Laser cooling yields Nobel in physics
(This story appeared on page 263 of the
Oct. 25, 1997, Science News.)
By Sid Perkins
Science News
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 in
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 where atoms 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 the
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, William D. 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 down 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, Claude 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 to 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 can be used in 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 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.
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Copyright 1997 by Science Service.
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