Ksenia Shagalov

Higher Josephson harmonics in a tunable double-junction transmon qubit

  1. Ksenia Shagalov,
  2. David Feldstein-Bofill,
  3. Leo Uhre Jakobsen,
  4. Zhenhai Sun,
  5. Casper Wied,
  6. Amalie T. J. Paulsen,
  7. Johann Bock Severin,
  8. Malthe A. Marciniak,
  9. Clinton A. Potts,
  10. Anders Kringhøj,
  11. Jacob Hastrup,
  12. Karsten Flensberg,
  13. Svend Krøjer,
  14. and Morten Kjaergaard
Tunable Josephson harmonics open up for new qubit design. We demonstrate a superconducting circuit element with a tunnel junction in series with a SQUID loop, yielding a highly magnetic-flux
tunable harmonic content of the Josephson potential. We analyze spectroscopy of the first four qubit transitions with a circuit model which includes the internal mode, revealing a second harmonic up to ∼10% of the fundamental harmonic. Interestingly, a sweet spot where the dispersive shift vanishes is achieved by balancing the dispersive couplings to the internal and qubit modes. The highly tunable set-up provides a route toward protected qubits, and customizable nonlinear microwave devices.
arxiv pdf