Control of Interaction-Induced Dephasing of Bloch Oscillations

We report on the control of interaction-induced dephasing of Bloch oscillations for an atomic Bose-Einstein condensate in an optical lattice under the influence of gravity. When tuning the strength of the interaction towards zero by means of a Feshbach resonance, the dephasing time is increased from a few to more than twenty thousand Bloch oscillation periods. We quantify the dephasing in terms of the width of the quasi-momentum distribution and measure its dependence on time for different values of the scattering length. Minimizing the dephasing allows us to realize a BEC-based atom interferometer in the non-interacting limit. We use it for a precise determination of a zero-crossing for the atomic scattering length and to observe collapse and revivals of Bloch oscillations when the atomic sample is subject to a spatial force gradient.

Spectroscopy of Ultracold, Trapped Cesium Feshbach Molecules

We explore the rich internal structure of Cs_2 Feshbach molecules. Pure ultracold molecular samples are prepared in a CO_2-laser trap, and a multitude of weakly bound states is populated by elaborate magnetic-field ramping techniques. Our methods use different Feshbach resonances as input ports and various internal level crossings for controlled state transfer. We populate higher partial-wave states of up to eight units of rotational angular momentum (l-wave states). We investigate the molecular structure by measurements of the magnetic moments for various states. Avoided level crossings between different molecular states are characterized through the changes in magnetic moment and by a Landau-Zener tunneling method. Based on microwave spectroscopy, we present a precise measurement of the magnetic-field dependent binding energy of the weakly bound s-wave state that is responsible for the large background scattering length of Cs. This state is of particular interest because of its quantum-halo character.

`Stuckelberg interferometry with ultracold molecules

We report on the realization of a time-domain `Stuckelberg interferometer, which is based on the internal state structure of ultracold Feshbach molecules. Two subsequent passages through a weak avoided crossing between two different orbital angular momentum states in combination with a variable hold time lead to high-contrast population oscillations. This allows for a precise determination of the energy difference between the two molecular states. We demonstrate a high degree of control over the interferometer dynamics. The interferometric scheme provides new possibilities for precision measurements with ultracold molecules.