Josephson Junction
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 wave function similar to a free particle wave function. In the DC Josephson effect, a current proportional to the phase difference of the wave functions 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.