C. Rich

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

Entanglement in a quantum annealing processor

  1. T. Lanting,
  2. A.J. Przybysz,
  3. A. Yu. Smirnov,
  4. F.M. Spedalieri,
  5. M.H. Amin,
  6. A.J. Berkley,
  7. R. Harris,
  8. F. Altomare,
  9. S. Boixo,
  10. P. Bunyk,
  11. N. Dickson,
  12. C. Enderud,
  13. J.P. Hilton,
  14. E. Hoskinson,
  15. M.W. Johnson,
  16. E. Ladizinsky,
  17. N. Ladizinsky,
  18. R. Neufeld,
  19. T. Oh,
  20. I. Perminov,
  21. C. Rich,
  22. M.C. Thom,
  23. E. Tolkacheva,
  24. S. Uchaikin,
  25. A.B. Wilson,
  26. and G. Rose
Entanglement lies at the core of quantum algorithms designed to solve problems that are intractable by classical approaches. One such algorithm, quantum annealing (QA), provides a promising
path to a practical quantum processor. We have built a series of scalable QA processors consisting of networks of manufactured interacting spins (qubits). Here, we use qubit tunneling spectroscopy to measure the energy eigenspectrum of two- and eight-qubit systems within one such processor, demonstrating quantum coherence in these systems. We present experimental evidence that, during a critical portion of QA, the qubits become entangled and that entanglement persists even as these systems reach equilibrium with a thermal environment. Our results provide an encouraging sign that QA is a viable technology for large-scale quantum computing.
arxiv pdf