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Stoppinglight

By Peter N. Spotts Staff writer of The Christian Science Monitor / January 25, 2001



Ever since Sir Isaac Newton sent sunlight streaming through a prism, showing there was more to light than meets the eye, scientists have bent it, focused it, amplified it, explained it, even slowed it from a torrid 670 million miles an hour to bicycle speeds.

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But never have they been able to stop it and bottle it for later use.

Until now.

In separate experiments, two teams of scientists say they have caught light in high-tech traps, then released it again unchanged. During its pause, its information was imprinted on atoms in a target gas.

It's a bit like watching a bullet train vanish into a paper-thin sheet of gauze for a period, only to reappear out the other side at full speed with cars in order and people in their original seats, notes Eric Cornell, a physicist at the University of Colorado in Boulder.

The results, researchers say, represent a key step toward building "quantum" computers that use molecules or atoms to store information and carry out calculations that would choke a conventional supercomputer.

This newfound ability to make light pause then release it "is one of the decisive points to make quantum computing possible," says Lene Hau, a Harvard University physicist and leader of one of the teams formally reporting results this week and early next.

And if the gravitas of quantum computing fails to capture some imaginations, there is the gee-whiz aspect of stopping the fastest known phenomenon dead in its tracks.

"That's not a thing to discount," acknowledges David Phillips, a member of the second team, "these are fun experiments to do."

Dr. Phillips, a physicist at the Harvard-Smithsonian Center for Astrophysics (CFA) in Cambridge, Mass., says his team's work came from research into making more-accurate atomic clocks. The clocks help astronomers synchronize and process signals coming in from radio-telescopes around the world observing the same object at the same time.

"One of our latest projects was to design a new set of rubidium clocks as small and as stable as existing clocks. The equipment you need to study the clocks is about equal to what you'd need to stop light," he says.

Last May, CFA physicist Mikhail Lukin proposed a way that the team could use the equipment to stop light.

The team filled a 3-in. by 3/4-in. glass tube with rubidium vapor and helium, then aimed a specially tuned laser at the target. The team first hit the target with a "control" burst that altered the rubidium atoms so they would not absorb the light in the manner they typically do. Then they fired the "signal" pulse that contained information they wanted to store.

The signal pulse lasted about 10 to 30 microseconds - long enough for the beam to stretch 2 kilometers if it had been shining in space. When the beam hit the rubidium, it was as though it hit light's equivalent of molasses. Its speed dropped to about 2,000 miles an hour. Meanwhile, the tail of the beam kept coming, compressing the signal beam within the rubidium cloud.

Then the team smoothly dimmed the control beam, shutting it down in about 3 microseconds. While the control beam's tail kept coming, the beam inside the rubidium effectively vanished - its information stored by uniform changes it made in a property of the atoms known as spin. When the team blasted the rubidium again with the control beam, the atoms released the information as a pulse of light identical to the pulse that entered the rubidium trap.

The portion of the beam trapped in the rubidium was squeezed into a length a hundred thousand times shorter than its free-space length, the team estimates.

"We were able to confine the beam for hundreds of microseconds," Phillips says.