A. Shabani

Observation of classical-quantum crossover of 1/f flux noise and its paramagnetic temperature dependence

  1. C. M. Quintana,
  2. Yu Chen,
  3. D. Sank,
  4. A. G. Petukhov,
  5. T. C. White,
  6. Dvir Kafri,
  7. B. Chiaro,
  8. A. Megrant,
  9. R. Barends,
  10. B. Campbell,
  11. Z. Chen,
  12. A. Dunsworth,
  13. A. G. Fowler,
  14. R. Graff,
  15. E. Jeffrey,
  16. J. Kelly,
  17. E. Lucero,
  18. J. Y. Mutus,
  19. M. Neeley,
  20. C. Neill,
  21. P. J. J. O'Malley,
  22. P. Roushan,
  23. A. Shabani,
  24. A. Vainsencher,
  25. J. Wenner,
  26. H. Neven,
  27. and John M. Martinis
By analyzing the dissipative dynamics of a tunable gap flux qubit, we extract both sides of its two-sided environmental flux noise spectral density over a range of frequencies around
2kBT/h≈1GHz, allowing for the observation of a classical-quantum crossover. Below the crossover point, the symmetric noise component follows a 1/f power law that matches the magnitude of the 1/f noise near 1Hz. The antisymmetric component displays a 1/T dependence below 100mK, providing dynamical evidence for a paramagnetic environment. Extrapolating the two-sided spectrum predicts the linewidth and reorganization energy of incoherent resonant tunneling between flux qubit wells.
arxiv pdf

Digitized adiabatic quantum computing with a superconducting circuit

  1. R. Barends,
  2. A. Shabani,
  3. L. Lamata,
  4. J. Kelly,
  5. A. Mezzacapo,
  6. U. Las Heras,
  7. R. Babbush,
  8. A. G. Fowler,
  9. B. Campbell,
  10. Yu Chen,
  11. Z. Chen,
  12. B. Chiaro,
  13. A. Dunsworth,
  14. E. Jeffrey,
  15. E. Lucero,
  16. A. Megrant,
  17. J. Y. Mutus,
  18. M. Neeley,
  19. C. Neill,
  20. P. J. J. O'Malley,
  21. C. Quintana,
  22. P. Roushan,
  23. D. Sank,
  24. A. Vainsencher,
  25. J. Wenner,
  26. T. C. White,
  27. E. Solano,
  28. H. Neven,
  29. and John M. Martinis
A major challenge in quantum computing is to solve general problems with limited physical hardware. Here, we implement digitized adiabatic quantum computing, combining the generality
of the adiabatic algorithm with the universality of the digital approach, using a superconducting circuit with nine qubits. We probe the adiabatic evolutions, and quantify the success of the algorithm for random spin problems. We find that the system can approximate the solutions to both frustrated Ising problems and problems with more complex interactions, with a performance that is comparable. The presented approach is compatible with small-scale systems as well as future error-corrected quantum computers.
arxiv pdf

Cavity-Assisted Monitoring of Dynamics for Complex Quantum Systems

  1. A. Shabani,
  2. J. Roden,
  3. and K. B. Whaley
Cavity and circuit quantum electrodynamics (CQED) technologies have progressed significantly during recent years, enabling real-time monitoring and control of quantum systems, yet their
potential for quantum tomography or spectroscopy is largely unexplored. We develop here a CQED formalism for monitoring the dynamics of a complex quantum system, deriving a set of Stochastic Hierarchy Equations of Motion to describe continuous measurement in presence of non-perturbative and non-Markovian decoherence effects. Using the cavity as a probe enables us to engineer a continuous measurement observable by tuning either the cavity frequency, the cavity drive phase or the detector phase. This turns the cavity into a tool for continuous quantum state tomography with no need for active control. We further demonstrate that time correlations of the detector record can provide a spectroscopic probe of the non-Markovian nature of the decoherence dynamics.
arxiv pdf