We demonstrate a juggling atomic fountain by launching two laser-cooled balls of 133 Cs atoms in rapid succession. The atoms collide near the top of the fountain and, by varying the delay between launches, we scan the collision energy from 19 to 150 mK. We measure the differential scattering cross section and isolate the contributions from different partial waves including their interferences. We observe the s-wave energy dependence and the p-wave scattering threshold. We infer a triplet s-wave scattering length of 2400a 0 and a p wave of ͑2107a 0 ͒ 3 for jF 4, m 4͘ 1 j3, 3͘.[S0031-9007 (98)08031-4] PACS numbers: 34.50. -s, 06.30.Ft, 32.80.Pj Many interesting effects of low energy quantum scattering have been beautifully demonstrated in electron-atom scattering [1] and in nuclear physics [2]. Here, we observe for the first time the scattering of multiple atomic partial waves, the interference between them, and the p-wave quantum scattering threshold. Low energy atomic scattering is currently important in many areas of modern atomic physics including Bose-Einstein condensation (BEC) [3], atom interferometers [4], and atomic clocks [5].Low energy atomic collisions have previously been observed in a variety of experiments. The first observations studied cryogenically cooled H [6] and later He [7] gases. More recently, cold collisions have played the crucial role of thermalizing dilute gases in magnetic traps [3,[8][9][10] during evaporative cooling to achieve BEC. In addition, inelastic cold collisions can inhibit the cooling to the BEC transition [11]. Another important and harmful effect of collisions is the large frequency shift they produce in laser-cooled clocks [5]. Finally, photoassociation spectroscopy [10,12] has provided detailed information to characterize cold collisions [13].All of the above experiments are largely insensitive to the angular distribution of the scattered atoms. Previously, we observed the angular distribution of Cs-Cs collisions and showed that collisions are 99.9(1)% s wave at 0.89 mK [14]. Here, we demonstrate a new technique, a juggling atomic fountain, where we scatter Cs atoms off of one another with a well-defined collision energy. We vary the collision energy from low energies, where the scattering is s wave, since the de Broglie wavelength is much longer than the scale of the interatomic potential [15], up to energies at which several partial waves are allowed.We juggle atoms by launching two balls of laser-cooled atoms with a short time delay Dt. The two balls are launched vertically at the same velocity and collide near the top of the fountain with a relative velocity y r Dt 3 g, where g 9.8 m͞s 2 . Launch delays of Dt 7 to 20 ms correspond to collision energies of E c ͞k B mg 2 Dt 2 ͞4k B 19 to 150 mK. We prepare each ball in a particular jF, m͘ state using optical pumping and a twophoton Raman transition [16]. After the atoms scatter, another two-photon Raman transition is used to detect the vertical velocity component, thereby measuring the angular distribution of the scatt...

We observe the angular distribution of s-wave scattering of Cs atoms in an atomic fountain by selecting and probing atomic velocities. At an average temperature of T = 0.89 p, K the scattering is predominantly s-wave and we have a sensitivity to p-wave cross sections as small as 0.1% of the swave. The cross section is 4 X 10 '' cm for atoms in the F = 3, mF = 0 state colliding with atoms distributed among the F = 4, m~states. We also demonstrate a multiply loaded magneto-optic trap and a temperature of 1.53 (7) p, K. PACS numbers: 34.50.s, 42.50.Vk, 32.80.Pj At p, K temperatures, quantum effects lead to novel phenomena in atom-atom interactions. These include quantum collective behaviors such as Bose-Einstein condensation, other indistinguishable

The collision of two ultracold atoms results in a quantum mechanical superposition of the two possible outcomes: each atom continues without scattering, and each atom scatters as an outgoing spherical wave with an s-wave phase shift. The magnitude of the s-wave phase shift depends very sensitively on the interaction between the atoms. Quantum scattering and the underlying phase shifts are vitally important in many areas of contemporary atomic physics, including Bose-Einstein condensates, degenerate Fermi gases, frequency shifts in atomic clocks and magnetically tuned Feshbach resonances. Precise experimental measurements of quantum scattering phase shifts have not been possible because the number of scattered atoms depends on the s-wave phase shifts as well as the atomic density, which cannot be measured precisely. Here we demonstrate a scattering experiment in which the quantum scattering phase shifts of individual atoms are detected using a novel atom interferometer. By performing an atomic clock measurement using only the scattered part of each atom's wavefunction, we precisely measure the difference of the s-wave phase shifts for the two clock states in a density-independent manner. Our method will enable direct and precise measurements of ultracold atom-atom interactions, and may be used to place stringent limits on the time variations of fundamental constants.