Credit: Department of Physics, University of Basel, Scixel
Electrons leave a (conventional) superconductor (S) only as pairs and only with opposite spins (arrow up or down, red or blue). If both electron paths are blocked for the same type of spin by parallel spin filters (here for spin down (blue)), a single electron with spin up (red) could in principle exit, but paired electrons from the superconductor are blocked, which ideally suppresses both electrical currents.