Josephsonkopplade

Josephsonkopplade refers to the coupling between superconducting elements mediated by the Josephson effect. It occurs when two superconductors are connected by a thin insulating barrier or another weak link, allowing a supercurrent to flow without a measurable voltage up to a characteristic critical current. Beyond this current, a voltage develops across the junction and the superconducting phase relation governs the dynamics.

The underlying mechanism is the phase coherence of the superconducting order parameter. The DC Josephson relation states that the supercurrent I through the junction equals Ic sin(phi), where phi is the phase difference between the superconductors. The AC Josephson relation links the time rate of change of the phase to the voltage via dphi/dt = 2eV/ħ. The barrier type (SIS, SNS, SFS) and material properties determine Ic and the frequency response of the device.

Josephson coupling is sensitive to temperature, barrier transparency, and magnetic fields. The current-voltage characteristic features a zero-voltage branch up to Ic, followed by a finite voltage region. The coupling strength decreases with barrier thickness and with temperature as superconductivity weakens. In practice, weak links behave as nonlinear inductors with a phase-dependent inductance that is central to superconducting circuit design.

Applications include superconducting quantum interference devices (SQUIDs) used for ultra-sensitive magnetometry; superconducting qubits for quantum computing (for example, transmon and phase qubits rely on Josephson junctions as nonlinear inductors); and rapid single flux quantum logic for digital superconducting electronics. The AC Josephson effect also provides a metrological voltage standard based on frequency measurement.