Putting the Brakes on Light Speed
Friday, January 19, 2007
Scientists said yesterday that they had achieved a long-sought goal of slowing waves of light to a relatively leisurely pace and using those harnessed pulses to store an image.
Physicists said the new approach to taming light could hasten the arrival of a futuristic era in which computers and other devices will process information on optical beams instead of with electricity, which for all its spark is still cumbersome compared with light.
Even the best fiber-optical systems today rely on intervening electrical signal processors, because no one has figured out a practical means of putting the brakes on light at critical junctions.
The new experiments bring scientists closer to that goal.
"We only have to turn a knob and it slows," said John C. Howell, the University of Rochester physicist who led the effort, described in the Jan. 22 online issue of the journal Physical Review Letters.
The achievement is the latest in the fast-paced field of "slow light" -- a discipline that barely existed a decade ago. While other researchers have dragged light to slower speeds than the Rochester scientists, who got it down to one-three-hundredth of its normal velocity, the new method is far simpler. That means the dream of domesticating one of nature's most feral forces for use in computing, image processing and a host of military and homeland security applications could be nigh.
"This is a big step toward bringing slow-light technology into practical usage," said Steve Harris, a professor of electrical engineering and applied physics at Stanford University.
As the fleetest form of energy in the universe, light has the potential to revolutionize a wide range of technologies. Pulses of light can substitute for the digital "ones" and "zeros" that are today conveyed by relatively massive electrons on silicon chips. Light waves can also carry detailed images, encrypted in their complex, self-propagating ripples.
But light's great attractions -- its outlandish speed and general unwillingness to slow down -- also pose a huge challenge. That's because to be useful, bits of information must coordinate their travel with countless other bits -- in some cases yielding to other data streams and in other cases merging to make a splintered message or image whole.
Chipmakers long ago perfected the art of traffic control for electricity, using transistors to halt and release electrons at microscopic gates. But light is not so easily regulated. Moreover, when light is slowed -- by passing through a dense material, for example -- its wave form changes, typically resulting in a loss of whatever information it was carrying.
"It's not enough to slow the light down," said Lt. Col. Jay Lowell, a program manager at the Defense Department's Defense Advanced Research Projects Agency, which has funded much of the latest work. Unless information can survive the ride, he said, "it's just going to be a scientific curiosity."
Howell and his colleagues created a four-inch-long chamber filled with cesium gas heated to about 212 degrees Fahrenheit. When they sent pulses of laser light through that gas, the cesium atoms put the brakes on the leading edge of that wave, creating a photonic traffic jam.
"Essentially, the light just piles up," Howell said.
Once the slowed light exits, it naturally resumes it normal velocity -- 300 million meters per second, or fast enough to circle the Earth seven times in one second.
Most important, the peaks and troughs of Howell's light waves remained in phase as they stacked up, meaning they did not get out of step and cancel one another out. That is key, because phase is one aspect of light that carries information.
To prove that their slowed light did not get scrambled, the team sent their beam through a tiny stencil, less than one-fourth inch on each side, with the block letters "UR" -- the university's initials. Like a shadow-puppet image, that "UR"-shaped beam passed through the chamber, slowed and then emerged with its block letter message intact, as detected by a camera at the end.
Unlike most other systems for slowing light, this one worked at very low light levels. In one experiment, the "UR" image was clear even when a single photon -- the smallest possible quantity of light -- was beamed through the stencil.
The cesium-induced delays were brief, on the order of a few billionths of a second each. Looked at differently, each instance amounted to a two-foot long beam of light being compressed to less than four inches.
But that seemingly small degree of braking is nothing less than a means of storing, or "buffering" waves of light. Moreover, the system is tunable. With the turn of a knob the temperature in the chamber can be changed, which alters how much the incoming light is slowed and stored.
"This is a tremendous boon," said Alan Willner of the University of Southern California, president of the Lasers and Electro-Optics Society of the Institute of Electrical and Electronics Engineers. "Unless you have control over time delays, you can't have the beauty of high-speed optical information processing."
The new work complements previous efforts. A few years ago, scientists at Harvard, Stanford and the Rowland Institute for Science in Cambridge, Mass., created a weird, plasmalike substance called a Bose-Einstein condensate, which is to light what quicksand is to feet. That slowed light to just 38 mph, about one-20-millionth its normal speed. But it required temperatures of about 450 degrees below zero, and the process was not friendly to carrying information.
More recently, researchers at IBM's T.J. Watson Research Center in Yorktown Heights, N.Y., etched a series of tiny "race tracks" -- 100 of them crammed within a tenth of a square millimeter -- on the surface of a silicon chip and forced beams of light to perform more than 50 laps around each track before each was allowed to move on. Those extra laps added up to a delay of about a half a billionth of second -- long enough for about 10 bits of information to pile up.
Optical image processing could allow automated comparisons of facial images from security cameras to images maintained by law enforcement officials. It could also become a valuable tool for scientists studying subtle changes in microbes or other kinds of cells over time.
Optical processing also is likely to ease the storage of holographic images directly on hardware and could lead to breakthroughs in cryptography, the science of making and breaking codes.