A Physics Nobel for the Light Fantastic

Three masters of manipulating atoms with light have received the 1997 Nobel Prize in Physics. The recipients--Steven Chu of Stanford University, Claude Cohen-Tannoudji of the École Normale Supérieure in Paris, and William Phillips of the National Institute of Standards and Technology in Gaithersburg, Maryland--showed how atoms could be chilled to millionths of a degree above absolute zero and trapped with beams of laser light. Aside from basic studies of atoms themselves, the work has led to dozens of novel applications, including astonishingly accurate clocks, high-precision devices for measuring the pull of gravity, and lasers made of coherent "waves" of atoms.

At room temperature, atoms zip around so fast that they are difficult to study and control. But atoms can be slowed with light by taking advantage of the so-called doppler effect, which raises or lowers the frequency of light seen by a moving atom, like a policeman's whistle heard from a passing car. If a laser beam is tuned to a frequency just below one that an atom naturally absorbs, a stationary atom will ignore the light. But if the atom is moving into the beam, the laser photons will be doppler shifted to the higher frequency the atom can absorb. The momentum of each absorbed photon slows the atom, like a tennis ball bouncing off an oncoming basketball.

In 1985, when Chu was at Bell Laboratories, he and his coworkers, acting on earlier suggestions by other groups, showed that with six laser beams, all pointing inward at a cloud of atoms, he could create an "optical molasses" that would slow their motion in all directions, cooling them. Phillips, Cohen-Tannoudji, and their colleagues then greatly extended the work by finding ways to confine the atoms in a long-lasting trap, as well as cool them to temperatures well below those in Chu's original setup.

Out of those successes has come an explosion of applications. In 1995, for example, other researchers used laser cooling in conjunction with other techniques to make atoms so cold that they formed a "Bose-Einstein condensate," in which their quantum-mechanical waves all overlapped to create a new state of matter. The steadiness of laser cooled atoms has led to new atomic clocks that are expected to keep time to one part in 1016. Other applications have turned up in biology, physics, and even sensing gravitational anomalies that can reveal underground oil fields.

The work has "opened up a new world, a new field," says Daniel Kleppner, a physicist at the Massachusetts Institute of Technology. "It's such an extraordinary situation to be able to manipulate atoms and get to these low temperatures."

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