  Time-dependent electron transport through a quantum dot and double quantum
dot systems in the presence of polychromatic external periodic quantum dot
energy-level modulations is studied within the time evolution operator method
for a tight-binding Hamiltonian. Analytical relations for the dc-current
flowing through the system and the charge accumulated on a quantum dot are
obtained for the zero-temperature limit. It is shown that in the presence of
periodic perturbations the sideband peaks of the transmission are related to
combination frequencies of the applied modulations. For a double quantum dot
system under the influence of polychromatic perturbations the quantum pump
effect is studied in the absence of source-drain and static bias voltages. In
the presence of spatial symmetry the charge is pumped through the system due to
broken generalized parity symmetry.
