Observation of transient momentum-space interference during scattering of a condensate from an optical barrier

Scattering theory traditionally deals with the asymptotic behavior of a system far removed from the actual scattering event. Here we present an experimental study of the one-dimensional scattering of a noninteracting condensate of Rb-87 atoms from a potential barrier in the nonasymptotic regime for which the collision dynamics are still ongoing. We show that, for near-transparent barriers, there are two distinct transient scattering effects that arise and dramatically change the momentum distribution during the collision: a deceleration of wave-packet components in midcollision and an interference between incident and transmitted portions of the wave packet. Both effects lead to the re-distribution of momenta giving rise to a rich interference pattern that can be used to reconstruct the single-particle phase profile.