Spectral properties of two superconducting artificial atoms coupled to a resonator in the ultrastrong coupling regime

We experimentally investigate a superconducting circuit composed of two flux qubits ultrastrongly coupled to a common LC resonator. Owing to the large anharmonicity of the flux qubits, the system can be described well by a generalized Dicke Hamiltonian containing spin-spin interaction terms. In the experimentally measured spectrum, we observed two key phenomena. First, an avoided level crossing provides evidence of the exotic interaction that allows the simultaneous excitation of two artificial atoms by absorbing one photon from the resonator. Second, we identified a pronounced spectral asymmetry that is a clear signature of light-matter decoupling. This multi-atom ultrastrongly coupled system opens the door to studying novel processes for quantum optics and quantum-information tasks on a chip.