Viewing, Jiggling a Novel State of Matter

I. Peterson

A dense cloud of sodium atoms chilled to a temperature barely above absolute zero acts like a lens. By slightly bending the path of laser light passing through it, such an aggregation enables researchers to obtain an image of the cloud.

This direct glimpse is one of the first insights into the behavior and characteristics of atoms in a remarkable state of matter known as a Bose-Einstein condensate. Created in the lab-oratory for the first time only last year, these unusual condensates consist of thousands or even millions of atoms in the same quantum state (SN: 7/15/95, p. 36).

"Several groups are now reliably making condensates, and they're starting to do science," says physicist Mark A. Edwards of Georgia Southern University in Statesboro. Researchers are checking theoretical predictions made years ago about what this state of matter would be like and are investigating various methods of manipulating such clumps of atoms.

A number of teams described their latest findings at an American Physical Society meeting held last week in Ann Arbor, Mich.

"Everybody was really quite excited to see how fast the field is moving," says Wolfgang Ketterle of the Massachusetts Institute of Technology. "All the experimental groups and theory groups reported major progress."

The physicists use laser and other techniques to cool atoms of an alkali metal, such as rubidium, sodium, or lithium, to temperatures below 2 microkelvins. When a gas of these atoms, all identical, gets sufficiently cold and dense, it reaches a state in which the quantum uncertainty in each atom's position is as large as the average distance between the atoms. Instead of becoming a liquid or solid, the entire clump behaves essentially as a single unit, a superparticle.

Bose-Einstein condensates display the same kind of quantum phenomena that characterize superconductors and superfluids. However, because these condensates are still gases, their behavior is, in principle, less complicated and easier to study than comparable effects in solids and liquids, where interactions between particles play a substantial role.

Eric A. Cornell and his colleagues at the University of Colorado and the National Institute of Standards and Technology (NIST), both in Boulder, were the first to achieve Bose-Einstein condensation. Using a refined technique to create condensates of rubidium atoms, the NIST-Colorado team has now verified a number of theoretical predictions, including how the condensate grows as the temperature decreases below the transition point. The researchers also demonstrated repulsive interactions between the atoms in the condensate.

"They found wonderful agreement with the theory," Edwards says.

Using a new, improved apparatus to trap 5 million sodium atoms, Ketterle and his coworkers presently hold the record for the largest number of atoms in a Bose-Einstein condensate (SN: 12/2/95, p. 373). Like the NIST-Colorado group, they checked various theoretical predictions, and they obtained similar results.

Ketterle and his colleagues were also able to generate the first direct images of Bose-Einstein condensation. They shone light of a wavelength that could penetrate the dense sodium cloud but was still slightly deflected by it.

"At first you see a diffuse cloud," Ketterle says. "Then you see a bright spot—an elongated droplet." The researchers could manipulate the droplet's shape by adjusting the magnetic field holding it in place.

Because this imaging technique does not disturb the condensate unduly, the researchers could, for the first time, view the same condensate twice. They may soon be able to take up to 100 images to track the condensate without destroying it, Ketterle remarks.

The MIT and NIST-Colorado collaborations also used magnetic pulses to disturb the condensate, making it ring like a bell. "We recorded the oscillations and measured their frequencies," says Ketterle. With better-defined pulses, Cornell and his coworkers discovered that only some types of excitations induce a response in the condensate. "One of the exciting things about this is that it could be used for controlling a condensate," Edwards says. "You're able to make [the condensate] dance the way you want."

From Science News, Volume 149, No. 21, May 25, 1996, p. 327.