Spin Rotations in a Bose-Einstein Condensate Driven by Counterflow and Spin-Independent Interactions

We observe spin rotations caused by atomic collisions in a nonequilibrium Bose-condensed gas of ⁸⁷Rb. Reflection from a pseudomagnetic barrier creates counterflow in which forward- and backward-propagating matter waves have partly transverse spin directions. Even though inter-atomic interaction strengths are state independent, the indistinguishability of parallel spins leads to spin dynamics. A local magnetodynamic model, which captures the salient features of the observed spin textures, highlights an essential connection between four-wave mixing and collisional spin rotation. The observed phenomenon is commonly thought not to occur in Bose condensates; our observations and model clarify the nature of these effective-magnetic spin rotations.