Ketterle Group Reports Observation of Vortex Phase Singularities in BEC

Bose-Einstein condensates of dilute atomic gases offer a unique opportunity to study quantum hydrodynamics. The low density of the gas allows direct comparison with first principle theories.

Recently, vortices in a Bose-Einstein condensate have been realized experimentally and are currently under intensive study. In most of this work, vortices were identified by observing the density depletion at the cores. The flow field of a vortex can be directly observed when the phase of the macroscopic wavefunction is measured using interferometric techniques. In the MIT work, Ketterle's group created one or several vortices in a condensate by moving a laser beam through it, and then imaged its phase by interfering that condensate with a second unperturbed condensate that served as a local oscillator. The characteristic signature of vortices was dislocations in the interference fringes. The "extra" fringe that terminates at the vortex core corresponds to one quantum of circulation h/m (where m is the atomic mass and h Planck's constant) or a phase change of 2pi integrated along a path around the vortex core.

Observation of the phase singularities of vortices Observation of the phase singularities of vortices created by sweeping a laser beam through a condensate. Without the sweep, straight fringes of about 20 µm spacings were observed (upper image), while after the sweep, fork-like dislocations appeared (lower image). The speed of the sweep was 1.1 µm/ms corresponding to a Mach number of 0.1. The field of view of each image is 1.1 mm x 0.38 mm.

These results are described in the paper "Observation of Vortex Phase Singularities in Bose-Einstein Condensates" with authors S. Inouye, S. Gupta, T. Rosenband, A.P. Chikkatur, A. Gšrlitz, T.L. Gustavson, A.E. Leanhardt, D.E. Pritchard, and W. Ketterle, published in Physical Review Letters 87, 080402 (2001).