Bose condensation and Bogoliubov excitation in resonator-embedded superconducting qubit network

  1. Patrick Navez,
  2. Valentina Di Meo,
  3. Berardo Ruggiero,
  4. Claudio Gatti,
  5. Fabio Chiarello,
  6. Alessandro D'Elia,
  7. Alessio Rettaroli,
  8. Emanuele Enrico,
  9. Luca Fasolo,
  10. Mikhail Fistul,
  11. Ilya Eremin,
  12. Alexandre Zagoskin,
  13. Paolo Vanacore,
  14. Paolo Silvestrini,
  15. and Mikhail Lisitskiy
Superconducting qubit networks (SQNs) embedded in a low-dissipative resonator is a promising device allowing one not only to establish the collective quantum dynamics on a macroscopic scale but also to greatly enhance the sensitivity of detectors of microwave photons. A quantum ac Stark effect provided by coupling between an SQN and microwave photons of a resonator, leads to a strong nonlinear interaction between photons. Here, we present a two-tone spectroscopy experiment in which a set of 10 superconducting flux qubits is coupled to the input R- resonator and the output T- transmission line. An external microwave pump field close to the resonance frequency populates macroscopically the resonator mode as a Bose-Einstein condensate, while a second probe beam scans the resonances referred also as Bogoliubov-like excitations. The corresponding excitation frequency measured from the transmission coefficient, |S21(f)| displays an abrupt change of the resonant dip position once the power of the pump field overcomes a critical value Pcr. This sharp shift occurs in a narrow region of pump frequencies, and can be tuned by an applied magnetic field. It is a signature of bistability of the photon number inside the resonator, in agreement with theory.
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