Why might you want to produce a single photon? Individual photons would be very useful for the development of quantum control, computing, and communication. Unfortunately, making one photon at a time in a controlled manner generally requires specialized nanomaterials and very cold temperatures. However, a group of researchers has achieved single-photon emission at room temperature using a modified diamond semiconductor device.

Diamonds composed of pure carbon are insulators, but introducing impurities—a process known as doping—can allow diamond to conduct electricity. N. Mizuochi et al. combined three different types of doped diamonds into a diode, including one with a nitrogen atom in place of one of the carbon atoms and a gap where a second carbon ordinarily would sit. The doping altered the electronic structure of the diamond so that single photons are produced under the influence of an electric current. While the physical mechanism for producing light appears to be much like a light-emitting diode (LED), the details of the electronic structure and the generation of single photons mark the diamond material as truly novel.



Semiconductors are insulators at low temperatures, but a slight increase in temperature boosts electrons into a higher energy state, allowing current to flow. Doping with atoms of a different type changes the electronic structure, allowing greater control over the motion of charge. The types of semiconductors are n-type, where electrons carry the current, and p-type, where the site of a missing electron (a "hole") carries the charge. Combining n-type and p-type semiconductors produces a diode. In these devices, current flows easily across the device in one direction, but faces a large resistance when flowing the other way.

Diamond, like graphite, graphene, and buckyballs, is a form of carbon. Unlike graphene, however, diamond is ordinarily an insulator. Doping with boron (to produce blue diamonds), phosophorous, or nitrogen (producing a yellow color) can change it into a semiconductor. In particular, a type of doping known as nitrogen-vacancy (NV) has interesting effects: one carbon atom is removed from the diamond lattice (the vacancy), and one of them is replaced by a nitrogen atom.

Mizuochi et al. constructed a diode from n-type and p-type diamond semiconductors and pure diamond, sandwiched between gold-platinum-titanium (Au/Pt/Ti) electrodes. They measured the telltale signature of electroluminescence: light emitted when electrons and holes recombine. The photon originated precisely from the NV site, indicating it was that particular doping that was responsible for the effect. In ordinary LEDs, the recombination process is continuous, producing a stream of photons, but only one photon is produced at a time from the NV diamond.

While the researchers are cautious about a theoretical explanation for this effect, it is still the first reported room-temperature electronic device to emit a single photon. (Another group has published results from independent experiments; see the Supplemental Link below.) They propose constructing better devices using nanowires to increase the efficiency of charge flow, which should bring the diamond diode closer to the level of performance exhibited by quantum dots. Improvements to the design should allow room-temperature quantum electronics for the first time—a significant accomplishment.

Nature Photonics, 2012. DOI: http://dx.doi.org/10.1038/nphoton.2012.75 (About DOIs).