Density (a) and phase (b) profile of a moving condensate with singly-charged vortex. The density profile shows the vortex core, whereas the phase pattern features a fork-like dislocation at the position of the vortex. Interference between two initially separated, freely expanding condensates produces exactly the same pattern as shown in (b), if one of the condensate contains a vortex.

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Observation of the phase singularities of vortices created by sweeping a laser beam through a condensate. Without the sweep, straight fringes of about 20 micrometer spacings were observed (a), while after the sweep, fork-like dislocations appeared (b-d). The speed of the sweep was 1.1 micrometer/ms, corresponding to a Mach number of 0.18. The field of view of each image is 1.1 x 0.38 mm. Fig. (d) shows a pair of dislocations with opposite circulation characteristic of a vortex pair. At the bottom, magnified images of the fork-like structures are shown (d1) with lines to guide the eye (d2).