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Q&A with Lene Hau

As the Danish physicist Lene Vestergaard Hau, a tenured professor at Harvard, explained to me how she slows light down -- and, when it's going at a "comfortable bicycling speed," does something to it that's weird even by the standards of quantum mechanics -- I couldn't resist blurting out the suspicion I'd harbored since reading about her work: "You're going to win a Nobel Prize if you're not careful." Hau, one of nine MacArthur Fellows picked by the MacArthur Foundation in 2005 to represent the history of its "genius" awards, just laughed and went back to energetically explaining the apparatus she had built to experiment with light.

Hau's feat, in 1999, of slowing light -- from what she called its "incomprehensible" rate of 186,000 miles per second to "something you can connect to as a human being" -- was unprecedented. Though Einstein had never declared that you couldn't slow light down -- stipulating only that you couldn't go faster -- the practical difficulties of reducing the speed of light to human scale, as Hau aimed to do, struck her peers as insuperable.

Hau succeeded by cooling sodium atoms down almost to absolute zero, creating what is known as a Bose-Einstein condensate. She then discovered that such a condensate works like "optical molasses," as she put it, on incoming light. (For more detail, see deas.harvard.edu/haulab.)

Hau was initially drawn to working with slow light because she expected it would be "new territory, a new regime of nature." Early this year she made a foray into that new territory with a widely-publicized experiment that can sound like death and resurrection on the quantum scale. After once again crashing a light beam into a sodium condensate, she extinguished the light. The condensate drifted into another such condensate. Then, stroking the second condensate with a laser, Hau released a light beam identical to the original pulse, and sent it on its high-speed way.

IDEAS: You clearly take pleasure in slowing light.

HAU: I absolutely do! There's a tremendous amount of work building the apparatus, getting the experiment to work. But sitting there late at night in the lab, and knowing light is going at bicycle speed, and that nobody in the history of mankind has ever been here before -- that is mind-boggling. It's worth everything.

IDEAS: What are the practical applications?

HAU: Perhaps you remember that when Charles Townes invented the laser, he was asked: What do you think the applications of this laser can be? He scratched his head, thought about it, and said: "Well, if you place a laser on the head of the typewriter, and make a typo, you can zap the typo." That was the wildest thing he could imagine for lasers. Think about what we do with lasers now.

We're bound to find new fundamental physics and new applications for slow light. What exactly? It's exciting to start exploring.

IDEAS: Are there implications for computing?

HAU: Sure. To get high data transfer rates in communicating information, you would love to use optical fibers. The problem is that light is extremely hard to manipulate. So we make a perfect copy of the information carried by the light. We transfer it to matter -- the condensate.

The matter copy is more long-lived. You have time to do all sorts of things to it. And matter is extremely easy to manipulate. You can grab onto it, massage it with laser beams, squish it, change it, and convert it back into light.

IDEAS: So you foresee your work playing a role in computers that perform or calculate with light rather than electricity. Are there also theoretical implications to your work?

HAU: By asking that, you are splitting, making a distinction between theory and experiment. To me, what makes physics physics is that experiment is intimately connected to theory. It's one whole. Physics is about questioning, studying, probing nature. You probe, and, if you're lucky, you get strange clues.

If that part is not there, it's mathematics.

IDEAS: The Large Hadron Collider, the most powerful particle accelerator ever, is about to go into operation in Switzerland. Will you be involved?

HAU: I work at the opposite end of the energy scale. Those guys are up here with billions of electron volts. I'm down here at a billionth of an electron volt.

The interesting thing --coming back to theoretical implications --is that in working with Bose-Einstein condensates we get a tremendous sensitivity. When we create slow light and odd states of matter, we rely on quantum mechanical effects that tend to be very fragile.

Our systems could be extremely sensitive detectors, giving us a different way of probing some of the fundamental questions of physics -- gravity, for example, and extra dimensions, if they exist.

IDEAS: It has been argued that string theory has a stranglehold on physics. As I understand it, string theory tries to unite the macrocosm, gravity, for example, with the microcosm, with quantum mechanics. Are there other models than string theory?

HAU: Even if there are no competing models, that doesn't mean that string theory is right. What is bothersome is you have many people working on it, but it doesn't seem to be testable. Until there are testable consequences, in some sense, it is math.

IDEAS: How did you get interested in physics?

HAU: I thought I would become a mathematician when I started college in Denmark. Studying quantum mechanics changed my mind. I thought it was absolutely fascinating. It hooked me on physics.

IDEAS: Quantum mechanics is full of paradox, isn't it? Aren't you ever disturbed by it?

HAU: Yeah. Then again I like to be disturbed.

IDEAS: You're a tenured professor of physics at Harvard, where there's been a lot of debate about women and science. What is your experience?

HAU: That's a hard one, a hard one. If you look around the other sciences, you see great progress for women, partly because of women entering colleges since the '60s. But physics somehow hasn't moved. Why is that? I don't know. To me, it's odd. It feels totally natural for me to be in physics.

IDEAS: Is it difficult for you to be in a department with mostly men?

HAU: Men have learned to deal with women as students or junior faculty. But there's a phase change when women are actually tenured faculty. Many men -- I shouldn't just say men, many women, many people -- have a hard time dealing with different backgrounds, different points of view.

I don't particularly want to have clones of me running around. That's pretty boring. I have been very surprised. I think I figured something out, I have the perfect argument. Then somebody who has also thought about it brings up an argument: Oh, I hadn't even thought about that.

IDEAS: Are you talking about your discipline or more generally?

HAU: More generally, about the openness I thought would be in academia. But academia is actually very conservative. You would have thought these scholars, particularly if they have tenure, would have the freedom to be free thinkers. But in many cases you see close-mindedness.

Harvey Blume is a writer based in Cambridge. His interviews appear regularly in Ideas. E-mail hblume@world.std.com.

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