Kondo resonance of a microwave photon

We emulate renormalization group models, such as the spin-boson Hamiltonian or the anisotropic Kondo model, from a quantum optics perspective by considering a superconducting device. The infrared confinement involves photon excitations of two tunable transmission lines entangled to an artificial spin-1/2 particle or double-island charge qubit. Focusing on the propagation of microwave light, in the underdamped regime of the spin-boson model, we identify a many-body resonance where a photon is absorbed at the renormalized qubit frequency and reemitted forward in an elastic manner. We also show that asymptotic freedom of microwave light is reached by increasing the input signal amplitude at low temperatures, which allows the disappearance of the transmission peak.