Programmable atom-photon quantum interface

We present the implementation of a programmable atom-photon quantum interface, employing a single trapped $^{40}\mathrm{Ca}^{+}$ ion and single photons. Depending on its mode of operation, the interface serves as a bidirectional atom-photon quantum-state converter, as a source of entangled atom-photon states, or as a quantum frequency converter of single photons. The interface lends itself particularly to interfacing ions with spontaneous parametric down-conversion--based single-photon or entangled-photon-pair sources.