Light crawls through cold-atom cloud

Science News, March 27, 1999 by P.W.

A bunch of cold atoms are dragging down light's speedy reputation. While many materials retard the velocity of a beam, a new experiment has slowed a traveling light pulse to an unprecedented crawl. It was sodium atoms fine-tuned by a laser and chilled to less than 50 nanokelvins that put on the brakes.

Usually, a light pulse rockets through space at 300 million meters per second. Passing through the laser-influenced atom cloud, it pokes along at just 17 meters per second. At that pace, it would hardly keep up with a hard-pumping bicyclist.

"This experiment is the stuff that Nobel prizes are made of," comments Marian O. Scully of Texas A&M University in College Station. "It's a giant step in quantum control."

Lene Vestergaard Hau of the Rowland Institute for Science in Cambridge, Mass., and her colleagues report the slowdown in the Feb. 18 Nature. "It's really opening up a lot of new exciting things you can start doing," she says.

For instance, it raises the prospect of using a few photons to control optical properties of materials, optics experts say. The new technique may also lead to novel telecommunications components, such as optical delay lines and switches.

To achieve the slowdown, Hau's team first induced the sodium atoms to form a superatom, or Bose-Einstein condensate (SN: 11/28/98, p. 342). The condensate is as opaque as lead until interaction with a specially tuned laser makes the cloud transparent by preventing the traveling light pulse from permanently losing energy to the atoms' electrons. The laser-atom system saps the pulse's energy in such a way that the pulse recovers its energy, and its speed, upon exiting the cloud, Hau explains.