Experimental Observation of Oscillating and Interacting Matter Wave Dark Solitons

We report on the generation, subsequent oscillation and interaction of a pair of matter-wave dark solitons. These are created by releasing a Bose-Einstein condensate from a double well potential into a harmonic trap in the crossover regime between one dimension and three dimensions. Multiple oscillations and collisions of the solitons are observed, in quantitative agreement with simulations of the Gross-Pitaevskii equation. An effective particle picture is developed and confirms that the deviation of the observed oscillation frequencies from the asymptotic prediction ${\nu }_z/\sqrt{2}$, where ${\nu }_z$ is the longitudinal trapping frequency, results from the dimensionality of the system and the soliton interactions.