Quantized Vortices in a Bose-Einstein Condensate

When a superfluid medium is forced into rotation, Quantum Mechanics imposes strong constraints on its velocity field. The superfluid is set into motion through the nucleation of lines of singularity or vortex lines, where the density vanishes and around which the circulation of the velocity is quantized. We report the observation of such vortices in a gaseous Bose-Einstein condensate of ⁸⁷Rb atoms, which is stirred using a rotating laser beam. We characterize the phase and the angular momentum of a single vortex, and we discuss the maximal number of vortices which can be nucleated in this system. Finally we address the problem of the critical rotation frequency at which the first vortex appears.