At first blush the idea of cooling groups of atoms, dramatically slowing their normally frantic motion by illuminating them with laser light, seems impossible. Normally, shining light on something heats it up.

Steven Chu at a press conference the morning he learned of winning the Nobel Prize in physics. (Image credit: L.A. Cicero)

In 1985, however, Steven Chu and his colleagues at AT&T Bell Laboratories found special circumstances where lasers formed what Chu dubbed “optical molasses,” a condition where the intense light acted to slow the motion of target atoms much like the motion of a marble is slowed when it plunges into actual molasses. They also developed the first atomic trap that held the chilled atoms in place, instead of letting them fall under the influence of gravity.

That research has now earned Chu, the Theodore and Frances Geballe Professor of physics and applied physics, a share of the 1997 Nobel Prize in physics. Co-recipients of the award are Claude Cohen-Tannoudji, a professor at the Collège de France and École Normale Supérieure in Paris, and William D. Phillips, who works at the National Institute of Standards and Technology in Gaithersburg, Maryland.

By slowing atoms down from typical speeds of 4,000 kilometers per hour to speeds of less than a tenth of a kilometer per hour, optical molasses had made these atoms much easier to study. Instead of rapidly disappearing, atoms caught in optical molasses form what to the naked eye looks like a glowing cloud the size of a pea. Previously, scientists could control the speed of electrically charged atoms by using electrical and magnetic fields. Optical molasses extended this capability to electrically neutral atoms for the first time.

As the Nobel prize committee mentioned, this technique has proven to be a powerful tool for increasing scientific knowledge about the interplay of light and matter. In particular, it has provided scientists with a greater understanding of the quantum-dynamical nature of gases at extremely low temperatures. Building on this work, for example, other scientists have been able to create a bizarre new state of matter, whose existence was originally postulated by Albert Einstein 70 years ago. In this state of matter, called a Bose-Einstein condensate, a group of atoms is chilled to such a low temperature that the atoms’ motion nearly stops and they begin acting like a single entity, a kind of super atom.

Scientists are using these techniques to design more precise atomic clocks for use in space navigation, atomic interferometers to provide ultra-precise measurements of gravitational forces, and atomic lasers, which might one day be used to manufacture extremely small electronic components.

Much of Chu’s work at Stanford has been to apply and extend these techniques in new areas. Among other things, Chu and his students have constructed an atomic fountain. Laser-cooled atoms are sprayed upward from an atomic trap like a jet of water. At the very top of the trajectory, the atoms are almost motionless for an instant. At that moment microwaves are beamed at them that provide information about the atoms’ inner structure. This may provide the basis for an atomic clock with a precision one hundredfold greater than at present.

Chu’s laboratory also has been applying an interesting spin-off of the technique to study the physical characteristics of individual polymer molecules: a marvelous tool called optical tweezers. It is a kind of microscopic version of a Star Trek tractor beam. The scientists can use laser light to grip and manipulate a number of different kinds of microscopic objects immersed in water.

In two papers published earlier this year, the scientist and his students have demonstrated that studying polymers one-by-one can provide important new insights into the way in which the properties of polymeric materials, like plastics and synthetic and biological fibers, arise from the collective action of large numbers of individual molecules. In one study, the researchers found that individual polymer molecules appear to express a surprising degree of individuality. When forced to unravel in a strong current, apparently identical molecules unwind in highly individual and unpredictable ways.