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.
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