The basic principle of operation is closely linked to flux quantisation. This is the phenomenon that the favoured states for a loop of superconductor are those where the flux inside is a multiple of the flux quantum.
Most SQUIDs are fabricated from lead or pure niobium. The lead is usually in the form of an alloy with 10% gold or indium, as pure lead is unstable when its temperature is repeatedly changed. The base electrode of the SQUID is made of a very thin niobium layer, formed by deposition, and the tunnel barrier is oxidised onto this niobium surface. The top electrode is a layer of lead alloy deposited on top of the other two, forming a sandwich arrangement.
SQUIDs are used to measure extremely tiny magnetic fields; they are currently the most sensitive such devices known, with noise levels as low as 3 fT/sqrt(Hz). Some processes in animals produce very small magnetic fields (typically sized between a billionth of a Tesla and a thousand billionth of a Tesla - a typical fridge magnet is a tenth of a Tesla), and SQUIDs are well suited to studying these. Magnetoencephalography (MEG), for example, uses measurements from an array of SQUIDs to make inferences about neural activity inside brains. Because SQUIDs can operate at acquisition rates much higher than the highest temporal frequency of interest in the signals emitted by the brain (kHz), MEG achieves good temporal resolution. Another application is the scanning SQUID microscope, which uses a SQUID immersed in liquid helium as the probe. The use of SQUIDs in oil prospecting, earthquake prediction and geothermal energy surveying is becoming more widespread as superconductor technology develops.