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 , 8 , 9 . Our scheme brings deterministic protocols of quantum information science with light and matter 10 , 11 , 12 , 13 , 14 , 15 , 16 closer to realization.