We report on experimental multi-photon spectroscopy of a hybrid quantum
system consisting of a superconducting phase qubit coherently coupled to an
intrinsic two-level defect. We directly probe hybridized states of the combined
qubit-defect system in the strongly interacting regime, where both the
qubit-defect coupling and the driving cannot be considered as weak
perturbations. This regime is described by a theoretical model which
incorporates anharmonic corrections, multi-photon processes and decoherence. We
present a detailed comparison between experiment and theory and find excellent
agreement over a wide range of parameters.Comment: 6 pages, 6 figure
