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How do SQUIDs detect very small magnetic fields?

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conductive ceramics Figure 1: Schematic diagram of a zirconia oxygen sensor used to monitor automobile exhaust gases. The sensor, approximately the size of a spark plug, is fitted into the exhaust manifold of an automobile engine. The thimble-shaped zirconia sensor, sandwiched between thin layers of porous platinum, is exposed on its interior to outside air and on its exterior to exhaust gas passing through slits in the sensor shield. The two platinum surfaces serve as electrodes, conducting a voltage across the zirconia that varies according to the difference in oxygen content between the exhaust gas and the outside air.

A SQUID (superconducting quantum interference device) detects extremely weak magnetic fields by utilizing a superconducting ring interrupted by a "weak link," or Josephson junction.1

This junction consists of an insulating barrier between two superconductors, allowing Cooper pairs of electrons to tunnel across the gap. The device operates on the principle of quantum interference, where the presence of an external magnetic field influences the flow of current through the junction. Specifically, the SQUID converts these magnetic field variations into a measurable voltage. Because the device is highly sensitive to these quantum-level changes, it can detect minute magnetic oscillations, such as those produced by the human body or the Earth's magnetic field.231