Nucleation of quantized vortices in a gaseous Bose-Einstein condensate

Using a focused laser beam we stir a Bose-Einstein condensate confined in a magnetic trap. When the stirring frequency lies near a critical value, com patible with the quadrupolar mode resonance, we observe the nucleation of a vortex even for a very small stirring anisotropy (of the order of 1%). The results: presented here demonstrate that the nucleation of vortices in the case of a trapped BEC is driven by the resonant excitation of the rotating quadrupolar mode. Using a percussive excitation of the condensate, we meas ure the angular momentum of the condensate with the vortex, and we find tha t it is similar to h per particle, as expected. For larger stirring frequen cies, regular arrays of vortices are observed. (C) 2001 Academie des scienc es/Editions scientifiques et medicales Elsevier SAS.