Generation of pure spin currents in a single electron transistor with a superconducting island

We describe a spin filter at the single-electron level that produces pure spin currents with no net bias. Our device is based on the ground-state energetics of a single-electron transistor comprising a superconducting island connected to normal leads via tunnel barriers with different resistances that break spatial symmetry. The current has opposite spin polarization when the current is reversed, which leads to a dc spin current when applying an alternating charge current with zero mean, as expected in a spin ratchet. We demonstrate spin transport and quantify the spin ratchet efficiency by means of ferromagnetic leads with known spin polarization.