A squid (“superconducting quantum interference device”) is an electronic system that uses a superconducting ring in which one or two small insulating layers have been inserted (see figure). This device based on the Josephson effect in the superconductor-insulator-superconductor sandwich and on the flux quantization in the ring makes it ultra-sensitive to any magnetic field.
Squids are hence the most efficient systems to measure magnetic fields with great accuracy, even the weaker ones. For instance, squids are sensitive enough to measure the magnetic activity of the human brain in real time, enabling very precise magnetoencephalographic measurements.
Squids are used whenever very powerful magnetometers are needed: in physics, archaeology, and geology… Most squids work with conventional superconductors, but some squids made of cuprates and working at the temperature of liquid nitrogen are starting to be marketed. Liquid nitrogen being easier to manipulate, these devices are more easily transportable and can be used outside a laboratory.
