Rice physicists observe new ‘atom wave’ phenomena

Trapped atoms from a Bose-Einstein Condensate form ‘soliton train’

Rice University physicists have shown for the first time that ultracold atoms can form bright “solitons,” localized bundles of waves that maintain a constant shape as they propagate. Solitons of light are used in ultra-high speed optical communication networks because they can carry data over great distances without the use of signal boosters. At the atomic level, solitons could further the development of new forms of atom lasers. The research is described in the May 9 issue of Nature.

The experiments involve a Bose-Einstein condensate, or BEC, a collection of atoms that is cooled to the point where the mysterious and counterintuitive forces of quantum mechanics take over, causing the atoms to lose their individual identities and behave not like individual particles, but as a single, collective wave. To create a BEC, physicists tightly confine atoms in magnetic fields and cool them using lasers and evaporation until they reach a temperature that is about one billion times colder than room temperature.

Like any confined wave, Bose-Einstein condensates are fragile and tend to disperse quickly when released from confinement. In the latest experiments, Rice scientists trapped atoms from a BEC in a narrow beam of light that only allowed the atoms to move in a single file line. By causing the atoms to attract each other, the physicists were able to create atomic solitons, atom waves whose self-attraction balances perfectly with their tendency to disperse. Solitons show up in a variety of other wave phenomena, but the first observation was of a non-spreading water wave in a canal in Scotland in 1834.

In the world of optics, solitons of light have been created by sending light pulses down specially designed optical fibers. Unlike typical data in telecommunications networks, which must be reinforced with “repeaters” that boost the signal at regular intervals, these signals don’t disperse and become weaker as they travel down the fiber.

In the latest experiments, Rice’s BEC researchers observed atomic “soliton trains,” groups of as many as 15 solitons lined up end-to-end. These solitons were observed to propagate for several seconds, an eternity for a localized wave bundle, without spreading.

The techniques that are being developed to control matter in BEC experiments could eventually be used to perform extremely precise measurements. For example, the same principle that makes lasers useful in interferometric fiber-optic gyroscopes could be applied with atom lasers to form instruments that are millions or perhaps billions of times more sensitive.

“Forty years ago, no one imagined that lasers would be used to play music in our cars or scan our food at the grocery store checkout,” said principal investigator Randall G. Hulet, Fayez Sarofim Professor of Physics and Astronomy. “BEC researchers are in a similar situation. We’re getting our first glimpse of a wondrous and sometimes surprising set of dynamic quantum phenomena, and there’s no way to know exactly what may come of it.”

In 1995, Hulet’s research group created the first BEC from lithium atoms, something some theorists had predicted could not be done because of the attractive nature of the atoms. Further study of this novel BEC system led to the direct observation of condensate growth and collapse. This provided new insights into weakly interacting Bose gases and laid the groundwork for the soliton experiments just completed.

The current research is described in detail in “Formation and Propagation of Matter Wave Soliton Trains,” by Kevin E. Strecker, Guthrie B. Partridge, Andrew G. Truscott, and Randall G. Hulet. Strecker and Partridge are graduate students at Rice. Truscott, formerly a post-doctoral researcher at Rice, is now on the faculty of the Australian National University in Canberra.

Hulet’s research is sponsored by the National Science Foundation, the National Aeronautics and Space Administration, the Office of Naval Research and the Welch Foundation.