Cold-collision properties derived from frequency shifts in a cesium fountain.

Abstract

We interpret recently measured collisional frequency shifts in a cesium atomic fountain using coupled-channel theory and a simple picture of hyperfine-coupled long-range molecular states. We pre­ dict the existence of CS2 singlet and triplet states very close to threshold with an associated strong influence on ultracold collisions and large negative values for the Cs+Cs scattering lengths. Our results are important for future precision experiments using Cs fountains, and may also have important conse­ quences for attempts to realize a Bose condensate in an ultracold atomic Cs vapor. PACS number(s): 32.80.Pj, 42.50.Vk The advances of techniques for using lasers to trap and manipulate atoms have, in the past few years, made it possible to cool atoms to temperatures near absolute zero and to examine unusual quantum states of matter under these circumstances. One of the most spectacular possi­ bilities now within sight is the observation of Bose­ Einstein condensation (BEC) in a weakly interacting va­ por of atoms, which is predicted to take place at such low temperatures and high densities that the atomic de Bro­ glie wavelength becomes comparable to the average dis­ tance between the atoms. Some groups are trying to achieve this phase transition in atomic hydrogen gas in a magnetic trap [1,2]. Others are attempting the same in a laser-cooled sample of alkali-metal atoms such as Cs [3] and Li [4]. Collisions between atoms playa crucial role in this en­ deavor. In two recent papers [5], the conditions for BEC of a cold Cs vapor were examined using the properties of Cs+ Cs ground-state collisions. Due to insufficient knowledge of the interatomic potentials it was only possi­ ble to reach rough conclusions. This situation has changed drastically with a recent cesium-fountain clock experiment in which two of the present authors measured large frequency shifts due to ultracold collisions of Cs atoms during their ballistic flight in the atomic fountain [6]. While these frequency shifts will be largely eliminat­ ed in future clocks [6], here we use the measured shifts to obtain information on the interaction potentials relevant for BEC. We find strong evidence for the existence of singlet and triplet states· close to the continuum with an associated large influence on ultracold collisions. The frequency shifts enable us to analyze collisions at the coldest temperatures (T~ 1.5 J.LK) at which atomic col­ lisions have ever been studied. We find the zero-energy elastic cross section for triplet scattering to be between