Bright Solitons in a Bose-Einstein Condensate

Bright solitons in a Bose-Einstein condensate have been created and observed for the first time, yielding a stunning new demonstration of the wavelike behavior of atoms and providing an important tool for eventual technological applications of BECs.

First observed on the surface of a narrow canal in 1834, a soliton is a group of waves combining in such a way to form a single composite wave that can travel for long distances without spreading out or losing its original shape.

Solitons can occur in all kinds of waves; for example, they have been thoroughly studied in sound waves and light waves. In fact, soliton light waves are currently employed in telecommunications.

Solitons can exist in BECs too. Since a BEC consists of ultracold atoms all in the same quantum state, it exhibits wavelike behavior and therefore can be considered as a single atom wave. However, the BEC atom wave usually spreads apart or "disperses" shortly after the BEC is released from a trap.

Nonetheless, in previous BEC experiments (such as Burger et al., Phys. Rev. Lett., 20 December 1999), researchers have observed "dark solitons," representing absences of atoms that can propagate without changing shape in a condensate.

Now, in a BEC of lithium atoms, a Rice University team (Randy Hulet, 713-348-6087, randy@atomcool.rice.edu) has produced "bright" solitons, each representing a condensate of actual atoms extracted from the main BEC. In effect, the bright solitons are individual atom waves broken off from the main BEC atom wave. Using a narrow laser beam to guide BEC atoms in a single-file line, the Rice team tailored the interactions between lithium atoms to be attractive so that the atoms' attraction for one another perfectly offset their predisposition to spread out. With their technique, the Rice researchers have created "trains" of up to 15 solitons (Strecker et al., Nature, 9 May 2002 print issue and see figure; this work will also be presented in papers G1.011 and R1.001 at the upcoming APS Division of Atomic, Molecular, and Optical Physics (DAMOP) meeting in Williamsburg from May 29-June 1.)

Whereas the Rice researchers studied a train of solitons over a long time scale, a collaboration between labs in France and Italy (contact Christophe Salomon, Laboratoire Kastler Brossel, Ecole Normale Superieure, Paris, 011-33-1-44-32-25-10, salomon@physique.ens.fr) observed and studied single-soliton formation and propagation over a macroscopic distance of more than one millimeter. They compared the behavior of their ultracold gas with an ideal gas, and found good agreement between their experimental observations and theory (L. Khaykovich et al., Science, 17 May 2002; also paper QPD7 at 2002 CLEO/QELS meeting in Long Beach, CA).

These atom-wave solitons will likely be a useful tool someday for BEC versions of gyroscopes for ultra-precise navigation and very accurate atomic clocks.