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.
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
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
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
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
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
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.