Electron shelving of a superconducting artificial atom

Interfacing stationary qubits with propagating photons is a fundamental problem in quantum technology. Cavity quantum electrodynamics (CQED) invokes a mediator degree of freedom in the form of a far-detuned cavity mode, the adaptation of which to superconducting circuits (cQED) proved remarkably fruitful. The cavity both blocks the qubit emission and it enables a dispersive readout of the qubit state. Yet, a more direct (cavityless) interface is possible with atomic clocks, in which an orbital cycling transition can scatter photons depending on the state of a hyperfine or quadrupole qubit transition. Originally termed "electron shelving", such a conditional fluorescence phenomenon is the cornerstone of many quantum information platforms, including trapped ions, solid state defects, and semiconductor quantum dots. Here we apply the shelving idea to circuit atoms and demonstrate a conditional fluorescence readout of fluxonium qubit placed inside a matched one-dimensional waveguide. 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 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.