Tuesday, October 20, 2009

Trapped Rainbows:Applications for Quantum Computing?

Light can be slowed down so that a beam of sunlight can travel at a leisurely stroll, be brought to a standstill, or even stored

for later use in the form of a rainbow.

Today, details of an exotic kind of material that can slow light from its top speed of around one million million metres per hour

so that it can be trapped as a crescent of colour is published by a team that suggests if could mark a revolution in computing.

This remarkable feat could allow "broadband storage" for "broadband computing" capable of much greater power than conventional

silicon chips because it can process information in the form of many light beams simultaneously, just as optical fibres can carry

lots of conversations simultaneously. And it could also mark an advance in quantum computing, named after the strange quantum

properties of matter at the atomic level, that could enhance the power of computers millions of times beyond anything available


The extraordinary feat of optical sorcery is described today in the journal Nature by Prof Ortwin Hess and Kosmas Tsakmakidis at

the University of Surrey, working with Professor Alan Boardman from Salford University.

Once theory is turned into reality, the technique will allow the use of light rather than electrons to store memory in devices

such as computers. The team predicts an increase in operating capacity of 1,000% over the use of conventional electronics by

exploiting light's broad spectrum to lay down lots of different information simultaneously in the first "optical capacitor."

Slow light could also, paradoxically, be used to increase the speed of optical networks, such as the Internet. At major

interconnection points, where billions of parcels of information from myriad phone calls arrive simultaneously, these materials

could be used to slow, divert and allow through information, working in the same way as traffic congestion calming schemes do on

motorways, when a reduction in the speed limit can lead to a swifter overall flow of traffic.

Previous attempts to slow and capture light have until now required extremely low temperatures, have been extremely costly, and

have only worked with one specific frequency of light at a time. The new technique proposed by Prof Hess and Kosmas Tsakmakidis

involves the use of exotic "metamaterials" with extraordinary optical properties.

These materials, which consist of carefully-designed shards of metal a billionth of a metre across (nanometres), have the strange

property of negative refraction, which means that light bends in the opposite direction to the way it shifts when passing from one

ordinary material into another (think of how a straight stick in water looks bent.) This means that, in theory at least, a

metamaterial could be designed so that light would curve around it, making the object invisible, an idea already under serious

study for cloaking devices. When combined with the "Goos Hänchen effect," where light can even go backwards, Prof Hess's team has

shown it is possible to use metamaterials to halt a light beam in its tracks.

As different component 'colours' of white light have different frequencies (colours) each individual frequency be stopped at a

different stage of a wedge of such material, he said, likening the way the light slows down to walking on shingle. At the point

that every step of the light beam forward leads to an equivalent slip backwards in the metamaterial, an effect that depends on the

colour of the light.

The result is a 'trapped rainbow'. "The key to understanding the trapping of the rainbow," he says, "is that every frequency of a

white light wave packet has in a tapered shape of metamaterial its own particular width where it eventually stops – the result is

the spatial spread of stopped/trapped light – the trapped rainbow." Prof Hess said that an onlooker could in an appropriate

arrangement of metamaterial layers (as shown here) see the rainbow of trapped light.

This ability to store light will conceivably provide a powerful new tool to control optical information, even harness the quantum

properties of atoms, and so exploit the possibilities of quantum computers that, in theory, will be able solve problems millions

of times faster than current machines.

The extraordinary properties of quantum computers were first explored by theorists such as the late Richard Feynman at Caltech and

David Deutsch of Oxford University. While a conventional PC shuffles information in the form of binary numbers, those containing

only the digits 1 and 0, which it remembers as the "on" and "off" positions of tiny switches, or "bits."

By contrast, the switches in a quantum computer can be both "on" and "off" at the same time. A "qubit" could do two calculations

at once, two qubits would do four and so on. Thus, it was theoretically possible to use quantum computers to explore vast numbers

of potential solutions to a problem simultaneously. The new work, which suggests a way to create optical qubits, adds to a range

of recent advances that make scientists confident that quantum computers will be feasible within a few years.

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