"Science cannot solve the ultimate mystery of Nature... because... we ourselves are part of [it]."
Max Planck, physicist
At the heart of a SQUID (Superconducting Quantum Interface Device) are two Josephson junctions. These consist of two superconductors separated by an insulating barrier that is so thin that electrons can "tunnel" through. If a voltage is applied, the current across the barrier starts to oscillate at a high frequency, this being influenced by the ambient magnetic field. In the process, the electrical resistance of the SQUID changes. These changes can be used to measure very small and weak magnetic fields.
The direct-current SQUID was invented by Robert Jaklevic, John Lambe, Arnold Silver, and James Mercereau in 1964, one year after the production of the first Josephson junction. The first models worked only at the temperature of liquid helium, -452°F (-269°C). The discovery of higher-temperature superconducting ceramics in 1987 meant that devices could be made to work at the temperature of boiling liquid nitrogen, at around -321°F (-196°C).
Magnetic fields that are a hundred billion times weaker than those exerted by a fridge magnet can easily be quantified. Magnetic fields produced in the brain, muscles, and nerves can be measured. SQUIDs also play a role in the testing of structures that contain metal. As non-natural objects affect their magnetic surroundings, SQUIDs can be used in delicate military surveillance tasks, as well as airport security.