Thursday, February 22, 2007

Light and Matter United

Albert Einstein and just about every other physicist insisted that light travels 186,000 miles a second in free space, and that it can't be speeded-up or slowed down. But in 1998, Lene Hau, for the first time in history, slowed light to 38 miles an hour, about the speed of rush-hour traffic.

From an article by William Cromie:

Two years later, she brought light to a complete halt in a cloud of ultracold atoms. Next, she restarted the stalled light without changing any of its characteristics, and sent it on its way.

Now her team has made a light pulse disappear from one cold cloud then retrieved it from another cloud nearby. In the process, light was converted into matter then back into light. For the first time in history, this gives science a way to control light with matter and vice versa.

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A weird thing happens to the light as it enters the cold atomic cloud, called a Bose-Einstein condensate. Atoms at room temperature move in a random, chaotic way. But when chilled in a vacuum to about 460 degrees below zero Fahrenheit, under certain conditions millions of atoms lock together and behave as a single mass. When a laser beam enters such a condensate, the light leaves an imprint on a portion of the atoms. That imprint moves like a wave through the cloud and exits at a speed of about 700 feet per hour. This wave of matter will keep going and enter another nearby ultracold condensate.

That's how light moves darkly from one cloud to another in Hau's laboratory. This invisible wave of matter keeps going unless it's stopped in the second cloud with another laser beam, after which it can be revived as light again.
Coherent control of optical information with matter wave dynamics. Naomi S. Ginsberg, Sean R. Garner, Lene Vestergaard Hau. Nature 445:623-626, 2007.