Cooper pairs

Cooper pairs are the bound couples of electrons that carry the resistance-free current in a superconductor. On the face of it this is paradoxical: electrons all carry negative charge and should repel one another, so how can two of them bind together? The answer is an indirect attraction. As one electron moves through the crystal it tugs slightly on the positive ions of the lattice, leaving a faint trail of extra positive charge that a second electron, passing later, is gently drawn toward.

Leon Cooper showed in 1956 that even an arbitrarily weak attraction of this kind is enough to bind electrons into pairs at low temperatures. The two partners need not be close together — they can be separated by thousands of atoms — and they pair with opposite momenta and opposite spins, forming a composite that behaves very differently from a lone electron. Crucially, while single electrons are fermions that refuse to share a state, a pair behaves like a boson, free to crowd into the same quantum state with countless others.

Because all the Cooper pairs occupy one and the same collective quantum state, they move in lockstep as a single coherent fluid. To slow one pair down by scattering, you would have to disturb the whole condensate at once, which costs more energy than ordinary jostling can supply. That is why the current flows without resistance. Cooper pairs are the microscopic heroes that turn a quantum subtlety into the macroscopic miracle of superconductivity.