Electron shelving of a superconducting artificial atom

Simultaneously maintaining high coherent stationary qubit and interacting with propagating photons is a fundamental problem in quantum technology. Conventional circuit quantum electrodynamics (cQED) introduce an extra far-detuned cavity mode to mediate high coherent qubit transition and enables a dispersive readout. Here we report experiment of a fluxonium artificial directly placed inside a matched one-dimensional waveguide. Without introducing extra degree of freedom, we apply the electron shelving idea and demonstrate a conditional fluorescence readout. Cycling the non-computational transition between ground and third excited states produces a microwave photon every 91 ns conditioned on the qubit ground state, while the qubit (transition between ground and first excited states) coherence time exceeds 50 us. The readout has a built-in quantum non-demolition property, allowing over 100 fluorescence cycles in agreement with a four-level optical pumping model. Our result introduces a resource-efficient alternative to cQED. It also adds a state-of-the-art quantum memory to the growing toolbox of waveguide QED.