  We have investigated the transport characteristics of an electron pump
consisting of an asymmetric double quantum dot at zero bias voltage which is
subject to electromagnetic radiation. Depending on the energies of the
intermediate states in the pumping cycle, electrons can be transferred through
the dots incoherently via sequential tunneling or coherently via co-tunneling.
The dc transport through the system can be controlled by the frequency of the
applied radiation. We concentrate on resonant tunneling peaks in the pumping
current that are interesting to observe experimentally.
