  We report time-resolved measurements of Landau-Zener tunneling of
Bose-Einstein condensates in accelerated optical lattices, clearly resolving
the step-like time dependence of the band populations. Using different
experimental protocols we were able to measure the tunneling probability both
in the adiabatic and in the diabatic bases of the system. We also
experimentally determine the contribution of the momentum width of the Bose
condensates to the width of the tunneling steps and discuss the implications
for measuring the jump time in the Landau-Zener problem.
