J. Dressel

Monitoring fast superconducting qubit dynamics using a neural network

  1. G. Koolstra,
  2. N. Stevenson,
  3. S. Barzili,
  4. L. Burns,
  5. K. Siva,
  6. S. Greenfield,
  7. W. Livingston,
  8. A. Hashim,
  9. R. K. Naik,
  10. J.M. Kreikebaum,
  11. K. P. O'Brien,
  12. D. I. Santiago,
  13. J. Dressel,
  14. and I. Siddiqi
Weak measurements of a superconducting qubit produce noisy voltage signals that are weakly correlated with the qubit state. To recover individual quantum trajectories from these noisy
signals, traditional methods require slow qubit dynamics and substantial prior information in the form of calibration experiments. Monitoring rapid qubit dynamics, e.g. during quantum gates, requires more complicated methods with increased demand for prior information. Here, we experimentally demonstrate an alternative method for accurately tracking rapidly driven superconducting qubit trajectories that uses a Long-Short Term Memory (LSTM) artificial neural network with minimal prior information. Despite few training assumptions, the LSTM produces trajectories that include qubit-readout resonator correlations due to a finite detection bandwidth. In addition to revealing rotated measurement eigenstates and a reduced measurement rate in agreement with theory for a fixed drive, the trained LSTM also correctly reconstructs evolution for an unknown drive with rapid modulation. Our work enables new applications of weak measurements with faster or initially unknown qubit dynamics, such as the diagnosis of coherent errors in quantum gates.
arxiv pdf

Violating the Bell-Leggett-Garg inequality with weak measurement of an entangled state

  1. T. C. White,
  2. J. Y. Mutus,
  3. J. Dressel,
  4. J. Kelly,
  5. R. Barends,
  6. E. Jeffrey,
  7. D. Sank,
  8. A. Megrant,
  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. P. Roushan,
  18. A. Vainsencher,
  19. J. Wenner,
  20. A. N. Korotkov,
  21. and John M. Martinis
Since the inception of quantum mechanics, its validity as a complete description of reality has been challenged due to predictions that defy classical intuition. For many years it was
unclear whether predictions like entanglement and projective measurement represented real phenomena or artifacts of an incomplete model. Bell inequalities (BI) provided the first quantitative test to distinguish between quantum entanglement and a yet undiscovered classical hidden variable theory. The Leggett-Garg inequality (LGI) provides a similar test for projective measurement, and more recently has been adapted to include variable strength measurements to study the process of measurement itself. Here we probe the intersection of both entanglement and measurement through the lens of the hybrid Bell-Leggett-Garg inequality (BLGI). By correlating data from ancilla-based weak measurements and direct projective measurements, we for the first time quantify the effect of measurement strength on entanglement collapse. Violation of the BLGI, which we achieve only at the weakest measurement strengths, offers compelling evidence of the completeness of quantum mechanics while avoiding several loopholes common to previous experimental tests. This uniquely quantum result significantly constrains the nature of any possible classical theory of reality. Additionally, we demonstrate that with sufficient scale and fidelity, a universal quantum processor can be used to study richer fundamental physics.
arxiv pdf