  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.
