Energy Spectrum of the Strongly Coupled Superconducting Artificial Atoms

We experimentally investigated the energy spectrum of two strongly coupled tunable transmon atoms with a coupling energy of 300MHz. The energy levels are dispersively probed through a readout resonator coupled to the atoms. The higher excited level energies could be probed by 2-photon or 3-photon processes. The power dependence of the readout transmission showed that the n-photon process satisfies the power law with a power of n - 1. We also observed the Autler-Townes splitting, by probing the Rabi frequencies resonant to one of the higher levels driven by an external microwave field. Coherent operations on the atoms reveal that the relaxation time (T1) and dephasing time (T2) are approximately 62ns and 51ns, respectively.