Single-photon-driven high-order sideband transitions in an ultrastrongly   coupled circuit quantum electrodynamics system
  We report the experimental observation of high-order sideband transitions at the single-photon level in a quantum circuit system of a flux qubit ultrastrongly coupled to a coplanar waveguide resonator. With the coupling strength reaching 10% of the resonator's fundamental frequency, we obtain clear signatures of higher-order red and first-order blue-sideband transitions, which are mainly due to the ultrastrong Rabi coupling. Our observation advances the understanding of ultrastrongly-coupled systems and paves the way to study high-order processes in the quantum Rabi model at the single-photon level. 
Zhen Chen, Yimin Wang, Tiefu Li, Lin Tian, Yueyin Qiu, Kunihiro   Inomata, Fumiki Yoshihara, Siyuan Han, Franco Nori, J. S. ...
quant-ph cond-mat.supr-con