  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.
