Lloyd,
  20. T. P. Orlando,
  21. I. Marvian,
  22. S. Gustavsson,
  23. and W. D. Oliver
A quantum algorithm consists of a sequence of operations and measurements applied to a quantum processor. To date, the instruction set which defines this sequence has been provided
by a classical computer and passed via control hardware to the quantum processor. Here, we demonstrate the first experimental realization of a quantum instruction set, in which a fixed sequence of classically-defined gates perform an operation that is fully determined only by a quantum input to the fixed sequence. Specifically, we implement the density matrix exponentiation algorithm, which consumes N copies of the instruction state ρ to approximate the operation e−iρθ (θ an arbitrary angle). Our implementation relies on a 99.7\% fidelity controlled-phase gate between two superconducting transmon qubits. We achieve an average algorithmic fidelity ≈0.9, independent of the setting of ρ, to circuit depth nearly 90. This new paradigm for quantum instructions has applications to resource-efficient protocols for validating entanglement spectra, principal component analysis of large quantum states, and universal quantum emulation.