| We report on two experiments with Bose-Einstein condensates (BEC) of Cs atoms loaded into optical lattices. In both experiments we exploit the tunability of the strength of the interaction near Feshbach resonances. In the first experiment we study the combination of nonlinear matter wave optics and matter-wave interferometry. We demonstrate macroscopic matter-wave interference that is driven and controlled by nonlinear particle interactions. By loading a (BEC) into a matter-wave interferometer, we find that the nonlinear phase evolution is highly coherent, leading to (coherent) wave function dephasing as seen in high contrast matter-wave interference patterns. The coherence gives rise to a surprisingly high degree of control over the system, in particular allowing us to reverse the interaction-induced dephasing in a matter-wave spin-echo-type experiment when the strength of the interaction is switched to zero. In the second experiment we realize a 1D quantum gas with strong attractive interactions in the presence of a 2D lattice. When the 3D scattering length is increased towards and beyond the length scale set be the tight transversal confinement, we observe a confinement-induced resonance (CIR) that allows us to tune the effective 1D-interaction parameter g1D to large positive and large negative values. We enter deeply into the Tonks-Girardeau (TG) regime (g1D>0) and from there into super-Tonks-Girardeau regime (g1D<0) as evidenced by a characteristic change of the breathing mode frequency along the 1D direction. We find, in contrast to the TG regime, that the release energy strongly depends on g1D. We also find that the sTG gas is surprisingly stable despite the fact that the interaction is strongly attractive. |
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