A light source that emits only one photon at a time would be an invaluable tool for quantum optics and quantum computing. Quasi-zero-dimensional systems such as atoms, nitrogen vacancies in diamond, and quantum dots emit photons one by one when excited by laser light. The reason for this is that when charge carriers are strongly confined, the multiple excited states necessary for emission of two photons do not exist.
In materials that are extended in one or more dimensions, multiple excited states (namely, electron-hole pairs) can coexist and these materials tend to be poor candidates for single-photon emitters. However, in the 27 May 2008 issue of Physical Review Letters, Alexander Högele, Christopher Galland, Martin Winger, and Ataç Imamo?lu report that at low temperatures, semiconducting carbon nanotubes act as single-photon emitters when excited by a laser beam.
The unexpected finding is due to a combination of effects that prohibit double occupancy of excited states: the electron-hole pairs are highly localized and Auger processes (in which electron-hole pairs recombine without photon emission) are strong. Fewer than 1 in 20 events are reported to be multiphoton emissions, making carbon nanotubes promising single-photon sources. - Daniel Ucko