Interferometric Determination of the s and d-Wave Scattering Amplitudes in ⁸⁷Rb

We demonstrate an interference method to determine the low-energy elastic scattering amplitudes of a quantum gas. We linearly accelerate two ultracold atomic clouds up to energies of 1.2 mK and observe the collision halo by direct imaging in free space. From the interference between s- and d- partial waves in the differential scattering pattern we extract the corresponding phase shifts. The method does not require knowledge of the atomic density. This allows us to infer accurate values for the s- and d-wave scattering amplitudes from the zero-energy limit up to the first Ramsauer minimum using only the van der Waals C₆ coefficient as theoretical input. For the ⁸⁷Rb triplet potential, the method reproduces the scattering length with an accuracy of 6%.