Ultrastrong light-matter interaction in a photonic crystal waveguide

The superconducting quantum circuits platform has developed into a rich playground for building synthetic quantum materials composed of interacting microwave excitations. In this work, we apply this toolbox for exploring the physics of an artificial atom coupled to the many modes of a photonic crystal. Recently, strongly coupling a transmon qubit to the band structure of a stepped impedance waveguide has led to the first observation of atom-photon dressed bound states. In this experiment, we use an emitter with a higher nonlinearity and push the coupling strength beyond the single-photon regime. Our platform consists of a fluxonium qubit galvanically coupled to a linear chain of microwave resonators. Transport measurements reveal how the propagation of a single photon becomes a many-body problem as multi-photon bound states participate in the scattering dynamics. The effective photon-photon interactions induced by the impurity emerge as we measure the inelastic scattering spectrum. Furthermore, we probe signatures of multi-mode entanglement from measured correlations in the emitted quadrature fields. This work opens a new avenue for future explorations in many-body quantum optics.