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The Sciences

Bose and Einstein in Boulder

By David H FreedmanJanuary 1, 1996 6:00 AM


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It was only a brief ghost on a TV screen, a video blob that came and went within a few seconds one day last June. But Carl Wieman and Eric Cornell had been trying to make that blob for years--had been competing with many other physicists to be first, in fact--and so to them it looked exquisite. Our initial reaction was that it was too good to be true, recalls Wieman.

The blob was an image of a few thousand atoms inside a tiny vial nearby. It meant that for a few seconds those atoms had coalesced into a new state of matter--a single, fuzzy megaparticle known as a Bose-Einstein condensate. For those few seconds, the vial in Wieman and Cornell’s lab was the coldest place that ever existed anywhere in the universe. We’re so close to absolute zero that the generally accepted definitions of temperature break down, says Wieman. But he and Cornell put the temperature at 20 nanokelvins, which is 36 billionths of a degree Fahrenheit above absolute zero.

At that temperature, the physicists showed, atoms smear. The culprit is the uncertainty principle of quantum mechanics. It says that if you’re fuzzy on an atom’s speed--as you inevitably are at room temperature, when atoms are buzzing around at hundreds of miles per hour--you can be precise about its location. But slow the atom down drastically--that is, make it cold--and its speed is determined: it is almost precisely zero. The atom’s location then becomes indefinite, wavelike--where the wave is a probability distribution reflecting the likelihood of the atom’s being in any particular spot.

At the extreme temperature of Wieman and Cornell’s vial, each atom came to a virtual standstill. As a result, each atom spread out until it occupied the same region as all the others. In effect the atoms in the vial condensed into a single entity, a sort of superparticle. Seventy years ago, Albert Einstein, relying on work by Satyendra Nath Bose, had predicted this new state of matter. Physicists had dreamed of creating it ever since.

Wieman, Cornell, and their students had been chasing the dream for the past six years in their Boulder lab, which is a joint operation of the University of Colorado and the National Institute of Standards and Technology. Their strategy was two-pronged. First they trained laser beams on the vial from all sides, bouncing particles of light off the atoms head- on to slow them down--much as you could gradually slow an oncoming bowling ball with a rapid-fire peashooter. Next, taking advantage of the fact that each atom is a tiny magnet, they applied external magnetic fields to cool the atoms further and to keep them packed together tightly enough for them to overlap.

Getting the magnetic fields right proved especially tricky. It would have been easy to confine the atoms to the center of the vial by simply putting a magnet right there. But then the atoms would have stuck to the magnet instead of forming a condensate. Instead, Wieman and Cornell had to devise an elaborate arrangement of overlapping external magnetic fields to trap their atoms. They further cooled the vial with the aid of an additional radio-frequency magnetic field tuned to resonate with the higher-energy atoms. This oscillating field caused the hot atoms to flip, reversing their magnetic orientation so that they were repelled out of the trap.

Even this ingenious setup, though, failed at first. Some cold atoms kept escaping, too, leaving behind too few to snuggle up and merge. The scientists traced the problem to weak spots in the trap caused by unavoidable fluctuations in the magnetic fields. Cornell then had a brainstorm: Why not spin the magnetic fields, like tops, at 8,000 revolutions per second? That way the weak spots would never stay in one place long enough for the cold atoms to pop through.

On June 5 the team finished fine-tuning their trap and loaded it with rubidium atoms, whose light-absorbing and magnetic properties make them easy to manipulate. The researchers switched on the cooling lasers and magnets for two minutes; switched off the lasers but let the magnets run another minute; and finally switched the magnets off too. Then they flashed an illuminating laser into the vial. That light would scatter the rubidium atoms almost instantly, but first it would allow a snapshot of them to be displayed on the video screen. In test runs, the swarm of atoms had shown up only as a faint dispersing fog. When we hit Bose condensation, Wieman had predicted a few years earlier, we’ll suddenly see this incredibly bright, shiny ball right in the middle of the cloud. On June 5 that’s what they got: a dense, light-reflecting core of 3,000 overlapping rubidium atoms at the center of the vial.

In the days that followed, Wieman and Cornell saw that condensate again and again. We spent a couple of days nervously checking everything, Wieman recalls. Each time, the researchers waited a little longer after turning off the cooling magnets to turn on the laser flash, counting the seconds to see how long the condensate would hold together. They found they could keep most of it intact for 15 seconds or longer. With more practice they think they’ll do much better.

What is a Bose-Einstein condensate good for? In practical terms, not much--at least not yet. Wieman suggests that the ultrastill atoms might someday play a part in new kinds of atomic clocks or supersensitive measuring devices. And he points out that the condensate is simply a bunch of atom waves that fall into lockstep--much like a laser, which consists of light waves doing the same thing. But it took a long time for people to realize how useful laser light could be, Wieman says, and it might take a long time to appreciate the atomic version.

Physicists, on the other hand, are cock-a-hoop right now. Now at last they can see and study the wave nature of atoms--that essential but forever mysterious tenet of quantum mechanics--with their own naked eyes, almost: the condensate measured two-thousandths of an inch across. And thanks to Wieman’s obsession with cheap equipment, they can do so economically. Instead of expensive industrial lasers, for example, Wieman and Cornell cooled their atoms with the same cheap lasers found in ordinary CD players. Their entire setup cost only about $50,000 to build--well within the reach of most university laboratories. Although it took physicists 70 years to figure out how to make a Bose-Einstein condensate, it almost seems easy now. It’s not impossible, says Wieman, that in a few years this could be the stuff of an undergraduate thesis.

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