Abstract

Bose-Einstein condensates (BEC) of weakly interacting gases allow for the new regime of nonlinear atom optics. Of special interest are macroscopically excited states like dark solitons or vortices. Solitons, as particle like excitations of matter wave fields, provide a link from condensate physics to fluid mechanics, nonlinear optics and fundamental particle physics. Dark solitons in matter waves are characterized by a local density minimum and a steep phase gradient of the wavefunction at the position of the minimum. We report on experimental and theoretical investigation of dark solitons in BEC’s of ⁸⁷Rb which are produced by the method of phase imprinting. A highly anisotropic confining potential (see [1]) allows us to be dose to the (quasi) 1D situation where dark solitons are expected to be dynamically stable. By monitoring the evolution of the density profile we observe the evolution of density minima travelling at a smaller velocity than the speed of sound. By comparison to analytical and numerical solutions of the 3D Gross-Pitaevskii equation for our experimental conditions we identify these density minima to be moving dark solitons [2], We have studied in detail the creation and the dynamics of dark solitons as a function of evolution time and imprinted phase. Fig. 1 shows the density profiles of atomic clouds for different evolution times in the magnetic trap, t_ev. The minimum moving to the negative x-direction is identified as a moving dark soliton. An additional structure is attributed to a density wave created in the phase imprinting process in combination with more complex dynamics during an additional free evolution time t_TOF. As predicted by theory, we see no indication for dynamical instability in the experiment. The lifetime of the soliton is sensitive to the gas temperature due to interaction with the thermal cloud offering a unique possibility for thermometry of BEC’s under conditions where the thermal cloud is not discernible.

© 2000 IEEE