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Former Bell Labs Scientist

Steven Chu Wins Nobel Prize


The idea of using lasers to trap and cool molecules for study began over a lunchtable conversation at Bell Labs in Holmdel, N.J. more than 10 years ago. Today, because of his idea, former Bell Labs researcher Steven Chu is one Nobel Prize in Physics richer.


Holmdel, N.J (October, 1997) -- An idea that sprang up over lunch at a Bell Labs cafeteria a little more than a decade ago has led Steven Chu to the most coveted honor in science. On Oct. 15, the 1997 Nobel Prize in Physics was awarded to Chu, now at Stanford University, and two others, William Phillips and Claude Cohen-Tannoudji, for their development of methods to cool and trap atoms with laser light.

[ Steven Chu ]

Steven Chu

The research that drew the attention of the Royal Swedish Academy of Sciences began at Bell Labs in Holmdel. A dozen years ago, Arthur Ashkin and Chu used to discuss physics regularly at the Holmdel cafeteria. They were interested in manipulating atoms at low temperatures.

An idea that arose during one of those lunches led to a series of experiments by Chu, Ashkin, John Bjorkholm, Alex Cable, and Leo Holberg. Chu left Bell Labs in 1987 to take up a professorship at Stanford, where he continued his work in low-temperature physics.

Ashkin Pioneered "Optical Trapping"

Ashkin, a physicist at Bell Labs, had developed the world's first methods of trapping atoms with the help of lasers. He is considered the father of optical trapping. Normally shining light on an object heats it up, but Ashkin had devised an ingenious method to trap small particles by carefully using the light of lasers.

Chu extended Ashkin's ideas for trapping atoms. Speaking of their work at Holmdel, Ashkin said, "Steve did some absolutely brilliant experiments and I am happy for him. I always thought he would win the Nobel Prize." Ashkin retired from Bell Labs in 1992 after a 40-year career in physics that included election to the National Academy of Sciences.

William Phillips of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., and Claude Cohen-Tannoudji of the Ecole Normale Superieure in Paris, extended the Bell Labs approach and made major contributions of their own. The Nobel Committee heaped praise on the three physicists, saying that they "have contributed greatly to increasing our knowledge of the interplay between radiation and matter."

The Eighth of Bell Labs Nobel Laureates

Bill Brinkman, vice president of Physical Sciences Research at Bell Labs, was enthusiastic about the award.

"This prize is a recognition of the high-quality work in optics that has been the characteristic of our Holmdel laboratory for the past 25 years," he said. He paused, then reflected, "With this latest award, we have eight Nobel laureates [now eleven as of 1998] who have done their prize-winning work at Bell Labs."

Freezing Molecules to Slow Them Down

The methods pioneered by Chu, Phillips, and Cohen-Tannoudji use laser beams to cool gases to temperatures just above absolute zero, the least possible temperature for all substances, which corresponds to -273.15 degrees Celsius or -459.67 degrees Fahrenheit. Once chilled to these very low temperatures, the atoms of the gas are trapped so that their physical properties can be studied in great detail.

At room temperature, the atoms and molecules of gases are in frenetic motion, ceaselessly bumping into one another at very high speeds. For example, molecules of air in a room typically zip across at speeds over 2,500 miles per hour; after going a short distance, they strike other molecules and veer off in different directions. This situation makes it very difficult for physicists to study individual molecules -- the molecules disappear from the area being observed too quickly.

Lowering the temperature reduces the speeds of molecules in gases but can introduce a different problem. When gases cool, they tend to condense into liquids and then freeze into solids. (In scientific parlance, each change in state is called a "phase change.") Every change of phase reduces the spacing between the molecules, making them more difficult to observe.

Avoiding 'Phase Change'

Researchers can avoid phase changes by keeping the density of the gas under observation so low that it is almost a vacuum. But even then, when the atoms and molecules have been cooled to -270 degrees Celsius (3 degrees over absolute zero), they travel at speeds around 250 miles per hour. It is only at temperatures just a few millionths of a degree over absolute zero that atoms and molecules move at speeds of less than a few miles per hour.

Thus, to study individual molecules and atoms, physicists need to cool gases to very low temperatures. Since temperature and the speed of atoms are related, a method that reduces one reduces the other. If you can slow the atoms in a gas down to a crawl, you get extremely low temperatures.

Just Above Absolute Zero

How cold is ten-millionths of a degree above absolute zero? The lowest natural temperature of the universe is the temperature of deep space. But even the frigid realms of interstellar space are almost a million times warmer; the cosmic microwave background that fills all space is at a temperature of about three degrees above absolute zero, or -454 degrees Fahrenheit. (It was discovering this background radiation -- regarded as confirmation of the "Big Bang" theory of the origin of the Universe -- that led to Bell Labs researchers Arno Penzias and Bob Wilson sharing a Nobel Prizes in Physics in 1978.

In 1985, the researchers at Holmdel found a way to use lasers to lower the temperature to just above zero. Lasers delivered intense beams of light that interacted with the atoms, impeding their progress. Since the way the laser light slowed down the atoms was analogous to how marbles are slowed down when falling through a viscous liquid like molasses, Chu dubbed the phenomenon "optical molasses."

Using Six Cooling Beams

Chu, Ashkin, Bjorkholm, Cable, and Holberg vaporized a sodium pellet with a laser and produced an atomic beam. The sodium atoms were hit by a laser beam traveling in the opposite direction, which slowed them down. Then the atoms were conducted to the intersection of six cooling laser beams.

The six beams were opposed in pairs and positioned in three directions at right angles to one another; the effect was that whichever direction the sodium atoms tried to move, they were slowed down and pushed back into the region where the six laser beams intersected. By this ingenious method, Chu and his colleagues managed to cool the sodium atoms to 240 millionths of a degree above absolute zero.

They also managed to trap the cooled atoms by shooting a powerful beam through the "optical molasses." Describing his work to the Bell Labs News right afterwards, Chu said, "Atoms in the molasses take what is called a random walk -- moving around aimlessly as they're hit from all sides by photons. The trap is a tempting resting place."

Developing a Magnetic Trap

William Phillips and co-workers at NIST developed a highly efficient magnetic trap that used it to cool atoms down to 40 millionths of a degree above absolute zero in 1988. Claude Cohen-Tannoudji had contributed greatly to the theoretical understanding of cooling experiments. Between 1988 and 1995, he and his group developed a new method, using helium atoms. They managed to cool the helium atoms produced to less than a millionth of a degree over absolute zero.

These techniques led to the discovery of the Bose-Einstein condensate in 1995. Named after physicists Satyendra Nath Bose and Albert Einstein who postulated its existence over seven decades ago, the Bose-Einstein condensate is an esoteric form of matter that has attracted great interest.

The methods developed by Chu, Phillips and Cohen-Tannoudji may give us atomic clocks that are a hundred-fold more precise, making space navigation much more accurate. Also, efforts are under way to design atomic lasers which may be used in the manufacture of very small electronic components.

Summing up the work, Chu told reporters last week, "It's remarkable how simple curiosity can lead to a lot of practical things." Not to mention the Nobel Prize.


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