Imaging ofsanddPartial-Wave Interference in Quantum Scattering of Identical Bosonic Atoms

Abstract

We report on the direct imaging of s and d partial-wave interference in cold collisions of atoms. Two ultracold clouds of 87 Rb atoms were accelerated by magnetic fields to collide at energies near a d-wave shape resonance. The resulting halos of scattered particles were imaged using laser absorption. By scanning across the resonance we observed a marked evolution of the scattering patterns due to the energy dependent phase shifts for the interfering s and d waves. Since only two partial wave states are involved in the collision process the scattering yield and angular distributions have a simple interpretation in terms of a theoretical model. 03.65.Nk, 32.80.Pj, 39.25.+k Collisions of atoms is a classic discipline of quantum mechanics [1,2]. With the advent of laser cooling [3], confining and cooling various atomic species to submillikelvin temperatures became possible and gave rise to a wealth of experiments in which quantum effects in collisions at very low energiescold collisions -were observed [4]. Knowledge about cold collision properties paved the way for exciting new developments in experimental atomic physics. It played a crucial role in achieving Bose-Einstein condensation [5] and Fermi degeneracy [6] in dilute atomic vapors by mediating thermalization during evaporative cooling and accounting for stability [7]. For atoms at temperatures associated with the quantum degenerate regime the essential interaction properties are determined by a single atomic parameter, the scattering length, because all elastic scattering has an isotropic (s-wave) nature at such low energies. The scattering length may exhibit a pronounced dependency on external magnetic fields giving rise to FIG. 1: (Color online) Illustration of the process of using absorption imaging for the detection of scattered particles. We present the case of pure d-wave scattering occurring at the origin for particles coming in along the z-axis. Scattered particles will be situated on an expanding sphere and distributed according the d-wave angular emission pattern |f (θ)| 2 . Absorption imaging along the x-axis projects this distribution onto the yz-plane. [9,10,11,12,13].To date, most experiments on cold collisions of atoms have been carried out using magneto-optical traps (MOTs) or magnetic traps which suffer from the disadvantages that no collision axis is singled out or the collision energy cannot be varied over a wide range [4,14]. If no fixed collision axis is present, anisotropic scattering, as occurs for collision energies above the s-wave regime, will be obscured by spatial averaging. One solution to this problem was provided in the "juggling" MOT experiment [15], where a cloud of cesium atoms was laser cooled to 3 µK and ejected vertically from a trap to collide with a previously launched cloud at energies up 160 µK. Scattered atoms were detected using a spectroscopic technique revealing interference between s and p partial waves. In experiments on BECs, a collision axis was also selected using Bragg scattering to accelerate part of t...

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