Little lamp may set quantum tech aglow

By implanting tiny atomic clusters in a commonplace electronic component, researchers have created a device that may hasten the arrival of exotic technologies that rely on quantum mechanics.

Under each hole in a tiny metal strip (arrow), an atomic cluster emits just one photon when excited by one voltage pulse. Yuan et al./Science

The new device is a type of light-emitting diode, or LED, the class of tiny, lamps found widely in electronic items and increasingly in electric equipment ranging from on-off switches to bus taillights. What sets the new LED apart is that it consistently emits a single photon of light in response to a single tiny electrical pulse.

That’s important because investigators have been hatching schemes for exploiting quantum mechanics to boost the performance of many technologies in fields including communications, navigation, and computing (SN: 12/8/01, p. 364: Gadgets from the Quantum Spookhouse). Some of the schemes demand dependable sources of single photons.

A few experimental sources of this sort exist, but external lasers drive them. In contrast, the new device spits out lone photons when stimulated by run-of-the-mill electric signals on microchips. That’s “the only way to go for practical devices,” claims Andrew J. Shields of Toshiba Research Europe Limited in Cambridge, England. He invented the device with colleagues there and at the University of Cambridge.

A related device reported by California researchers several years ago (SN: 2/13/99, p. 102) also generated solitary photons in response to voltage pulses–but only about a third of the time. Moreover, it required temperatures near absolute zero. While the new device has so far only been tested at such temperatures, it probably doesn’t require that much cooling, Shields says.

The novel photon emitter, which Shields says could reach the market in 3 years, can help ensure that certain quantum-cryptography systems convey messages impossible to decipher, he claims. Such emitters might also contribute to optical quantum computers capable of vast calculations, such as factoring numbers so huge that today’s computers could never finish the task.

In an upcoming issue of Science, Shields and his colleagues describe how they modified a conventional LED design, which consists of three slightly different layers of the semiconductor gallium arsenide. In ordinary LEDs, abundant electrons in the central layer combine with so-called holes, or absences of electrons, to release a blaze of light.

In its design, the Cambridge team added to the middle layer an archipelago of cylindrical, nanometer-scale islands, each composed of only a few thousand indium and arsenic atoms. Because of quantum mechanical effects associated with those quantum islands, or dots (SN: 6/17/00, p. 392), modest voltage pulses could reliably force only one electron-hole pair into a dot at any instant. Under these conditions, the pair meets and immediately emits a photon.

Ata Imamoglu of the University of California, Santa Barbara calls the new diode “a really remarkable achievement.”

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