The laser-cooling technology devised by Nobel Laureate Steven Chu will let us build instruments that exploit quantum precision to attain unparalleled sensitivity.
obel-Prize winning physicists are different from you and me. They have more
brains. Those of us who fall on the big part of the bell curve are curious
about the mental processes of heavyweight brainiacs. Do they dream in
mathematical equations? What kinds of thoughts fill their waking moments? Do
they watch TV? Do they have favorite cereals?
We tracked down Steven Chu at home near
Stanford University where he is a professor of applied physics. He spends part
of his time teaching quantum mechanics to graduate students but devotes the
better part of his energies to techniques for controlling and studying matter at
the atomic level.
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Chu's most famous work -- the work that won him the 1997 Nobel Prize in Physics -- has centered on
cooling and trapping sodium atoms. It recognized him for pioneering a novel
technique for cooling them down to 240-millionth of a degree above absolute
zero. What takes the feat beyond ordinary comprehension is how it was done: by
firing an array of six lasers at the target atoms.
To try to diminish the energy of atoms by bombarding them with beams of electromagnetic energy is totally counter-intuitive to anyone with a smattering of physics knowledge. But hardcore physicists like Chu know that matter and energy follow
different rules on the atomic level. While ordinary objects appear to absorb
energy along an uninterrupted continuum, individual atoms are governed by the
laws of quantum mechanics which restrict energy changes to discrete jumps. Chu
ingeniously exploited Doppler shifts in wavelengths produced by moving objects to create what he calls an "optical molasses" effect. The lasers are tuned to a wavelength which, when doppler-shifted, can be absorbed by atoms moving toward the laser, thereby progressively slowing them until most of their kinetic energy has been lost. The laser has no effect on atoms with little motion along the laser's axis.
What's the point of supercooling atoms? For people like Chu, there is no higher
purpose than to show himself and fellow physicsts that it can be done. In
devising his elegant solution to the cooling problem, he also attained something
of a holy grail of atomic physics -- a method for trapping high densitities of
atoms into relatively small spaces. Chu is credited with the simple but novel
approach of cooling atoms first before trying to trap them. This was a
revelation to scores of scientists who had been trying without success to do it
in the reverse order.
By doing the intellectual heavy lifting of conceptualizing and proving out novel
techniques in an important area of atomic physics, Chu has also created a
bonanza for inventors and engineers working on everything from
more precise atomic clocks to better equipment for locating mineral deposits
from airplanes. Laser-cooling is still in its infancy, but its potential to
create and revolutionize new industries could ultimately rival the laser.
Steve Chu was born February 28, 1948 in St Louis, Missouri where his China-born
father was teaching chemical engineering. He grew up in Garden City, New York.
After kindergarten he began building model airplanes, then graduated to erector
sets in the fourth grade. That led to chemistry experiments and model rocketry.
Chu did also participate in sports like touch football, baseball, basketball and
tennis, but his life work would be an extension of his boyhood love of
tinkering.
As a freshman at Rochester he assumed that he would become a theoretical
physicist, generally considered the field's more glamorous side. For graduate school
Chu chose Berkeley with an eye toward following in the footsteps of the great
theoretical physicists, especially Feynman. As he entered the upper division he
reconciled himself to his natural love for experimental physics with all its extravagant tinkering. One of his
post-doc Berkeley experiments involving table-top lasers ignited his interest in
working with what was then an exciting experimental technology.
After finishing both his graduate and post-doc, Chu moved to Bell Laboratories in
1978. There he enjoyed the luxury of being able to devote all his energies to
devising and carrying out blue-sky experiments without the tight funding
reins imposed on most commercial-world experimenters. His first
experiment focused on energy transfer in ruby. By 1983 he had established
himself as head of Bell Labs' Quantum Electronics Research Department at
Holmdel, New Jersey. That was where Steve Chu met Art Ashkin. Ashkin had
pursued efforts to trap atoms with lasers, but had had funding cut four years
earlier. Chu took up
the mantle and, after pursuing it for a number of years at Stanford, devised an
ingenious solution to the goal Ashkin had been pursuing. This work
would ultimately win him the 1997 Nobel prize in physics.
GS: Did you ever think you might win a Nobel Prize for your work on using lasers to cool sodium atoms?
SC: Not in the very beginning.
GS: At what point did you begin to feel it might be significant enough for a Nobel?
SC: I would say in the early '90s. There are two types of Nobel prizes. One is a completely unexpected discovery and one is a method of technology or set of
advances that end up being used very widely in the scientific community.
Laser-cooling trapping was of that variety. Some people thought you couldn't do
it. Once they thought about it, most people thought it might be possible. But
once it was done and once people started to see the impact it could have in
other branches of science as well as in atomic physics and optical science, then
it catches on, has a life of its own.
GS: According to your autobiography, the basic idea for using lasers to cool atoms originated from someone named Art Ashkin.
SC: That's correct. I have a Golden Rule: No one invents anything out of whole
cloth. There are always precursors to everything. I think Art Ashkin -- and
even before Art Ashkin... well Ashkin was thinking of ways to hold onto atoms as
was Leticoff. Ashkin in particular showed that it could be possible under
certain circumstances to do this with macroscopic objects. Macroscopic means
micron-sized objects. But the idea and focus at that time was to try to hold
onto atoms with light.
GS: When you say macroscopic, micron-sized, you mean that it's bigger than an atom.
SC: Yes.
GS: So it was thought that it would not be possible to do it with something as small as an atom?
SC: At the time I remember that Art Ashkin among others had published papers saying
it may be possible to do this but no one was actually acting on this because if
you looked at the numbers, things looked bad. When I was doing this in 1983,
1984, it looked like you couldn't hold onto atoms for very long periods of time
before the light would heat them up, so the scattering of photons from the laser
beam used to hold onto the atoms would heat them up and they'd boil out of the
trap. So it didn't look as though, first, that it would be good for much. But
people weren't really focusing on that. The question then was, could you do it.
And it looked like... in his papers [Ashkin] tried to highlight types of traps,
what we would call large volume traps, traps that had large enough volume that
you could pack an appreciable number of atoms in there at any given time so you
could detect what you've done. So things looked pretty bleak in the late 70s,
early 80s, especially after one began to appreciate the heating effect on the
atoms absorbing rescatterin photons.
GS: Why would anyone think of using light to trap atoms?
SC: That's actually not a bad choice because what you normally think of using are
electric, magnetic or electro-magnetic fields to trap atoms. Those are the only
forces at our disposal.
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obel-Prize winning physicists are different from you and me. They have more
brains. Those of us who fall on the big part of the bell curve are curious
about the mental processes of heavyweight brainiacs. Do they dream in
mathematical equations? What kinds of thoughts fill their waking moments? Do
they watch TV? Do they have favorite cereals?