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Steven Chu
Nobel Physicist


The laser-cooling technology devised by Nobel Laureate Steven Chu will let us build instruments that exploit quantum precision to attain unparalleled sensitivity.

by Tom Kagy

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Steven Chu
Laser Super-Cooler

obel-Prize winning physicists are different from you and me. They have morebrains. Those of us who fall on the big part of the bell curve are curiousabout the mental processes of heavyweight brainiacs. Do they dream inmathematical equations? What kinds of thoughts fill their waking moments? Dothey watch TV? Do they have favorite cereals?

    We tracked down Steven Chu at home nearStanford University where he is a professor of applied physics. He spends partof his time teaching quantum mechanics to graduate students but devotes thebetter part of his energies to techniques for controlling and studying matter atthe atomic level.

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    Chu's most famous work -- the work that won him the 1997 Nobel Prize in Physics -- has centered oncooling and trapping sodium atoms. It recognized him for pioneering a noveltechnique for cooling them down to 240-millionth of a degree above absolutezero. What takes the feat beyond ordinary comprehension is how it was done: byfiring 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 followdifferent rules on the atomic level. While ordinary objects appear to absorbenergy along an uninterrupted continuum, individual atoms are governed by thelaws of quantum mechanics which restrict energy changes to discrete jumps. Chuingeniously 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 higherpurpose than to show himself and fellow physicsts that it can be done. Indevising his elegant solution to the cooling problem, he also attained somethingof a holy grail of atomic physics -- a method for trapping high densitities ofatoms into relatively small spaces. Chu is credited with the simple but novelapproach of cooling atoms first before trying to trap them. This was arevelation to scores of scientists who had been trying without success to do itin the reverse order.

    By doing the intellectual heavy lifting of conceptualizing and proving out noveltechniques in an important area of atomic physics, Chu has also created abonanza for inventors and engineers working on everything frommore precise atomic clocks to better equipment for locating mineral depositsfrom airplanes. Laser-cooling is still in its infancy, but its potential tocreate and revolutionize new industries could ultimately rival the laser.

    Steve Chu was born February 28, 1948 in St Louis, Missouri where his China-bornfather was teaching chemical engineering. He grew up in Garden City, New York.After kindergarten he began building model airplanes, then graduated to erectorsets in the fourth grade. That led to chemistry experiments and model rocketry.Chu did also participate in sports like touch football, baseball, basketball andtennis, but his life work would be an extension of his boyhood love oftinkering.

    As a freshman at Rochester he assumed that he would become a theoreticalphysicist, generally considered the field's more glamorous side. For graduate schoolChu chose Berkeley with an eye toward following in the footsteps of the greattheoretical physicists, especially Feynman. As he entered the upper division hereconciled himself to his natural love for experimental physics with all its extravagant tinkering. One of hispost-doc Berkeley experiments involving table-top lasers ignited his interest inworking with what was then an exciting experimental technology.

    After finishing both his graduate and post-doc, Chu moved to Bell Laboratories in1978. There he enjoyed the luxury of being able to devote all his energies todevising and carrying out blue-sky experiments without the tight fundingreins imposed on most commercial-world experimenters. His firstexperiment focused on energy transfer in ruby. By 1983 he had establishedhimself as head of Bell Labs' Quantum Electronics Research Department atHolmdel, New Jersey. That was where Steve Chu met Art Ashkin. Ashkin hadpursued efforts to trap atoms with lasers, but had had funding cut four yearsearlier. Chu took upthe mantle and, after pursuing it for a number of years at Stanford, devised aningenious 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 ofadvances that end up being used very widely in the scientific community.Laser-cooling trapping was of that variety. Some people thought you couldn't doit. Once they thought about it, most people thought it might be possible. Butonce it was done and once people started to see the impact it could have inother branches of science as well as in atomic physics and optical science, thenit 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 wholecloth. There are always precursors to everything. I think Art Ashkin -- andeven before Art Ashkin... well Ashkin was thinking of ways to hold onto atoms aswas Leticoff. Ashkin in particular showed that it could be possible undercertain circumstances to do this with macroscopic objects. Macroscopic meansmicron-sized objects. But the idea and focus at that time was to try to holdonto 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 sayingit may be possible to do this but no one was actually acting on this because ifyou 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 timebefore the light would heat them up, so the scattering of photons from the laserbeam used to hold onto the atoms would heat them up and they'd boil out of thetrap. So it didn't look as though, first, that it would be good for much. Butpeople 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 thatyou could pack an appreciable number of atoms in there at any given time so youcould 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 theatoms 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 areelectric, magnetic or electro-magnetic fields to trap atoms. Those are the onlyforces at our disposal.PAGE 2

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“When I was doing this in 1983,1984, it looked like you couldn't hold onto atoms for very long periods of timebefore the light would heat them up.”


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