A research team at California Institute of Technology in Pasadena has demonstrated that an optically trapped cesium atom can be induced to emit single photons. The work may have applications in quantum cryptography, in distributed quantum networking and in quantum computing, all of which require reliable sources of single photons.The approach employs a single atom in a standing-wave dipole trap within a high-finesse optical cavity. Illuminating the atom with a laser pulse at approximately 850 nm drives a "dark state" transfer between two hyperfine ground states, leading to the emission of a photon. A second laser pulse returns the atom to the initial ground state so that the process can be repeated.Because the setup traps the atom for approximately 0.14 seconds, the scientists calculate that each atom generates 14,000 single-photon pulses. Approximately 3 percent of the time, two atoms are trapped in the cavity, yielding a two-photon event. They suggest that either real-time monitoring of trap loading or heating the trap may eliminate such events.