This provides a unique situation for studying the interactions of the atoms in this rare state of matter. For this reason, the Rice team is taking an especially close look at the mechanics of how their condensate forms and the special properties it possesses. Their findings contribute to a basic understanding of interactions on the atomic level.
A Bose-Einstein condensate, independently postulated by Indian physicist Satyendranath Bose and Albert Einstein in the 1920s, is a rare state of matter in which the atoms are so cold and move so slowly that they undergo a phase transition, as when water freezes to ice. But in this phase transition, the atoms condense into a special gas which behaves as a single unit. This occurs at fractions of a degree above absolute zero, or minus 460 degrees Fahrenheit.
In the Feb. 10 issue of Physical Review Letters, the Rice team, led by Randall Hulet, professor of physics, reports that when the condensate is formed with attractive atoms, specifically lithium, there is a ceiling to the number of atoms before the condensate maxes out and collapses.
Quantum theoreticians predicted that condensates with atoms that attract each other are limited to about 1,400 atoms. The Rice group found a maximum between 650 and 1,300 atoms, which is consistent with the predictions.
The limit on the number of atoms is in contrast to condensates formed with atoms that repel each other--there is no known limit to the number of atoms possible in those, and as many as five million atoms have been reported.
Since the Rice group first indirectly observed a Bose-Einstein condensate in 1995, Hulet and graduate students Curtis Bradley and Charles "Cass" Sackett have been working to improve their lab apparatus so they could directly see the condensate peak and distribution of the atoms. When they first saw the state of matter, they had to infer its existence from distortions it produced in a laser beam passed through it.
Using technology borrowed from biologists, Hulet, Bradley and Sackett were able to adapt what is called phase-contrast imaging to get their measurements. Biologists use this technique to look at transparent objects such as cell membranes. Light passing through the membrane is made to interfere with light not passing through it, producing an image of the transparent material.
"We have developed a similar technique," Hulet says, "that makes use of the phase shift of the atoms. Now there's no distortion, and we get a cleaner picture of the condensate."
Currently, the group is busy studying the interactions of the atoms in the condensate--how it forms, how it begins to decay, and how it ultimately collapses.
"What happens when it collapses is similar to the way a star collapses," says Hulet. "At some point, the attractive interactions overwhelm the 'quantum pressure' that stabilizes the condensate and it implodes. We want to watch this happening. This is an example of a macroscopic quantum mechanical process, which is rare in physics."
Editors: For more information about the Bose-Einstein condensation project at Rice see: <http://atomcool.rice.edu/>. Text of the published paper can be accessed at <http://ojps.aip.org/PRLTAO-bt/current.html>.
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