By Dan Vergano, USA TODAY Scientists have unveiled a miniature device that emits light particles, or photons, one at a time, an accomplishment which could pave the way for impregnable coded messages and electronic commerce in coming decades. In theory, such a single light particle offers benefits to people hoping to create secure communications, including bankers in need of secure pathways for transactions or governments relaying secret diplomatic documents. Writing in Friday's Science, researchers note that such fundamental particles cannot be examined without altering their physical characteristics, part of the physics theory known as quantum mechanics. To create an unbreakable code, physicists plan to send a stream of single photons imprinted with the key to a later coded message. If transmitted via fiber-optic cable, a third party could not intercept the particles without scrambling the key. Alerted to the transgression, the target of the eavesdropping would halt transmission of the coded message. ''I think this work represents a very nice advance towards that goal,'' says David Peter DiVincenzo, an encryption expert at IBM's T.J. Watson Research Center in Yorktown Heights, N.Y. But a number of technological hurdles need to be overcome before so-called ''quantum cryptography'' becomes a reality, he cautions. To create a photon ''turnstile,'' a University of California-Santa Barbara research team placed ''quantum dots'' Ñ crystals containing confined groups of negative- and positive-charged atoms Ñ onto a mushroom-shaped semiconductor. When pulsed with a laser, the structure releases a single photon, the team reports in Science. Even lasers, which represent the best control of light emission today, release tens of thousands of photons at once, whereas control of single photons has long been a scientific goal. ''Really, it's about the ultimate control of light,'' DiVincenzo says. At least four other teams are pursuing single-photon devices, he adds. In September, for example, Stanford University researchers reported a way to produce single photons that is 84% efficient. Now, ''we can produce a photon every time, so the efficiency is 100%,'' says scientist Pierre Petroff of UC-Santa Barbara, a member of the device team led by his colleague Atac Imamoglu. In the next year, the photon turnstile team hopes to raise the temperature at which their device functions above its current limit of minus-321 degrees Fahrenheit. They hope to produce the effect at room temperatures. Support for development of the photon turnstile came from the Defense Advanced Research Projects Agency, a Defense Department agency charged with advancing information technology.
