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Toying With Temperature
12 September 2003 (All day)
For 2 decades, physicists have chilled gases of atoms ever closer to absolute zero in a quest to reveal the quantum nature of matter. As they've pressed down to a few billionths of a degree, researchers have netted two Nobel Prizes and produced a weird new state of matter. Now, a team has found a clever way to cool that strange atomic soup past the billionth-of-a-degree mark--using a technique that works like a popular toy.
In 1995, physicists produced a new state of matter called a Bose-Einstein condensate (BEC), in which ultracold atoms crowd into a single quantum state. Using laser light, researchers coaxed each atom in a sample to gyrate so that it could lower its energy by migrating to the weak spot in the center of a complicated magnetic field. They then shined microwaves on the huddling atoms to change the spin of the most energetic ones. The microwaves effectively blew these extra-energetic atoms out of the magnetic field, thus cooling those left behind. Using this technique and others, physicists have since cooled BECs to a few billionths of a degree kelvin.
But a much simpler trap suffices to cool atoms past the billionth-of-a-kelvin milestone, report Wolfgang Ketterle, Aaron Leanhardt, and colleagues at the Massachusetts Institute of Technology in Cambridge. Instead of using an assemblage of coils to squeeze the atoms between strong magnetic fields above and below, the researchers used a single coil to generate a field that supported their sample of sodium atoms against the pull of gravity. The coil lay directly beneath the sample and produced a field that repelled the atoms, each of which spun like a top. So the atoms levitated just as a spinning top hovers over a magnetic base in a desk-top novelty known as the Levitron.
The "gravito-magnetic" trap holds atoms much less tightly than a purely magnetic trap, so it allows researchers to work with lower density samples and achieve lower temperatures, says Ketterle, who shared the 2001 Nobel Prize in physics for the discovery of the BEC. The researchers produced low-density samples that underwent Bose-Einstein condensations at 480 picokelvin--less than a half a billionth of a kelvin--they report in the 12 September issue of Science.
Physicists ought to be able to reach even lower temperatures by using more massive atoms such as rubidium or cesium, says Rudi Grimm of the University of Innsbruck in Austria. "This is just the beginning," Grimm says. "I think with these kinds of experiments, a few picokelvins is possible." At very low temperatures and densities, researchers might study subtle quantum effects such as the reflection of Bose-Einstein condensates off solid surfaces.