APPLIED PHYSICS As the field of quantum optics develops, so does the requirement for a light source that can emit photons one at a time and on demand. One approach to fabricating a single-photon source is to attenuate the light source so that on average only a single photon will reach its destination. More control can be achieved by using an excited organic molecule and observing its fluorescence as it relaxes. Such molecules do emit single photons, but suffer from a limited lifetime. Kurtsiefer et al. show that a single nitrogen vacancy in diamond, which combines the robustness of the diamond structure with the fluorescent properties of the vacancy itself, is an effective candidate for a stable, single-photon light source. Their experiments reveal that when the vacancy is excited with green light, photons are emitted one at a time over the red to near-infrared wavelengths as it relaxes. Even after the emission of more than 1013 single photons, there were no observable changes in the emission characteristics. — ISO Phys. Rev. Lett. 85 , 290 (2000).