A. Fowler

Chiral groundstate currents of interacting photons in a synthetic magnetic field

  1. P. Roushan,
  2. C. Neill,
  3. A. Megrant,
  4. Y. Chen,
  5. R. Babbush,
  6. R. Barends,
  7. B. Campbell,
  8. Z. Chen,
  9. B. Chiaro,
  10. A. Dunsworth,
  11. A. Fowler,
  12. E. Jeffrey,
  13. J. Kelly,
  14. E. Lucero,
  15. J. Mutus,
  16. P. J. J. O'Malley,
  17. M. Neeley,
  18. C. Quintana,
  19. D. Sank,
  20. A. Vainsencher,
  21. J. Wenner,
  22. T. White,
  23. E. Kapit,
  24. and J. Martinis
The intriguing many-body phases of quantum matter arise from the interplay of particle interactions, spatial symmetries, and external fields. Generating these phases in an engineered
system could provide deeper insight into their nature and the potential for harnessing their unique properties. However, concurrently bringing together the main ingredients for realizing many-body phenomena in a single experimental platform is a major challenge. Using superconducting qubits, we simultaneously realize synthetic magnetic fields and strong particle interactions, which are among the essential elements for studying quantum magnetism and fractional quantum Hall (FQH) phenomena. The artificial magnetic fields are synthesized by sinusoidally modulating the qubit couplings. In a closed loop formed by the three qubits, we observe the directional circulation of photons, a signature of broken time-reversal symmetry. We demonstrate strong interactions via the creation of photon-vacancies, or „holes“, which circulate in the opposite direction. The combination of these key elements results in chiral groundstate currents, the first direct measurement of persistent currents in low-lying eigenstates of strongly interacting bosons. The observation of chiral currents at such a small scale is interesting and suggests that the rich many-body physics could survive to smaller scales. We also motivate the feasibility of creating FQH states with near future superconducting technologies. Our work introduces an experimental platform for engineering quantum phases of strongly interacting photons and highlight a path toward realization of bosonic FQH states.
arxiv pdf

Measuring and Suppressing Quantum State Leakage in a Superconducting Qubit

  1. Zijun Chen,
  2. Julian Kelly,
  3. Chis Quintana,
  4. R. Barends,
  5. B. Camppbell,
  6. Yu Chen,
  7. B. Chiaro,
  8. A. Dunsworth,
  9. A. Fowler,
  10. E. Lucero,
  11. E. Jeffrey,
  12. A. Megrant,
  13. J. Mutus,
  14. M. Neeley,
  15. C. Neill,
  16. P. J. J. O'malley,
  17. P. Roushan,
  18. D. Sank,
  19. A. Vainsencher,
  20. J. Wenner,
  21. T. C. White,
  22. A. N. Korotkov,
  23. and John M. Martinis
Leakage errors occur when a quantum system leaves the two-level qubit subspace. Reducing these errors is critically important for quantum error correction to be viable. To quantify
leakage errors, we use randomized benchmarking in conjunction with measurement of the leakage population. We characterize single qubit gates in a superconducting qubit, and by refining our use of Derivative Reduction by Adiabatic Gate (DRAG) pulse shaping along with detuning of the pulses, we obtain gate errors consistently below 10−3 and leakage rates at the 10−5 level. With the control optimized, we find that a significant portion of the remaining leakage is due to incoherent heating of the qubit.
arxiv pdf