Tiny turnstile spits out solo photons.

Photons are party animals, compelled by quantum mechanics to roam in crowds. The gregarious habits of these fundamental particles of light have hindered attempts to use them one at a time for tasks such as computing or sending secrets. The challenge has been to guarantee that only one photon arrives on the scene at any given moment.

A new device made from the semiconductor gallium arsenide, however, forces photons to sally forth one by one. The pillar-shaped component--an exotic light-emitting diode--acts as a turnstile with electrons as tokens, say its inventors at Stanford University, Hamamatsu (Japan) Photonics, and NTT Basic Research Laboratories in Atsugi, Japan.

Ideally, each electron that enters the component chucks out one photon from the top of the device. The turnstile, described in the Feb. 11 NATURE, approaches that goal when operated at a frigid 55 millikelvins. It ejects a photon for at least a third of the electrons, timed to a precision of tens of billionths of a second. Unfortunately, the device's builders note, the photons go off in all directions, making them difficult to capture in an optical fiber and to pipe to downstream circuits.

"If they're up to 33 percent, that's a significant breakthrough and improvement," comments Michael G. Raymer of the University of Oregon in Eugene. If they can further boost that percentage and steer the photons to a useful destination, "then the device would have very important applications in quantum information processing," he adds.

Quantum computing, for example, attempts to exploit the quantum mechanical nature of tiny particles, such as photons or atoms, to surpass conventional computers in certain tasks (SN: 3/1/97, p. 135). After researchers first used photons as bits four years ago, further progress using photons has stalled. One reason is the difficulty of guaranteeing that a photon would be present when needed.

Because the new device provides timed release of lone photons, it may overcome that limitation, says Oliver J. Benson, one of the Stanford researchers. Adds Jungsang Kim, also of the Stanford team, the device may also aid the use of photons in secure communications. It would help eliminate unintended, redundant photons carrying copies of a message. Those copies may allow a spy to pilfer information from extra photons without being detected.

The researchers are making a new version of the turnstile in a tiny cluster of semiconductor atoms, or quantum dot, hoping to boost photon output and operating temperature.