Taming the Speed of Light to Aid Computing

In Nascent Chip Technology,
More Data Could be Stored
While Using Less Power

Light, as every physics student knows, is the fastest thing there is. But now International Business Machines Corp. researchers may have found an effective way to carry and store data by slowing light down.

The IBM researchers report, in a paper published today in the journal Nature, that they have developed a method to use light as an effective data carrier on the standard silicon chips that are at the heart of practically every computer and electronic device.

The breakthrough makes it more feasible for computer and other high-tech companies to follow in the path of the telecommunications industry, which is abandoning traditional copper wires for cables that transmit data on the back of light waves.

Tom Theis, director of physical sciences at IBM's T.J. Watson Research Center, says the work is "exploratory research," but "the physics that is demonstrated is key to building several small optical communications devices." Any applications are likely years away, says Fred Zieber, president of Pathfinder Research Inc., a semiconductor consulting and analyst firm in San Jose, Calif. Still, he adds, "these are important building blocks to putting light-based systems together."

Scientists are intrigued by using this method in computers because light pulses can encode more data than an electric charge and can be sped along with far less power. Using light pulses at Internet traffic junctions also could speed the flow of information around the Web.

Applying light to silicon is particularly attractive commercially because silicon dominates the semiconductor universe; high-tech companies have dedicated billions of dollars and decades of research to silicon-chip manufacturing.

Nevertheless, the circuitry that rules the roost inside microprocessors and on computer motherboards is electrical, relying on charges and currents, not light, to speed the flow of information. That is because electrons can easily be stopped, stored and switched around different paths, an agility necessary for the intricate workings of complex processors.

Light is a difficult beast to harness. In a vacuum, it zips around at 186,282 miles a second. It would traverse a one-inch-wide chip in 85 trillionths of a second. While speed is desirable, light is so fast that it would overwhelm the equipment that tells the pulses where to go in the system. There's also no easy way to "store" a light particle, or photon, while it waits to be directed to its destination.

The IBM researchers report that light can be reined in, at least somewhat, on a silicon chip built on regular semiconductor-manufacturing equipment. They slowed it by a factor of 300 and demonstrated a method to control the light and rapidly adjust its speed -- a key result for computing applications.

Light slows when passing through some materials. Through glass at normal temperatures, for instance, visible light slows down by about a third. In recent years, experiments have shown that light pulses can be tamed to the speed of a leisurely stroll, or even brought completely to a halt. These tricks, however, require exotic laboratory concoctions, such as clouds of gas cooled to very low temperatures, to impede the pulse's flow.

The IBM researchers, though, found a way to slow light pulses with silicon. Head researcher Yurii Vlasov and his team built "photonic crystal waveguides," silicon membranes 223 nanometers thick (less than a hundred-thousandth of an inch) and riddled with tiny holes. The waveguides trap the light pulses and slow them down. At 1/300th its normal speed in a vacuum, light cruises along at more than 600 miles a second. But that is slow enough, Mr. Vlasov says, that if 10 billion bits of information encoded in light are coursing each second through a one-millimeter long waveguide, three of those bits will be in the waveguide at once. That is a significant step toward optical storage: The bits in the waveguide are delayed, perhaps while other traffic zooms past at higher speeds.

Using light on silicon chips -- a field called "silicon photonics" -- holds out several areas of interest for computer and communications engineers. Data packets traversing the Internet as pulses of light on fiber-optic cables are converted to electricity, then back to light, at each of the routers directing traffic along the way. The process would be speedier and smoother if the pulse could remain as light. On a smaller scale, light-based "interconnects" promise faster transfer of data between the various chips inside a system. Several major technology companies, including chip giant Intel Corp., are at work in this area.

Silicon photonics is "very, very hot," says Michal Lipson, a Cornell University researcher. Prof. Lipson and her colleagues showed last year that it is possible to use one light beam to directly switch another beam on and off inside a silicon chip, an important precursor to routing network traffic without converting signals to electricity. The field has seen a flurry of research in the past year. Prof. Lipson's work is funded by government grants, but several companies, she says, have expressed interest in it.

Write to Charles Forelle at charles.forelle@wsj.com