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College Row


Assistant Professor of Physics David Hall (left) works on his cooling device with a student, Ted Reber ’03. The cooling chambers are at the far end of the machine; the elements in the foreground tune and direct laser beams.

Redefining cold

It’s cold at the end of the cosmos, in the blackness beyond the last star. But on most days David Hall’s laboratory in the Merrill Science Center is colder than that—several billion times colder. It is, in fact, the coldest place in the universe, at least for 30 seconds at a time.

The device that creates these low temperatures looks like a Rube Goldberg fantasy, and was hand-built by Hall ’91 (now an assistant professor of physics at Amherst) and his students entirely from scratch, using various electronic, optical and mechanical parts, carefully aligned and optimized to create and handle delicate atomic samples. Despite its homemade appearance, it is a supremely sophisticated piece of equipment, one that accomplishes something that scientists have been trying to do for 70 years. There are only about 50 such machines in the world, and the one at Amherst is the only one at an under­graduate institution.

Its purpose is to create a new form of matter called a Bose-Einstein condensate. The condensate, predicted by Albert Einstein in 1924, allows physicists for the first time to see atomic processes. Einstein calculated that if one could cool a dilute gas to a temperature a few tens of billionths of a degree above absolute zero (the point where all molecular motion stops—zero on the Kelvin scale; minus 459.67 degrees Fahrenheit), most of the atoms in that gas would drop to the lowest possible energy level. Moreover, they would all be identical, and, Einstein predicted, they would condense to a single point. In doing so, the atoms would reinforce each other, acting like a single, gigantic “superatom,” which would allow scientists to directly observe its peculiar quantum-mechanical properties.


This image shows a Bose-Einstein condensate created by David Hall’s machine. As a condensate emerges from confinement in the magnetic trap, it rebounds into this signature ovoid shape. The false colors indicate the density of atoms — white being most dense and blue the least dense.

That’s just what Hall is doing with his machine. The first time anyone managed to make a Bose-Einstein condensate (Bose is the physicist whose research prompted Einstein’s predictions) was at the University of Colorado in 1995. Hall worked with the Colorado team as a post-doctoral researcher in 1997, and when he came to Amherst in 1999, he decided to build his own machine. Hall’s device, which has no official name, uses two different techniques to cool gases. First, six laser beams capture, confine and cool a dilute rubidium gas (among the most obliging of the few elements that will condense this way—“God’s atom,” in Hall’s words). A photon of light from a precisely tuned laser hits a rubidium atom, which absorbs the photon and then releases it again, losing some of its own energy. As the process is repeated with each succeeding photon, the atom moves more and more slowly and becomes increasingly cold. The laser assembly can cool the rubidium gas to about 50 millionths of a degree Kelvin: extremely cold, but not enough to condense.

To further cool the gas, it is transferred to a chamber where it is exposed to the second technique, forced evaporative cooling. This is the same thing that happens when you blow on a bowl of hot soup: the most energetic molecules jump up above the surface of the soup, where your breath carries them away before they can fall back and reintroduce their energy. In the case of Hall’s machine, the bowl is a magnetic field, and rather than blowing on it, the computers controlling the process gradually lower the sides of the magnetic bowl, letting the most energetic atoms escape. The ones that remain are about 0.00000005 degrees Kelvin—cold enough to condense in the Bose-Einstein pattern. (For comparison, the coldest spot in outer space is estimated to be 2.7 degrees Kelvin.)

The resulting condensate is about one-fifth the thickness of a sheet of paper, which is enormous by atomic standards—large enough to be photographed in images like the one shown here. The matter exists for only about 30 seconds, so Hall and his students (currently Mark Wheeler ’04) use the images to study its properties.

So far, Hall says, they are just exploring the terrain of this new world; no one is yet dealing with practical applications for Bose-Einstein condensates. But the basic science is valuable on its own terms, helping scientists flesh out what had been only theoretical assumptions. “The system is richer than the theories,” he says. “You can come up with some theory that seems to be perfect and explains every

aspect, but nature isn’t exactly like that. When you throw in reality, you find quite a number of phenomena that you might never have suspected would exist from the simplest models.”

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Photo: Frank Ward

 
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