A powerful set of universal relations, centered on a quantity called the contact, connects the strength of short-range two-body correlations to the thermodynamics of a many-body system with delta-function interactions. We report on measurements of th
e contact, using RF spectroscopy, for an $^{85}$Rb atomic Bose-Einstein condensate (BEC). For bosons, the fact that contact spectroscopy can be used to probe the gas on short timescales is useful given the decreasing stability of BECs with increasing interactions. A complication is the added possibility, for bosons, of three-body interactions. In investigating this issue, we have located an Efimov resonance for $^{85}$Rb atoms with loss measurements and thus determined the three-body interaction parameter. In our contact spectroscopy, in a region of observable beyond-mean-field effects, we find no measurable contribution from three-body physics.
We demonstrate a photon-counting technique for detecting Bragg excitation of an ultracold gas of atoms. By measuring the response of the light field to the atoms, we derive a signal independent of traditional time-of-flight atom-imaging techniques. W
ith heterodyne detection we achieve photon shot-noise limited detection of the amplification or depletion of one of the Bragg laser beams. Photon counting for Bragg spectroscopy will be useful for strongly interacting gases where atom-imaging detection fails. In addition, this technique provides the ability to resolve the evolution of excitations as a function of pulse duration.
We report on the observation of controllable spatial separation in a dual-species Bose-Einstein condensate (BEC) with $^{85}$Rb and $^{87}$Rb. Interparticle interactions between the different components can change the miscibility of the two quantum f
luids. In our experiments, we clearly observe the immiscible nature of the two simultaneously Bose-condensed species via their spatial separation. Furthermore the $^{85}$Rb Feshbach resonance near 155 G is used to change them between miscible and immiscible by tuning the $^{85}$Rb scattering length. Our apparatus is also able to create $^{85}$Rb condensates with up to $8times10^4$ atoms which represents a significant improvement over previous work.
