Interface between Trapped-Ion Qubits and Traveling Photons with Close-to-Optimal Efficiency

Experimental results are presented on the efficiency limits for a quantum interface between a matter-based qubit and a photonic qubit. Using a trapped ion in an optical cavity, we obtain a single ion-entangled photon at the cavity output with a probability of 0.69(3). The performance of our system is shown to saturate the upper limit to photon-collection probability from a quantum emitter in a cavity, set by the emitter's electronic structure and by the cavity parameters. The probability for generating and detecting the ion-entangled fiber-coupled photon is 0.462(3), a fivefold increase over the previous best performance. Finally, the generation and detection of up to 15 sequential polarized photons demonstrates the ability of a trapped ion to serve as a multiphoton source. The comparison between measured probabilities and predicted bounds is relevant for quantum emitters beyond trapped ions, in particular, for the design of future systems optimizing photon collection from, and absorption in, quantum matter.