  We demonstrate that the tunnel oscillations of a biased double quantum dot
can be employed as driving source for a quantum ratchet. As a model, we use two
capacitively coupled double quantum dots. One double dot is voltage biased and
provides the ac force, while the other experiences the ac force and acts as
coherent quantum ratchet. The current is obtained from a Bloch-Redfield master
equation which ensures a proper equilibrium limit. We find that the
two-electron states of the coupled ratchet-drive Hamiltonian lead to unexpected
steps in the ratchet current.
