Light computer runs quantum algorithm

By Eric Smalley, Technology Research News

Although the incentive to build quantum computers is strong -- they are potentially many orders of magnitude faster than today's classical computers -- no one is sure yet that they can be built.

A team of researchers from the University of Rochester has set its sights a little lower with a prototype computer that has some elements in common with quantum computing, but operates using interfering lightwaves rather than quantum particles.

While the theory of quantum computers has been proven, there is still a large and potentially lengthy challenge in figuring out how to build them. Wave-based computers, on the other hand, could be built right away. The challenge is in proving that there's any reason to build them.

Although the lightwave computers use some of the same types of algorithms as quantum computers, they can be built today using technology found in telecommunications networks. "The nice thing about optical interference is the technology is very mature, and it's a lot easier to manipulate light than it is to manipulate individual atoms," said Ian A. Walmsley, a professor of optics at the University of Rochester.

The new type of computer would be as fast as quantum computers because its light beams could be used to examine all at once every possible answer to a given problem. The question is whether they would be efficient enough to be practical, meaning they wouldn't require huge amounts of hardware or energy.

Although they would not ever reach the efficiency of quantum computers and so could not be scaled up to handle the largest problems, they might be more efficient than classical computers, making them useful for some problems that are beyond the reach of classical computers.

Lightweight interference computing could be used for some quantum computing applications like search engines that could answer queries to large databases almost instantaneously, according to Walmsley.

The researchers tested their prototype with a version of a quantum search algorithm that uses different colors in a beam of light to query every item in a 50-item database at the same time. They split the beam in two and sent one of the beams through a modulator, which shifted the phase, or position of the wave crest of one of the colors corresponding to the target of the query. The researchers then recombined the two beams and detected which color experienced interference due to the phases being out of alignment.

The algorithm did not search any more efficiently than is possible on a classical computer, but the experiment showed that computations performed using interference between the quantum mechanical wave states of particles can also be done using classical lightwave interference, Walmsley said. The next step for the researchers is finding an algorithm that would make classical lightwave computing more efficient than classical computing, he said.

Most quantum algorithms that are more efficient than classical algorithms require the most counterintuitive aspect of quantum mechanics -- entanglement. In these cases, the quantum computer performs a series of operations on a set of entangled particles. The key to entanglement's usefulness in computing is that groups of particles have an exponential number of possible states and so a relatively small number of them can represent very large numbers.

Because lightwaves do not have this property, they cannot use these algorithms. But there are others that do translate to classical physics. "Some of the quantum algorithms do not rely on entanglement, at least for their speed of operation," said Paul Kwiat, a physics professor at the University of Illinois. "You can certainly do things like the search algorithms just using interference, and as soon as you can do it just using interference then quantum or classical, it doesn't make any difference."

However, the efficiency of most quantum algorithms comes from entanglement. "If you want to have exponential savings in resources so that you don't need to have a gazillion beam splitters on your table or you don't need to have all these different distinguishable frequencies, then entanglement becomes important," he said.

But, not every application needs the power of a full-blown quantum computer. "A dictionary... is a reasonable kind of database that people search that has a pretty finite number of elements in it," Kwiat said. "You might imagine somehow eventually extending these [wave computing] techniques -- it's not exactly clear how -- but somehow encoding your dictionary in some sort of hardware, some sort of fixed system which you could then search very, very quickly," he said.

Researchers are working on quantum algorithms that do not require entanglement but are more efficient than classical algorithms. The University of Rochester researchers are planning to adapt these algorithms to their wave-based computing scheme, said Walmsley.

The challenge for wave-based computing is filling in the conceptual blanks, he said. "Can we actually implement these efficient algorithms using interference? How far can we push into the quantum regime? It may be that we can only get a factor of two or three in efficiency, in which case it's hardly worth investing a lot of money into this stuff. But maybe we can go further," Walmsley said

Once the appropriate algorithms are written, it should take a year or less to implement them using optical technology, he said.

Walmsley's research colleagues were Christophe Dorrer, Matt Anderson, Pablo Londero, Sascha Wallentowitz and Konrad Banaszek of the University of Rochester. They presented the research at the Lasers and Electro-Optics/Quantum Electronics and Laser Science conference held in Baltimore the week of May 7, 2001. The research was funded by the Department of Defense (DoD).

Timeline:   > 1 year
Funding:   Government
TRN Categories:   Quantum Computing; Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Computing with Waves: All-Optical Single-Query 50-Element Database Search," Lasers and Electro-Optics/Quantum Electronics and Laser Science conference, Baltimore, May 7-11, 2001


May 23, 2001

Page One

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Light computer runs quantum algorithm

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Prototype shows electronic paper potential

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