Resolving Fock states near the Kerr-free point of a superconducting resonator

  1. Yong Lu,
  2. Marina Kudra,
  3. Timo Hillmann,
  4. Jiaying Yang,
  5. Hangxi Li,
  6. Fernando QuijandrĂ­a,
  7. and Per Delsing
We have designed a tunable nonlinear resonator terminated by a SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement). Such a device possesses a sweet spot in which the external
magnetic flux allows to suppress the Kerr interaction. We have excited photons near this Kerr-free point and characterized the device using a transmon qubit. The excitation spectrum of the qubit allows to observe photon-number-dependent frequency shifts about nine times larger than the qubit linewidth. Our study demonstrates a compact integrated platform for continuous-variable quantum processing that combines large couplings, considerable relaxation times and excellent control over the photon mode structure in the microwave domain.