Light Flips Transistor Switch Photons Emerge As Competitors to Electrons in Computer Circuits

IN FOCUS NEWS PHOTONICS Light flips transistor switch Photons emerge as competitors to electrons in computer circuits. BY DEVIN POWELL Researchers have succeeded in using a single photon to switch o TURN OFF THE LIGHT a beam of light, a key step in demonstrating an optical transistor. ransistors, the tiny switches that flip on and off inside computer chips, have Cold caesium atoms Switching photon long been the domain of electricity. excites atoms TBut 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 Strong Light beam building a transistor that is switched by a sin- light beam blocked gle photon. Conventionally, photons are used only to deliver information, racing along fibre- classical computing, but a fuzzy bit of quantum markets, such as equipment for scrambling optic cables with unparalleled speed. The information. cable channels and military technologies that first commercial silicon chip to include opti- A more practical optical transistor debuted could benefit from light’s imperviousness to an cal elements, announced last December, did in April 2012 at Purdue University in West electromagnetic attack. little to challenge the status quo. The on-board Lafayette, Indiana, where electrical engineer Routers that guide information through beams of light in the device, developed at IBM’s Minghao Qi has made one that is compatible the Internet could also be amenable to opti- research centre in Yorktown Heights, New with the semiconductor industry’s existing cal transistors and switches. At present, these York, merely shuttle data between computer manufacturing techniques1. “The advantage stopping points in the network convert opti- chips. of our device is that cal signals travelling through fibre-optic cables Now, Wenlan Chen of MIT and her col- we have it on a silicon “Making into electrical signals; these are then processed, leagues have taught light some new tricks, chip,” says Qi. an optical converted back to light and sent on their way. using a cloud of chilled caesium atoms sus- In this case, the transistor that A router in which one beam of light pushes pended between two mirrors. Their transis- beam of light to be really satisfies another in the appropriate direction — with tor is set to ‘on’ by default, allowing a beam of switched on and off the necessary no conversions involved — could in principle light to sail through the transparent caesium enters and exits along criteria is be faster and consume less energy. cloud unmolested. But sending in a single a channel, etched in very hard.” A popular candidate for such switches are ‘gate’ photon turns the switch off, thanks to the silicon, that sits quantum dots, small semiconductor crystals an effect called electromagnetically induced next to a parallel channel. In between the two that behave like atoms. In one particularly transparency. The injected photon excites the rails is an etched ring. When a weaker light sensitive quantum-dot switch, a beam of light caesium atoms, rendering them reflective to beam courses through the second optical line, is first guided along a material dotted with light trying to cross the cloud (see ‘Turn off the ring heats up and swells, interfering with holes, called a photonic crystal. The light can the light’). One photon can thus block the pas- the main beam and switching off the transistor. pass through a quantum dot placed in its path sage of about 400 other photons, says Chen, This switch can flip on and off up to 10 billion without changing course. But if a pulse of light who presented the result on 7 June at a meet- times per second. is sent in just ahead of that beam, it can induce ing of the American Physical Society’s Division And the output beam can fan out and drive an interaction between the dot and the crystal of Atomic, Molecular and Optical Physics in two other transistors, meeting one of the estab- that scatters the beam and sends it on a differ- Quebec City, Canada. lished requirements2 for an optical transistor ent path. The ability to turn a strong signal on and off set out in 2010 by David Miller, a physicist at Reported in May 2012 by Edo Waks of the using a weak one fulfils a key requirement of Stanford University in California. Other cri Joint Quantum Institute at the University of an optical transistor. “Nothing even came close teria include matching the frequency of the Maryland in College Park and colleagues3, it before,” says physicist Ataç İmamoğlu of the exiting signal to the input frequency and keep- switches when struck by a pulse of 140 pho- Swiss Federal Institute of Technology Zurich, ing the output clean, with no degradation that tons. In principle, that is a small enough who called the experiment “a true break- could cause errors. “Making an optical transis- amount of energy to rival conventional routers. through”. In theory, the hundreds of photons, tor that really satisfies the necessary criteria is But the switch still faces a practical obsta- controlled by the triggering photon, could fan very hard,” says Miller. cle common to all of these emerging optical out and switch off hundreds of other transis- Still, Qi does not expect to challenge the technologies. The lasers that supply the devices tors in an optical circuit. electronic transistor with his optical analogue, with light consume considerable energy, offset- With its exotic clouds of atoms and bulky which consumes a lot more power and runs ting any savings. “Right now,” says Waks, “the equipment, the proof-of-principle transistor much more slowly. “We overhead is what’s killing us.” ■ is unlikely to become a component in every- NATURE.COM want to complement For more on 1. Varghese, L. T. et al. Preprint at http://arxiv.org/ day computers. But it could be a useful tool the Intel transistor,” he abs/1204.5515 (2012). for studying how photons interact at the quan- alternative says. “We don’t want to 2. Miller, D. A. B. Nature Photon. 4, 3–5 (2010). tum level — potentially leading to a quantum transistors, see: replace it.” He hopes to 3. Bose, R., Sridharan, D., Kim, H., Solomon, G. S. & transistor that flips, not a one or a zero as in go.nature.com/r6dl2q find a foothold in niche Waks, E. Phys. Rev. Lett. 108, 227402 (2012). 13 JUNE 2013 | VOL 498 | NATURE | 149 © 2013 Macmillan Publishers Limited. All rights reserved.

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