We have performed precision microwave spectroscopy on ultra-cold 87 Rb confined in a magnetic trap, both above and below the Bose-condensation transition. The cold collision frequency shifts for both normal and condensed clouds were measured, which allowed the intra-and inter-state density correlations (characterized by sometimes controversial "factors of 2") to be determined. Additionally, temporal coherence of the normal cloud was studied, and the importance of meanfield and velocity-changing collisions in preserving coherence is discussed.PACS numbers: 03.75. Fi, 05.30.Jp, 32.80.Pj, 34.20.Cf With the advent of modern cooling and trapping techniques, the cold collision regime has become readily accessible. In the cold collision regime quantum statistical effects due to particle indistinguishability dominate scattering processes. The symmetrization requirement for identical particles in an ultra-cold Bose gas enhances the probability of finding two particles nearby, causing density fluctuations. At lower temperatures the statistical nature of the Bose gas causes the phenomena of BoseEinstein condensation, where all atoms in the condensate share the same wavefunction, suppressing the density fluctuations found in a noncondensed sample.Suppression of second-order density fluctuations in a condensate has been measured through analysis of the expansion energy of condensates [1,2]. In a separate experiment, the suppression of third-order density fluctuations was probed by comparing the three-body loss rate of a condensate to that of a normal cloud [3]. The effect of cold collisions has also been measured as a densitydependent energy shift in atomic fountain clocks [4,5,6]. These shifts are quite small (∼0.1-10 mHz) due to the low densities at which the clocks operate, but are measurable because of their high precision. The uncertainty associated with these collisional shifts can be problematic; in fact the next generation of atomic fountain clocks are based on 87 Rb rather than 133 Cs because the collisional shift of 87 Rb is ∼ 30 times smaller. In recent ultra-cold hydrogen experiments the cold collision shift provided the signature of Bose-condensation; below the transition a large frequency shift of the 1S-2S transition was seen, reflecting the high density of the condensate [7].In this paper we report precision microwave spectroscopy performed on ultra-cold and condensed 87 Rb atoms confined in a magnetic trap. Due to the high densities achievable in a magnetic trap, the collisional energy shifts were 10 5 greater than those in 87 Rb atomic clocks, allowing a high-precision measurement of the shifts of the magnetically trappable states to be made with relative ease. The collisional shifts for both a normal and condensed sample were measured, providing a useful probe of the quantum statistics of the system. Additionally, magnetic confinement permits long interrogation times, allowing us to characterize temporal coherence of the normal cloud under various experimental conditions. Comparison of measured coherenc...
We have performed a measurement of the Casimir-Polder force using a magnetically trapped 87 Rb Bose-Einstein condensate. By detecting perturbations of the frequency of center-of-mass oscillations of the condensate perpendicular to the surface, we are able to detect this force at a distance ∼5 µm, significantly farther than has been previously achieved, and at a precision approaching that needed to detect the modification due to thermal radiation. Additionally, this technique provides a limit for the presence of non-Newtonian gravity forces in the ∼1 µm range.
We observe counter-intuitive spin segregation in an inhomogeneous sample of ultra-cold, noncondensed Rubidium atoms in a magnetic trap. We use spatially selective microwave spectroscopy to verify a model that accounts for the differential forces on two internal spin states. In any simple understanding of the cloud dynamics, the forces are far too small to account for the dramatic transient spin polarizations observed. The underlying mechanism remains to be elucidated.
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