Electric current is able to pass through a thin layer of insulator between two superconductors. A pair of contacts of this type allows you to measure the intensity of the magnetic field with the highest accuracy.
The Bardeen-Cooper-Schrieffer theory of superconductivity explains why at ultra-low temperatures the electrical resistance of a number of substances drops to almost zero, so that the electric current can circulate in them without loss for a very long time. This mechanism is based on electron pairing according to Cooper, the meaning of which is that paired electrons with oppositely directed spin practically cease to experience resistance from the side of the conductor, in contrast to single electrons, which provide electrical conductivity under normal conditions.
In 1962, Brian Josephson – then just an undergraduate student – realized that two superconducting layers, separated by a tiny insulator layer only a few atoms thick, would behave like a single system. Applying the principles of quantum mechanics to such a system, he showed that Cooper pairs will overcome this barrier (now it is usually called Josephson junction) even in the absence of voltage applied to them. The existence of an electric current of this kind was soon confirmed experimentally, and the effect itself was also called stationary Josephson effect…
If a constant voltage is applied on both sides of the junction, quantum mechanics predicts that Cooper pairs of electrons will begin to move through the barrier, first in one direction and then in the opposite direction, resulting in an alternating current, the frequency of which increases as the voltage rises. This effect is called nonstationary Josephson effect… Since the frequency of the current can be measured with great accuracy, the AC effect is now used for highly accurate voltage calibration.
However, perhaps the most common practical application of the Josephson effect stems from another prediction made by quantum mechanics. If we make a small superconducting circuit with two embedded Josephson junctions at each end, and then pass a current through it, we get a device called a superconducting quantum interferometer, or SQUID (from the English SQUID – Superconducting QUantum Interference Device). Depending on the intensity of the external electromagnetic field, the current in its circuit can vary from zero (when the currents coming from two transitions mutually cancel out) to a maximum (when they are unidirectional and reinforce each other).
The superconducting quantum interferometer is the most accurate device for measuring magnetic fields today, and at the same time it is very compact. It finds the widest practical application in a wide variety of fields, from earthquake prediction to medical diagnostics (cm. picture). And the history of the Josephson effect teaches us that the most abstract, seemingly physical discovery can bring enormous practical benefits.