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Light Stands Still in Atom Clouds

5:14pm, January 29, 2001

Two independent teams of physicists have overcome the restless nature of light and stopped laser pulses in their tracks. A proposed method, now in tests by a third team, may even make light pulses creep backwards.

It's not normal for photons to stop, let alone to back up. Those quintessential particles of light usually zip straight through a vacuum at 300,000 kilometers per second and traverse other materials at slower but still dazzling speeds.

In a sense, stopping light is a trivial feat, comments Michael Fleischhauer of Kaiserslautern University in Germany, who helped develop the theory behind the new stopping techniques. "Take a black piece of paper and you can stop light very easily," he notes. However, the photons absorbed by that paper are destroyed.

What's remarkable about the new experiments is that the researchers stop a light pulse without destroying the photons, he explains. In each experiment, a so-called coupling laser illuminates a gaseous group of

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