Photons can now be emitted one at a time, an advance towards quantum computers and cryptography A tiny disc that emits light in an orderly procession of single photons, and with no background noise, has been created. The "photon turnstile" could ultimately transmit the keys to secret codes safely past prying eyes or help computer scientists exploit the strange laws of quantum mechanics. Researchers have long tried to develop devices that emit one photon at a time. In September, researchers reported that a single molecule could do the trick. But the material surrounding the molecule produced a lot of noise - an unwanted second photon popped out about 20% of the time. However, the new semiconductor disc, made by a team from the University of California, Santa Barbara, avoids this problem. "Our principle improvement is that we were able to remove this background," says applied physicist Atac Imamoglu. Quantum dot Imamoglu and colleagues embedded a "quantum dot" inside a 200-nanometer-thick disc of gallium arsenide. The dot is a dab of indium arsenide less than 50 nanometres across and 3 nanometres thick. They cooled the device to about 25 Kelvin and shone pulses of laser light on it. The light sent electrons cruising through the gallium arsenide. These left behind vacancies or "holes" that acted like positive charges floating through the disk. An electron-hole pair would quickly settle onto the quantum dot, where they would recombine to give off one photon of a particular wavelength. In the meantime, all the other electron-hole pairs would annihilate in ways that produce no light, or light of the wrong wavelength. In this way, the researchers obtained precisely one of the desired photons per laser pulse. Extraneous photons didn't crop up because the disk was so small, Imamoglu says. "The microdisc helps by reducing the volume of material that is excited and thereby reducing the background," he says. Keys and Q-bits A stream of single photons could safely transmit code keys because a spy would have to measure each photon. According to the laws of quantum mechanics, this would alter its state, says team member Christoph Milcher, now at the University of Bremen, in Germany. "You can send a key with single photons so that no-one else can detect it without you knowing," he says. Two photons might also be joined through a process called entanglement, Imamoglu says, to form a logic bit that can be 0, 1, or 0-and-1 at the same time. Such "q-bits" are essential for quantum computing. But currently, Imamoglu and colleagues capture only one in every 10,000 single-photon pulses. They hope to adjust the shape of their tiny device to quickly raise that to a more practical one in every 100. More at: Science (vol 290, p 2282), Nature (vol 407, p 491) Correspondence about this story should be directed to latestnews@newscientist.com 1136 GMT, 22 December 2000 Adrian Cho