In an emptying bathtub, water forms a whirlpool around the drain. But circular flow can't persist to the very center of the vortex; there must be a water-free funnel. In 1985 Wojciech Zurek, following on work of Tom Kibble, suggested that "topological defects" analogous to the whirlpool could be generated spontaneously in a system undergoing a second-order phase transition. For a fast enough process in a large enough system, small regions independently change state, being unable to communicate with other, relatively far off regions. That independence allows parameters such as the quantum-mechanical phase angle to arrange themselves in vortex structures. Researchers have seen spontaneous vortex formation in, for example, superfluid helium-3, nonlinear optical systems, and superconductors (see the article by Kibble, PHYSICS TODAY, September 2007, page 47). Now a new system can be added to the list: the Bose-Einstein condensate. Deliberately inducing a vortex in a BEC is nothing new, but recent joint experimental work at the University of Arizona and numerical work at the University of Queensland in Australia represents the first study of spontaneous vortex formation in that particularly clean system. In the experiment, Chad Weiler and colleagues tweaked standard procedures to maximize the chance of their observing spontaneously formed vortices. After a trapped atomic gas transitioned to a BEC over the course of a few seconds, the group removed the trapping potential and imaged the escaping condensate. The vortices are revealed by dark, zero-density spots in the figure; the rightmost image shows two vortices, the others a single vortex. Continuing experiment and simulation together, Weiler and colleagues hope, will shed light on the universality of spontaneous topological defect formation in phase transitions. (C. N. Weiler et al., Nature 455, 948, 2008.) — Steven K. Blau
Related links: Bose Einstein Condensation Lab at the University of Arizona College of Optical Sciences Centre for Quantum-Atom Optics at the University of Queensland
