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elements of superconducting devices. They are most commonly used in SQUIDs and Rapid Single Flux Quantum integrated circuits.

A Josephson junction is the interface between two superconducting materials separated by a non-superconducting barrier. A current may flow freely within the superconductors, but the barrier prevents the current from flowing freely between them. However, the supercurrent may tunnel through the barrier, depending on the quantum phase of the superconductors. The amount of supercurrent that may tunnel through the barrier is restricted by the size and substance of the barrier. The maximum value the supercurrent may attain is called the critical current of the Josephson junction, and is an important phenomenological parameter of a junction.

Josephson junctions have two basic electrical properties. The first is that the junctions have an inductive reactance. That is, similarly to inductors, the voltage difference across a junction is related to the time rate of change of the current. The second is that a constant voltage across a junction will produce an oscillating current through the barrier, and vice versa. Thus, Josephson junctions convert a direct-current voltage to an alternating-current current.

There are two general types of Josephson junctions: overdamped and underdamped. In overdamped junctions, the barrier is conducting ( normal metal or superconductor bridge). The effects of the junction's internal electrical resistance will be large compared to its small capacitanceA capacitor (historically known as a "condenser") is a device that stores energy in the electric field established between a pair of conductors on which equal but opposite electric charges have been impressed. Historically, capacitors have taken the form. An overdamped junction will quickly reach a unique equilibriumFor the 2002 science fiction movie see Equilibrium (2002 movie Equilibrium or balance is any of a number of related phenomena in the natural and social sciences. In general, a system is said to be in a state of equilibrium if all influences on the system state for any given set of conditions. The barrier of an underdamped junction is an insulatorInsulators are materials which prevent the flow of heat (thermal insulators) or electrical current (electrical insulators). The opposite of electrical insulators are conductors and semiconductors, which permit the flow of current (Note: a semiconductor is. The effects of the junction's internal resistance will be minimal. Underdamped junctions do not have unique equilibrium states, but are hystereticHysteresis is a property of systems (usually physical systems) that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state: that is, systems whose states depend on their immediate history..