  We experimentally and numerically investigate the quantum accelerator mode
dynamics of an atom optical realization of the quantum delta-kicked
accelerator, whose classical dynamics are chaotic. Using a Ramsey-type
experiment, we observe interference, demonstrating that quantum accelerator
modes are formed coherently. We construct a link between the behavior of the
evolution's fidelity and the phase space structure of a recently proposed
pseudoclassical map, and thus account for the observed interference
visibilities.
