Laser cooling yields Nobel in physics.

Using high-intensity lasers to cool atoms may seem paradoxical, but it works -- and works well enough to earn a Nobel prize.

Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips have been awarded the 1997 Nobel Prize in Physics for developing methods of using laser light to chill gases to within a few millionths of a degree of absolute zero. By probing atoms' behavior in this supercold realm, researchers have been able to observe odd quantum effects not apparent in the everyday world. Harnessing such effects may allow more accurate measurement of time and gravity.

Because photons of light carry momentum, they alter the speed and direction of any atom with which they interact. In particular, an atom that repeatedly absorbs a photon head-on and then emits a similar photon in a random direction will be slowed considerably.

This year's physics laureates devised ways of tapping the momentum of laser beams to slow atoms from room temperature, where they travel at speeds of a little more than a kilometer per second, to supercold conditions, in which they move at a glacial few centimeters per second (cm/s).

Chu, now at Stanford University, developed a method of slowing atoms in 1985. He and his colleagues at Bell Laboratories in Holmdel, N.J., used an array of six lasers that converged at a single point in space to create a region they called "optical molasses." The researchers then steered sodium atoms into this space, where they became stuck.

With this technique,, Chu and his team reduced the average speed of atoms to about 30 cm/s, which corresponds to a temperature of about 240 microkelvins. This agreed well with a theoretical calculation of the lowest temperature obtainable by laser cooling -- but it turned out that the theory behind the calculation was incomplete.

In 1988, Phillips and his coworkers at the National Institute of Standards and Technology in Gaithersburg, Md., found that they could use the same method to cool sodium atoms to 43 microkelvins, well below the supposed theoretical limit (SN: 7/23/88, p. 52).

Scientists later realized that they had based their predictions on a simplified model of the sodium atom and that they had not accounted for variations in the laser-induced electric fields within the optical molasses. Complex interactions with those electric fields caused the atoms to slow down, and cool off, more than expected.

Subsequently, Cohen-Tannoudji and his colleagues at the College de France and Ecole Normale Superieure in Paris developed a way to cool atoms even further (SN: 7/16/94, p. 47). By converting the slowest atoms in the optical molasses into a "dark" state, in which they no longer absorb photons, the team was able to chill helium atoms to a mere 180 billionths of a degree above absolute zero, where the atoms moved at the terrapinlike pace of 2 cm/s.

Laser cooling may not lead to colder or more efficient household refrigerators, but it has a remarkable variety of other applications. "This is a scientifically rich area," says Daniel Kleppner, a physicist at the Massachusetts Institute of Technology who served as Williams' thesis adviser in the early 1970s.

The technique has already proven itself in the lab, Kleppner says. It could lead to atomic clocks some 100 times more precise than those currently in use and to supersensitive instruments capable of detecting subtle changes in the gravitational field above mineral or oil deposits.

In 1995. researchers used laser cooling to achieve a milestone in physics, producing so-called Bose-Einstein condensates (SN: 7/15/95, p. 36). More than 70 years earlier, Albert Einstein and Indian physicist Satyendra Nath Bose had predicted such a low-temperature condition, in which atoms fall into the same quantum state and essentially behave as a single atom.