Demonstration of nonstoquastic Hamiltonian in coupled superconducting flux qubits

  1. I. Ozfidan,
  2. C. Deng,
  3. A. Y. Smirnov,
  4. T. Lanting,
  5. R. Harris,
  6. L. Swenson,
  7. J. Whittaker,
  8. F. Altomare,
  9. M. Babcock,
  10. C. Baron,
  11. A.J. Berkley,
  12. K. Boothby,
  13. H. Christiani,
  14. P. Bunyk,
  15. C. Enderud,
  16. B. Evert,
  17. M. Hager,
  18. J. Hilton,
  19. S. Huang,
  20. E. Hoskinson,
  21. M.W. Johnson,
  22. K. Jooya,
  23. E. Ladizinsky,
  24. N. Ladizinsky,
  25. R. Li,
  26. A. MacDonald,
  27. D. Marsden,
  28. G. Marsden,
  29. T. Medina,
  30. R. Molavi,
  31. R. Neufeld,
  32. M. Nissen,
  33. M. Norouzpour,
  34. T. Oh,
  35. I. Pavlov,
  36. I. Perminov,
  37. G. Poulin-Lamarre,
  38. M. Reis,
  39. T. Prescott,
  40. C. Rich,
  41. Y. Sato,
  42. G. Sterling,
  43. N. Tsai,
  44. M. Volkmann,
  45. W. Wilkinson,
  46. J. Yao,
  47. and M.H. Amin
Quantum annealing (QA) is a heuristic algorithm for finding low-energy configurations of a system, with applications in optimization, machine learning, and quantum simulation. Up to
now, all implementations of QA have been limited to qubits coupled via a single degree of freedom. This gives rise to a stoquastic Hamiltonian that has no sign problem in quantum Monte Carlo (QMC) simulations. In this paper, we report implementation and measurements of two superconducting flux qubits coupled via two canonically conjugate degrees of freedom (charge and flux) to achieve a nonstoquastic Hamiltonian. Such coupling can enhance performance of QA processors, extend the range of quantum simulations. We perform microwave spectroscopy to extract circuit parameters and show that the charge coupling manifests itself as a YY interaction in the computational basis. We observe destructive interference in quantum coherent oscillations between the computational basis states of the two-qubit system. Finally, we show that the extracted Hamiltonian is nonstoquastic over a wide range of parameters.

Tuning a 3D Microwave Cavity via Superfluid Helium at MilliKelvin Temperatures

  1. F. Souris,
  2. H. Christiani,
  3. and J.P. Davis
Frequency tunability of 3D microwave cavities opens up numerous possibilities for their use in hybrid quantum systems and related technologies. For many applications it is desirable
to tune the resonance at cryogenic temperatures without mechanical actuation. We show that a superconducting 3D microwave cavity can be tuned at the percent level by taking advantage of the dielectric properties of superfluid 4He at milliKelvin temperatures, without affecting its intrinsic quality factor — reaching 3×105 in the present experiment.