Quantum-state transfer from an ion to a photon

One model for quantum networks^1,2 is based on the probabilistic measurement of two photons, each entangled with a distant node, e.g., an atom or atomic ensemble^3-7. A second, deterministic model transfers information directly from an atom onto a cavity photon, which carries it to a second node⁸, as recently demonstrated with neutral atoms⁹. In both cases, the challenge is to transfer information efficiently while preserving coherence. Here, following the second scheme, we map the quantum state of an ion onto a photon within an optical cavity. Using an ion enables deterministic state initialization^10,11, while the cavity provides coherent coupling to a well-defined output mode^12-15. Although it is often assumed that a cavity-based quantum interface requires the strong-coupling regime, we show transfer fidelities of 92% in the presence of non-negligible decoherence and characterize the interplay between fidelity and efficiency. Our time-independent mapping process offers a promising route toward ion-based quantum networks.