As a result of the discovery of Bose-Einstein condensation of atomic gases, there has been a new impulse in the study of quantum fluids. Of the many questions that can be studied, superfluidity is one of the most intriguing and fascinating. Originally discovered and studied with liquid Helium and later in the context of superconductors, superfluidity is a hallmark property of interacting quantum fluids and encompasses a whole class of fundamental phenomena. One striking consequence of superfluidity is the response of such a system to rotating peturbations. In contrast to a normal fluid, which at thermal equilibrium will rotate like a solid body with the peturbation, a superfluid will not circulate unless the rotation frequency of the perturbation is sufficiently large. Moreover, when the superfluid does circulate, it can only do so by forming vortices in which the condensate density vanishes and for which the velocity field flow evalutated around a closed contour is quantized.
The ENS group (formed by Vincent Bretin, Frederic Chevy, Kirk Madison, and Jean Dalibard) has observed experimentally the nucleation of such quantized vortices in a stirred gaseous condensate of atomic rubidium. A rotating, non-axisymmetric potential created by a laser beam is superimposed onto a 87Rb condensate confined in a magnetic trap, and the formation of vortex filaments is observed when the stirring frequency is above a critical frequency. The angular momentum of a single vortex has been measured, and found to be of the order of Planck's constant per particle. When several vortices are present, they form a Abrikosov-like lattice, similar to those observed with type II supra-conductors.
F. Chevy, K.W. Madison, and J. Dalibard., Phys. Rev. Lett. 85, 2223 (2000)