Bose-Einstein Condensates Studied with a Linear Accelerator

We present a stand-alone interference method for the determination of the s- and d-wave scattering amplitudes in a quantum gas. Colliding two ultracold atomic clouds we observe the halo of scattered atoms in the rest frame of the collisional center of mass by absorption imaging. The clouds are accelerated up to energies at which the scattering pattern shows the interference between the s- (l = 0) and d- (l = 2) partial waves. With computerized tomography we transform the images to obtain the angular distribution, which is directly proportional to the differential cross section. This allows us to measure the asymptotic phase shifts of the s- and d-wave scattering channels. The method does not require knowledge of the atomic density. It allows us to infer accurate values for the s- and d-wave scattering amplitudes from the zero-energy limit up to the first Ramsauer-Townsend minimum using only the Van der Waals C₆ coefficient as theoretical input. For the ⁸⁷Rb triplet potential, the method yields an accuracy of 6%.