Electrical effects observed when two superconducting materials (at low temperature) are separated by a thin layer of insulating material (typically a layer of oxide less than 10⁻⁸ m thick). If normal metallic conductors are separated by such a barrier it is possible for a small current to flow between the conductors by the tunnel effect. If the materials are superconductors (see superconductivity), several unusual phenomena occur:

1. (1) A supercurrent can flow through the barrier; i.e. it has zero resistance.

2. (2) If this current exceeds a critical value, this conductivity is lost; the barrier then only passes the ‘normal’ low tunnelling current and a voltage develops across the junction.

3. (3) If a magnetic field is applied below the critical current value, the current density changes regularly with distance across the junction. The net current through the barrier depends on the magnetic field applied. As the field is increased the net current increases from zero to a maximum, decreases to zero, increases again to a (lower) maximum, decreases, and so on. If the field exceeds a critical value the superconductivity in the barrier vanishes and a potential difference develops across the junction.

4. (4) If a potential difference is applied across the junction, a high-frequency alternating current flows through the junction. The frequency of this current depends on the size of the potential difference.

A junction of this type is called a Josephson junction; two or more junctions joined by superconducting paths form a Josephson interferometer. Such junctions can be used in measuring fundamental constants, in defining a voltage standard, and in the highly accurate measurement of magnetic fields. An important potential use is in logic components in high-speed computers. Josephson junctions can switch states very quickly (as low as 6 picoseconds). Moreover they have very low power consumption and can be packed closely without generating too much heat. The effects are named after Brian Josephson (1940– ), who predicted them theoretically in 1962.