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Optical Circuits: Single Photon Flips Transistor Switch

Photons emerge as competitors to electrons in new computer circuits

Transistors, the tiny switches that flip on and off inside computer chips, have long been the domain of electricity. But scientists are beginning to develop chip components that run on light. Last week, in a remarkable achievement, a team led by researchers at the Massachusetts Institute of Technology (MIT) in Cambridge reported building a transistor that is switched by a single photon.

Conventionally, photons are used only to deliver information, racing along fiber-optic cables with unparalleled speed. The first commercial silicon chip to include optical elements, announced last December, did little to challenge the status quo. The on-board beams of light in the device, developed at IBM’s research center in Yorktown Heights, New York, merely shuttle data between computer chips.

Now, Wenlan Chen of MIT and her colleagues have taught light some new tricks, using a cloud of chilled caesium atoms suspended between two mirrors. Their transistor is set to ‘on’ by default, allowing a beam of light to sail through the transparent caesium cloud unmolested. But sending in a single ‘gate’ photon turns the switch off, thanks to an effect called electromagnetically induced transparency. The injected photon excites the caesium atoms, rendering them reflective to light trying to cross the cloud (see ‘Turn off the light’). One photon can thus block the passage of about 400 other photons, says Chen, who presented the result on 7 June at a meeting of the American Physical Society’s Division of Atomic, Molecular and Optical Physics in Quebec City, Canada.

Image: Courtesy of Nature magazine

The ability to turn a strong signal on and off using a weak one fulfills a key requirement of an optical transistor. “Nothing even came close before,” says physicist Ataç İmamoğlu of the Swiss Federal Institute of Technology Zurich, who called the experiment “a true breakthrough”. In theory, the hundreds of photons, controlled by the triggering photon, could fan out and switch off hundreds of other transistors in an optical circuit.

With its exotic clouds of atoms and bulky equipment, the proof-of-principle transistor is unlikely to become a component in everyday computers. But it could be a useful tool for studying how photons interact at the quantum level — potentially leading to a quantum transistor that flips, not a one or a zero as in classical computing, but a fuzzy bit of quantum information.

A more practical optical transistor debuted in April 2012 at Purdue University in West Lafayette, Indiana, where electrical engineer Minghao Qi has made one that is compatible with the semiconductor industry’s existing manufacturing techniques. “The advantage of our device is that we have it on a silicon chip,” says Qi.

In this case, the beam of light to be switched on and off enters and exits along a channel, etched in the silicon, that sits next to a parallel channel. In between the two rails is an etched ring. When a weaker light beam courses through the second optical line, the ring heats up and swells, interfering with the main beam and switching off the transistor. This switch can flip on and off up to 10 billion times per second.

And the output beam can fan out and drive two other transistors, meeting one of the established requirements for an optical transistor set out in 2010 by David Miller, a physicist at Stanford University in California. Other cri­teria include matching the frequency of the exiting signal to the input frequency and keeping the output clean, with no degradation that could cause errors. “Making an optical transistor that really satisfies the necessary criteria is very hard,” says Miller.

Still, Qi does not expect to challenge the electronic transistor with his optical analogue, which consumes a lot more power and runs much more slowly. “We want to complement the Intel transistor,” he says. “We don’t want to replace it.” He hopes to find a foothold in niche markets, such as equipment for scrambling cable channels and military technologies that could benefit from light’s imperviousness to an electromagnetic attack.

Routers that guide information through the Internet could also be amenable to optical transistors and switches. At present, these stopping points in the network convert optical signals travelling through fiber-optic cables into electrical signals; these are then processed, converted back to light and sent on their way. A router in which one beam of light pushes another in the appropriate direction — with no conversions involved — could in principle be faster and consume less energy.

A popular candidate for such switches are quantum dots, small semiconductor crystals that behave like atoms. In one particularly sensitive quantum-dot switch, a beam of light is first guided along a material dotted with holes, called a photonic crystal. The light can pass through a quantum dot placed in its path without changing course. But if a pulse of light is sent in just ahead of that beam, it can induce an inter­action between the dot and the crystal that scatters the beam and sends it on a different path.

Reported in May 2012 by Edo Waks of the Joint Quantum Institute at the University of Maryland in College Park and colleagues, it switches when struck by a pulse of 140 photons. In principle, that is a small enough amount of energy to rival conventional routers.

But the switch still faces a practical obstacle common to all of these emerging optical technologies. The lasers that supply the devices with light consume considerable energy, offsetting any savings. “Right now,” says Waks, “the overhead is what’s killing us.”

This article is reproduced with permission from the magazine Nature. The article was first published on June 12, 2013.

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