In situ imaging of vortices in Bose-Einstein condensates

We present an application of dark-field imaging that enables in situ detection of two-dimensional vortex distributions in single-component Bose-Einstein condensates (BECs). By rotating a Rb-87 BEC in a magnetic trap, we generate a triangular lattice of vortex cores in the BEC, with core diameters on the order of 400 nm and cores separated by approximately 9 mu m. We have experimentally confirmed that the positions of the vortex cores near the BEC center can be determined without the need for ballistic expansion of the BEC. Our imaging method should allow for the determination of arbitrary distributions of vortices and other superfluid density defects in cases where expansion of the BEC is either impractical or would significantly alter the physical characteristics and appearance of vortices or defects. Our method is also a step toward real-time measurements of complex two-dimensional vortex dynamics within a single BEC.