The Josephson junction is named after the 1973 physics Nobel Prize winner Brian Josephson who carried out extensive theoretical work on its properties. The junction itself consists of two superconducting metals separated by a thin insulating oxide layer usually only 10-20 angstroms, or equivalently about 30 atomic layers, in width. The remarkable properties of the junction are due to a superconducting current that can flow through the oxide layer even at zero voltage difference between the outer plates. The origin of this current depends crucially on the quantum mechanical principles underlying superconductivity.

In a superconducting metal, electrons exert forces on the crystal lattice of the metal. This force displaces positive ions in the lattice and sets up a polarization that can in turn exert attractive forces on the original electrons. This net force is ordinarily much weaker than the usual Coulomb repulsion between electrons, but this Coulomb force...