Brownian motion of solitons in a Bose–Einstein condensate
Solitons — solitary waves that behave more like discrete particles than waves — occur in diverse physical systems, from water in a canal to light waves in optical-fiber telecommunications. They can also exist in Bose-Einstein condensates, manifesting as stable density waves.
In a pristine environment, BEC solitons can last for several seconds, oscillating regularly through the atomic cloud. Now, a team of PFC-supported researchers have investigated how impurities affect the movement and lifetime of BEC solitons.
The researchers began by cooling down a cloud of rubidium atoms. Right before the gas became a homogenous quantum fluid, a radio-frequency magnetic field coaxed a handful of these atoms into remaining classical. Those atoms behave as impurities in the atomic mix. The scientists then used laser light to push apart atoms in one region of the fluid, creating a solitary wave of low density—a “dark” soliton.
In the presence of the impurities, the dark soliton behaves as if it were a heavy particle, with lightweight impurity atoms bouncing off of it. These collisions make the dark soliton’s movement more random, an effect reminiscent of Brownian motion. Unlike Brownian motion, where collisions these collisions also accelerated the soliton to a point of destabilization, leading to shorter lifetimes.
These results, published in the Proceedings of the National Academy of Sciences on March 7, 2017, may cast light on a similar problem with solitons in optical fibers, where random noise can disrupt the precise timing needed for communication over long distances.