Josephsonjunctions

A Josephson junction is a quantum electronic device formed by two superconductors separated by a thin non-superconducting barrier, such as an insulating oxide or a normal metal. The barrier allows Cooper-pair tunneling, enabling a supercurrent to flow across the junction without any voltage drop up to a critical current I_c. The junction’s behavior is governed by its critical current, resistance in the normal state, and capacitance, and it acts as a nonlinear, non-dissipative element at low temperatures.

The key relations governing Josephson junctions are the Josephson equations. The direct current relation states that the supercurrent I across the barrier is I = I_c sin(φ), where φ is the phase difference of the superconducting order parameters on the two sides. The second relation links the time evolution of the phase to the voltage via dφ/dt = 2eV/ħ. Together, these relations connect electrical signals to quantum phase dynamics.

In circuit models, the resistively and capacitively shunted junction (RCSJ) model describes the total current as I = I_c sinφ + V/R + C dV/dt, incorporating the nonlinear Josephson current, dissipation, and parasitic capacitance. This framework captures the rich dynamics of junctions, including Josephson oscillations and chaotic regimes under drive.

Josephson junctions come in several realizations, notably SIS (superconductor–insulator–superconductor) tunnel junctions and SNS (superconductor–normal–superconductor) weak links. Materials commonly used include aluminum-based SIS junctions with thin AlOx barriers, as well as Nb- and NbN-based devices.

Applications span superconducting qubits (such as transmons and phase qubits), superconducting quantum interference devices (SQUIDs) for ultra-sensitive magnetometry, voltage standards based on the AC Josephson effect, microwave detectors, and rapid single flux quantum logic.

Josephson junctions were predicted by Brian D. Josephson in 1962 and quickly confirmed experimentally, becoming foundational elements in superconducting electronics and quantum information science.