A single-photon server with just one atom

Neutral atoms are ideal objects for the deterministic processing of quantum information. Entanglement operations have been carried out by photon exchange(1) or controlled collisions(2), and atom - photon interfaces have been realized with single atoms in free space(3,4) or strongly coupled to an optical cavity(5,6). A long-standing challenge with neutral atoms, however, is to overcome the limited observation time. Without exception, quantum effects appeared only after ensemble averaging. Here, we report on a single-photon source with one, and only one, atom quasi-permanently coupled to a high-finesse cavity. 'Quasi-permanent' refers to our ability to keep the atom long enough to, first, quantify the photon-emission statistics and, second, guarantee the subsequent performance as a single-photon server delivering up to 300,000 photons for up to 30 s. This is achieved by a unique combination of single-photon generation and atom cooling(7-9). Our scheme brings deterministic protocols of quantum information science with light and matter(10-16) closer to realization.