M. Babcock

Probing Environmental Spin Polarization with Superconducting Flux Qubits

  1. T. Lanting,
  2. M.H. Amin,
  3. C. Baron,
  4. M. Babcock,
  5. J. Boschee,
  6. S. Boixo,
  7. V. N. Smelyanskiy,
  8. M. Foygel,
  9. and A. G. Petukhov
We present measurements of the dynamics of a polarized magnetic environment coupled to the We present measurements of the dynamics of a polarized magnetic environment coupled to the
flux degree of freedom of rf-SQUID flux qubits. The qubits are used as both sources of polarizing field and detectors of the environmental polarization. We probe dynamics at timescales from 5\,μs to 5\,ms and at temperatures between 12.5 and 22 mK. The measured polarization versus temperature provides strong evidence for a phase transition at a temperature of 5.7±0.3 mK. Furthermore, the environmental polarization grows initially as t√, consistent with spin diffusion dynamics. However, spin diffusion model deviates from data at long timescales, suggesting that a different phenomenon is responsible for the low-frequency behavior. A simple 1/f model can fit the data at all time scales but it requires empirical low- and high-frequency cutoffs. We argue that these results are consistent with an environment comprised of random clusters of spins, with fast spin diffusion dynamics within the clusters and slow fluctuations of the total moments of the clusters.
arxiv pdf

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.
arxiv pdf