“Because they can carry data over great distances without the use of signal boosters, solitons of light are used in ultra-high speed optical communication networks,” says ONR program manager Peter Reynolds. “Such atomic level soliton trains could further the applications of already-new developments like atom lasers.”
The experiments involve a Bose-Einstein condensate (BEC), first achieved in 1995 under ONR support. A BEC is 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 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 manipulating the forces between the atoms in such a way as to cause 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. Such a balance between competing effects is a hallmark of solitons. 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 or become weaker as they travel down the fiber. In the present experiments, the researchers observed atomic “soliton trains” — groups of as many as 15 solitons lined up end-to-end. These solitons were observed to propagate without spreading for several seconds.
“Several seconds is an eternity for a localized wave bundle,” notes Reynolds.
The techniques that are being developed to control matter in BEC experiments could eventually be used to perform extremely precise measurements… which is exactly why the Navy is interested. The same principle that makes lasers useful in interferometric fiber-optic gyroscopes could be applied with atom lasers to form matter-wave gyros that are millions or perhaps billions of times more sensitive. In the world of submarines, gyroscopes are an essential ingredient in navigation because frequent access to GPS is not always possible.
“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 ONR principal investigator Randall G. Hulet, Fayez Sarofim Professor of Physics and Astronomy at Rice. “BEC researchers are currently in a similar situation.” “We’re getting our first glimpse of a wondrous world of quantum phenomena at the atomic scale. There’s no way to know exactly what may come of it,” adds Hulet.
“In terms of potential Naval applications, we’re just scratching the tip of the iceberg,” says Reynolds.
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 attraction between 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 also sponsored by the National Science Foundation, the National Aeronautics and Space Administration, and the Welch Foundation.