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In the fall of 1987, Steven Chu had just taken a position as professor of physics at Stanford University. Writer Keith Wailoo (now a professor in the Departments of Social Medicine and History at the University of North Carolina at Chapel Hill) dropped by Chu's offices at Bell Laboratories to interview the young scientist, who had developed an optical technique for slowing down atoms so that they could be studied-work that has led to many applications and was antecedent to the recent creation of a new state of matter called the Bose-Einstein condensate. Atoms in a gas at room temperature zip around at about 4,000 kilometers per hour, but Chu uses the light from lasers to "cool" gases to temperatures in the microkelvin range, which slows the atoms to speeds of a few tens of centimeters per second. Chu says that laser light acts as a sort of optical molasses. Associate Editor Mike May recently interviewed Chu again, after he was selected, along with Claude Cohen-Tannoudji and William Phillips, to receive the 1997 Nobel Prize in Physics. We begin with a few excerpts from the interview published in our January-February 1988 issue. The sketches are from Steven Chu's notebooks.
1988
Could you explain something about the physics of slowing those atoms down-of creating what you came to call optical molasses?
It's very simple. When an atom absorbs a photon from the laser, it absorbs the momentum of the photon. And every time it absorbs a photon, its velocity slows down by three centimeters a second. It then reradiates the photon with no preferred direction; what this means is that if you average over many photon absorptions, the atom constantly gets momentum kicks in the direction of the laser beam.... [T]he atom can absorb a photon and reradiate it in about 30 nanoseconds. So in the course of one second you can get a pretty substantial force on the atom, something like 100,000 times the force of gravity. This means that over the course of a millisecond you can slow an atom to an insect's crawl.
What are the actual dynamics of the molasses?
When I was making a back-of-the-envelope V"calculation of how long the molasses would keep the atoms in this region, I was thinking...