Cavity-free circuit quantum electrodynamics: interfacing a high-coherence qubit with propagating photons

We report experiments with a fluxonium artificial atom directly connected to a 1D transmission line. Unlike conventional circuit quantum electrodynamics (cQED), here there is no extra cavity mode degree of freedom buffering the qubit from the readout environment. Thanks to the highly anharmonic fluxonium spectrum, we simultaneously achieved a strong coupling of a high-frequency “cycling” readout transition (0→3 or 1→2) to the traveling waves and a nearly complete suppression of the spontaneous emission of the low-frequency ``clocking" qubit transition (0→1). Notably, the cycling dynamics during the readout is confined to a small Hilbert space of a single quantum degree of freedom, and it can be understood within a minimal optical pumping model. Our system realizes the simplest possible interface between a highly-coherent qubit and propagating photons for constructing quantum communication networks.