JosephsonJunctionen

A Josephson junction is a quantum device consisting of two superconductors separated by a thin barrier through which Cooper pairs can tunnel. The prototype is an SIS junction, where a thin insulating barrier allows tunneling, but SNS junctions with a normal-metal barrier between superconductors are also common. The concept was proposed by Brian D. Josephson in 1962 and later confirmed experimentally, forming a cornerstone of superconducting electronics.

Two fundamental relations govern a Josephson junction. The DC Josephson relation states that a supercurrent I can flow across the barrier without any voltage, following I = I_c sin(phi), where I_c is the critical current and phi is the phase difference between the superconductors. The AC Josephson relation links a constant voltage V to the time evolution of the phase, dphi/dt = 2eV/ħ, which implies that a fixed voltage drives oscillations at a frequency f = (2e/h)V. These effects enable precise frequency-to-voltage conversions used in metrology.

Junctions are realized in various materials. Aluminum-based junctions with Al2O3 barriers and niobium-based junctions with native or engineered barriers are common in research and industry. SIS devices typically use a thin insulating layer, while SNS devices insert a normal metal like copper or gold between superconductors such as Nb or NbTiN. Long and short junctions, as well as arrays, enable diverse circuit functionalities.

Key parameters include the critical current I_c, the normal-state resistance R_n, and the I_c R_n product, which characterizes the junction’s energy scale. The Josephson energy E_J = ħ I_c/(2e) governs quantum behavior and is related to the superconducting phase dynamics.

Applications span highly sensitive magnetometers called SQUIDs, superconducting qubits for quantum computing (including various transmon and phase qubit implementations), voltage standards via the AC Josephson effect, and rapid single flux quantum logic in digital circuits. Josephson junctions remain central to quantum information science and precision metrology.