Superconducting quantum interference device

A superconducting quantum interference device (SQUID) is a magnetometer made from a superconducting loop interrupted by one or more Josephson junctions. Magnetic flux through the loop changes its quantum-interference response, enabling exceptionally sensitive measurements.

Josephson junction: A weak link between superconductors that carries current through quantum tunneling or another coherent coupling mechanism. Its phase-dependent current makes the loop’s response sensitive to magnetic flux.
Magnetoencephalography: A noninvasive technique that maps magnetic fields produced by electrical activity in the brain. SQUID sensor arrays detect the brain’s extremely weak magnetic signals.
Superconductivity: A state of matter with zero electrical resistance and magnetic-field expulsion below a material-dependent critical temperature. The loop must be superconducting to sustain coherent currents and quantized flux.
Hall effect sensor: A magnetic-field sensor that measures a voltage generated when current-carrying charge carriers experience a magnetic field. It is compact and operates without cryogenic cooling, but is generally less sensitive.
Brian Josephson: A British physicist who predicted the coherent tunneling effects now known as the Josephson effects. His prediction supplied the junction physics on which SQUIDs depend.
DC SQUID: A SQUID with two Josephson junctions in a superconducting loop, usually operated with a bias current. Its two junctions produce an output voltage that varies periodically with applied flux.
Superconducting gravimeter: An instrument that measures changes in gravity using a superconducting test mass and magnetic suspension. SQUID readout can measure minute motion or force in superconducting gravimeter systems.
Cooper pair: A pair of electrons bound into a correlated state in a superconductor. Cooper pairs carry the supercurrent whose phase governs junction interference.
Fluxgate magnetometer: A vector magnetometer that detects fields through nonlinear magnetization of a periodically driven ferromagnetic core. It offers sensitive room-temperature measurements without superconducting circuitry.
John Lambe: An American physicist who helped develop early superconducting quantum interference devices. His work with Robert Jaklevic and colleagues demonstrated early point-contact SQUIDs.

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