  A Bose-Einstein condensate (BEC) of rubidium atoms is prepared in one of two
degenerate energy minima in the second Bloch band of an optical square lattice.
A subsequent oscillation of the BEC between the two energy minima is observed,
which is driven by two distinct collision processes: the conventional
Hubbard-type on-site collision and a collision process that changes the orbital
flavor. The oscillation frequency scales with the relative strength of these
collisional interactions, which can be readily tuned via an experimentally well
controlled distortion of the unit cell. The observations are compared to a
quantum model of two single-particle modes and to a semi-classical multi-band
tight-binding simulation of 12x12 tubular sites of the lattice. Both models
reproduce the observed oscillatory quantum many-body dynamics and show the
correct dependence of the oscillation frequency on the ratio between the
strengths of the on-site and flavor-changing collision processes.
