Two superconductors separated by a thin insulating layer can experience tunneling of Cooper pairs of electrons through the junction. The Cooper pairs on each side of the junction can be represented by a wavefunction similar to a free particle wavefunction. In the DC Josephson effect, a current proportional to the phase difference of the wavefunctions can flow in the junction in the absence of a voltage. In the AC Josephson effect, a Josephson junction will oscillate with a characteristic frequency which is proportional to the voltage across the junction. Since frequencies can be measured with great accuracy, a Josephson junction device has become the standard measure of voltage.

The wavefunction which describes a Cooper pair of electrons in a superconductor is an exponential like the free particle wavefunction. In fact, all the Cooper pairs in a superconductor can be described by a single wavefunction in the absence of a current because all the pairs have the same phase - they are said to be "phase coherent" (Clarke). If two superconductors are separated by a thin insulating layer, then quantum mechanical tunneling can occur for the Cooper pairs without breaking up the pairs. Clarke envisions this condition as the wavefuntions for Cooper pairs on each side of the junction penetrating into the insulating region and "locking together" in phase. Under these conditions, a current will flow through the junction in the absence of an applied voltage (the DC Josephson effect).

Superconductivity concepts

Reference Ohanian

Clarke