Measurement of the angular momentum of a rotating Bose-Einstein condensate

We study the quadrupole oscillation of a Bose-Einstein condensate of Rb-87 atoms confined in an axisymmetric magnetic trap, after it has been stirred by an auxiliary laser beam. The stirring may lead to the nucleation of one or more vortices, whose presence is revealed unambiguously by the precessio n of the axes of the quadrupolar mode. For a stirring frequency Omega below the single vortex nucleation threshold Omega(c), no measurable precession occurs. Just above Omega(c), the angular momentum deduced from the precessi on is similar to h. For stirring frequencies above Omega(c) the angular mom entum is a smooth and increasing function of Omega, until an angular freque ncy is reached at which the vortex lattice disappears.