E. Kapit

Versatile parametric coupling between two statically decoupled transmon qubits

  1. X. Y. Jin,
  2. K. Cicak,
  3. Z. Parrott,
  4. S. Kotler,
  5. F. Lecocq,
  6. J. Teufel,
  7. J. Aumentado,
  8. E. Kapit,
  9. and R. W. Simmonds
Parametric coupling is a powerful technique for generating tunable interactions between superconducting circuits using only microwave tones. Here, we present a highly flexible parametric
coupling scheme demonstrated with two transmon qubits, which can be employed for multiple purposes, including the removal of residual ZZ coupling and the implementation of driven swap or swap-free controlled-Z (cZ) gates. Our fully integrated coupler design is only weakly flux tunable, cancels static linear coupling between the qubits, avoids internal coupler dynamics or excitations, and operates with rf-pulses. We show that residual ZZ coupling can be reduced with a parametric dispersive tone down to an experimental uncertainty of 5.5 kHz. Additionally, randomized benchmarking reveals that the parametric swap cZ gate achieves a fidelity of 99.4% in a gate duration of 60 ns, while the dispersive parametric swap-free cZ gate attains a fidelity of 99.5% in only 30 ns. We believe this is the fastest and highest fidelity gate achieved with on-chip parametric coupling to date. We further explore the dependence of gate fidelity on gate duration for both p-swap and p-swap-free cZ gates, providing insights into the possible error sources for these gates. Overall, our findings demonstrate a versatility, precision, speed, and high performance not seen in previous parametric approaches. Finally, our design opens up new possibilities for creating larger, modular systems of superconducting qubits.
arxiv pdf

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