Scientists in two labs coax very cold atoms to move in trains

05/20/2002

By ALEXANDRA WITZE / The Dallas Morning News

Physicists have coaxed ultracold atoms to do an ultracool trick – to travel in small bunches, just as light waves do down fiber-optic cables.

These atomic groupings, or solitons, could one day serve as the basis for extraordinarily precise navigational instruments, scientists say. But for now, the solitons remain the latest discovery in one of physics' hottest fields.

The field involves Bose-Einstein condensates, a form of matter in which some atoms, when chilled to fractions of a degree above absolute zero, lose their individual identities and begin to behave as a collective mass. Scientists first created Bose-Einstein condensates in 1995 – earning last year's Nobel Prize in physics for the work – and have been playing atomic tricks with the stuff ever since.

This month, researchers from Houston and Paris independently reported making solitons out of Bose-Einstein condensates.

RANDALL HULET / Rice University A 3-D rendering depicts an image of solitons -- a kind of wave -- of ultracold atroms created in a Rice University laboratory in Houston. Such solitons could someday be useful for atom lasers in high-precision instruments.

Unlike other atom clusters, which spread apart and disintegrate as they travel, solitons move without changing size or shape, says Randall Hulet of Rice University, leader of one of the teams. Fiberoptic cables use solitons of light because they can carry information over long distances without the signal losing quality.

Solitons are stable-looking waves that show up in many corners of physics and mathematics. "Anywhere you find waves you find solitons," says Dr. Hulet.

That includes water, where scientist John Scott Russell discovered solitons in 1834. He watched a barge come to a halt in a canal near Edinburgh, setting off a "solitary wave" that moved mysteriously and without changing shape for more than a mile.

Atoms in Bose-Einstein condensates can be made to behave like that Scottish wave, says Dr. Hulet, whose work appeared this month in the journal Nature.

His team started with about half a million lithium atoms, cooled in a special apparatus to as close as possible to absolute zero, or minus 459.7 degrees Fahrenheit. The researchers forced the atoms into a line, then caused them to become attractive to each other. The cloud of atoms clumped into 15 solitons, each containing roughly 6,000 atoms. (The rest of the cloud was lost in other interactions in a complicated process, says Dr. Hulet.)

The scientists then watched the soliton "train" move through their laboratory apparatus for about three seconds.

"That's an extraordinarily long time for a matter wave," says Dr. Hulet. A Bose-Einstein condensate that wasn't a soliton, he says, might break apart in a few hundredths of a second.

In France, a competing research team observed its own soliton for just 10 thousandths of a second before it disappeared. The way that experiment was set up caused the soliton to slip out of view quickly, says team member Lev Khaykovich.

Unlike the Texas group, which made 15 solitons at once, the French team created only one. Dr. Khaykovich says it's easier that way, because the researchers don't have to worry about how different solitons might interact.

"If you have as simple a situation as possible, you can better compare it to the theory," he says. The team, led by Christophe Salomon of the École Normale Supérieure, reported its findings last week in Science.

Solitons behave as a sort of atom laser, in which pulses of atoms substitute for the pulses of light in an optical laser, says Rice's Dr. Hulet. Someday, atom lasers could be used to make ultraprecise instruments such as gyroscopes, he says.

In other related work, a third team has taken a major step toward improving atom lasers. In last week's Science, researchers from the Massachusetts Institute of Technology reported fashioning a sort of bottomless well of Bose-Einstein condensed atoms.

That well could serve as a reservoir to constantly replenish an atom laser, allowing it to spit out atom pulses for as long as its designer wanted, says lead author Ananth Chikkatur. So far, atom lasers have worked only until their original atom supply ran out.

The MIT group designed a special chamber to hold the ultracold sodium atoms after they were made into Bose-Einstein condensates. "That was a key technology for this to work," says Mr. Chikkatur. The team manipulated light beams to create a pair of optical "tweezers" that can move the atoms into the reservoir.

Mr. Chikkatur compares the achievement to moving from a dripping faucet – a pulsed source of water – to the continuous water source of a bucket. He says it's now just a short step to get the atoms in the bucket to come out as an atom laser that could, in theory, run forever.

E-mail awitze@dallasnews.com