Observation of Sine-Gordon-like Solitons in a Spinor Bose-Einstein Condensate.

We experimentally generate sine-Gordon-like solitons in a spin-1 spinor BEC utilizing a robust and reproducible local phase-imprinting scheme. We find that the soliton velocity can be tuned by the effective quadratic Zeeman shift. This enables the investigation of controlled soliton interactions, in which we observe the characteristic elastic collision behavior of the integrable sine-Gordon model. The spatial displacement-the so-called phase shift-between incoming and outgoing solitons, the signature of their pairwise interaction, is found to be in quantitative agreement with numerical spin-1 simulations within the error bars. These results establish spinor BECs as a highly controllable experimental platform for studying aspects of the dynamics of sine-Gordon-like models.