Multi-photon sideband transitions in an ultrastrongly-coupled circuit quantum electrodynamics system

  1. Zhen Chen,
  2. Yimin Wang,
  3. Tiefu Li,
  4. Lin Tian,
  5. Yueyin Qiu,
  6. Kunihiro Inomata,
  7. Fumiki Yoshihara,
  8. Siyuan Han,
  9. Franco Nori,
  10. J. S. Tsai,
  11. and J. Q. You
Ultrastrong coupling in circuit quantum electrodynamics systems not only provides a platform to study the quantum Rabi model, but it can also facilitate the implementation of quantum logic operations via high-lying resonator states. In this regime, quantum manifolds with different excitation numbers are intrinsically connected via the counter-rotating interactions, which can result in multi-photon processes. Recent experiments have demonstrated ultrastrong coupling in superconducting qubits electromagnetically coupled to superconducting resonators. Here we report the experimental observation of multiphoton sideband transitions of a superconducting flux qubit coupled to a coplanar waveguide resonator in the ultrastrong coupling regime. With a coupling strength reaching about 10% of the fundamental frequency of the resonator, we obtain clear signatures of higher-order red-sideband transitions and the first-order blue-sideband transition in a transmission spectroscopic measurement. This study advances the understanding of driven ultrastrongly-coupled systems.
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