(a) A singly occupied quantum dot, shown with the circle to the left, acts as a spin-1/2 impurity. Once well coupled to a normal electrode N, the spin on the dot (blue arrow) can be screened by the spins (red arrows) of the conducting electrons present at the Fermi level of the contact. (b) When the singly occupied quantum dot is coupled instead to a superconducting electrode S, the screening problem becomes complex. In a superconductor, electrons with antiparallel spin are paired to form Cooper pairs, which cannot screen the spin-1/2 impurity on the dot. The system will then have to choose between an unscreened QD with an undisturbed superconductor or a screened QD with a disturbed superconductor.