Cavity-less circuit quantum electrodynamics of a fluxonium artificial atom – Design

In circuit quantum electrodynamics (cQED), a high-quality factor cavity plays a central role: it protects a qubit from spontaneous emission and acts as a buffer system enabling a quantum non-demolition (QND) dispersive readout of the qubit state. Here we experiment with a fluxonium artificial atom capacitively connected to a 1D transmission line. We simultaneously achieved a strong coupling of a high-frequency “cycling” transition (0→3 or 1→2) to the traveling waves and a complete suppression of the spontaneous emission of the low-frequency qubit transition (0→1). This allowed implementing the fluorescence “shelving” readout of a highly-coherent and fully controllable qubit. Unlike in conventional cQED, here the cycling dynamics during the readout is confined to a small Hilbert space and can be understood within a simple optical pumping model. Our system realizes a hardware-efficient interface between qubits and photons and hence can be useful in constructing quantum networks. It can also help understanding the processes leading to the loss of QND-ness in cQED at relatively low photon numbers.