  We analyze transport through both a double quantum dot and a triple quantum
dot with inhomogeneous Zeeman splittings in the presence of crossed dc and ac
magnetic fields. We find that strongly spin-polarized current can be achieved
by tuning the relative energies of the Zeeman-split levels of the dots, by
means of electric gate voltages: depending on the energy level detuning, the
double quantum dot works either as spin-up or spin-down filter. We show that a
triple quantum dot in series under crossed dc and ac magnetic fields can act
not only as spin-filter but also as spin-inverter.
