17 May 2001
Light Computer Runs At Quantum Speeds
by Kate Melville 
A simple computer that marries the mind-boggling computing power of quantum mechanics with the ease of manipulating light has been built by researchers at the University of Rochester. The device proves that a specific quirk of atoms, which lets scientists conduct huge computations almost instantly, can be perfectly mimicked by light, which is much more practical to control than individual atoms. The result could be a computer that performs some tasks a billion times faster than today's supercomputers, using relatively simple technology that's already well understood. The research behind the device was revealed at the Lasers and Electro-Optics Quantum Electronics and Laser Science Conference in Baltimore, Md. The device mimics quantum interference, an important property that makes quantum computers exponentially faster at tasks such as breaking encryption codes or searching huge databases. Instead of interference, conventional computers use particles called electrons to perform tasks sequentially, like a librarian looking for a book by inspecting the entire library one volume at a time. Interference essentially allows you to make clones of that librarian—one librarian for every book—and set them all loose at once. The new device proves that using light interference is just as effective as quantum interference in retrieving items from a database. "There's a big push to explore information processing based on quantum mechanics," says Ian Walmsley, professor of optics at the University of Rochester, who lead the team that invented the device. "You can do things with quantum mechanics that are impossible on classical machines.
What we've shown here is that if you have a quantum computer that is based entirely on quantum interference, we can build you a computer that is equally efficient, based entirely on light interference. And light is a whole lot easier to manipulate than quantum systems." One of the biggest limitations of quantum computers had always been thought to be their need for entanglement—a condition where different particles become linked, sharing many similar properties like the librarian clones sharing similarities with each other. Entanglement is difficult to achieve, and so far it has not been done for more than a few particles at a time. Scientists then found that entanglement may not be necessary for operations such as database searches if quantum interference were used. When Walmsley heard this, he was sure he could build a computer that used light interference instead of subatomic particle interference. "We wanted to show that the implementations which have been done with quantum computing have an exact analogy that is just as effective in light-based processes," says Walmsley. Walmsley's device uses a piece of transparent tellurium dioxide called an acousto-optic modulator. This acts as the database by storing the information in the form of acoustic waves. A transducer vibrates against one side of the modulator, sending waves through it much like a stereo speaker would send sound waves through the air. The waves slightly compress some parts of the modulator and slightly expand others, creating a pattern in which the database information resides. To search the database, Walmsley directs a beam of light toward the modulator. The light is first split into two, with one part traveling through a prism so that a rainbow of different frequencies of light shines on the modulator. Each frequency shines through a different compressed or expanded part of the tellurium dioxide, which bends that frequency of light the way a straw appears bent when sticking out of a glass of water. The rainbow of frequencies is then recombined into a single beam. By mixing the new beam with the original beam that entered the device, a single frequency will emerge as having been altered by its trip through the database. So in the case of Walmsley's device, 50 different frequencies of light shine through the modulator, and if the 20th frequency is the altered one, then Walmsley knows that the bit of information he was searching for is located at position 20 in the database. A conventional computer would have had to check 20 times to find the location. If the database in question were the Manhattan phone book, the search for a single phone number could take a conventional computer several million searches, while a light-based device could pinpoint the number in just one. What makes the device particularly attractive is that it is so simple in comparison to quantum computers. Engineers have had decades of experience precisely manipulating light and all the concepts in the device are based on well-known, 19th-century classical physics—though as Walmsley points out, the technology to carry out the experiment only became available in the last 10 years. "In effect, we are leveraging new physics on the back of optical technology; a synergy that is particularly easy at Rochester, and illustrates the close links between basic science and engineering."
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