Entanglement between a trapped ion qubit and a 780-nm photon via quantum frequency conversion

Abstract: Future quantum networks will require the ability to produce matter-photon entanglement at photon frequencies not naturally emitted from the matter qubit. This allows for a hybrid network architecture, where these photons can couple to other tools and quantum technologies useful for tasks such as multiplexing, routing, and storage, but which operate at wavelengths different from that of the matter qubit source, while also reducing network losses. Here, we demonstrate entanglement between a trapped ion and a 780 nm photon, a wavelength which can interact with neutral-Rb-based quantum networking devices. A single barium ion is used to produce 493 nm photons, entangled with the ion, which are then frequency converted to 780 nm while preserving the entanglement. We generate ion-photon entanglement with fidelities $\geq$ 0.93(2) and $\geq$ 0.84(2) for 493 nm and 780 nm photons respectively with the fidelity drop arising predominantly from a reduction in the signal-noise of our detectors at 780 nm compared with at 493 nm.