  We investigate parametric pumping of a spin-polarized current through a
nearly-closed quantum dot in a perpendicular magnetic field. Pumping is
achieved by tuning the tunnel couplings to the left and right lead - thereby
operating the quantum dot as a turnstile - and changing either the magnetic
field or a gate-voltage. We analyze the quantum dynamics of a pumping cycle and
the limiting time scales for operating the quantum dot turnstile as a pure spin
pump. The proposed device can be used as a fully controllable double-sided and
bipolar spin filter and to inject spins "on demand".
