  For a symmetric bridge coupled to infinite leads, in the presence of a
dipole-coupled external ac-field with harmonic mixing, we solve the
Schr\"odinger equation in the time-domain using open boundary conditions as
well as in the energy-domain using Floquet scattering theory. As this potential
breaks parity and generalized parity, we find a non-vanishing average current.
We then optimize the relative amplitude ratio between the fundamental and the
second harmonic leading to a maximum in the pump current.
