In 1964, Jaklevic, Lambe, Silver and Mercereau demonstrated quantum interference in a superconducting ring containing two Josephson tunnel junctions. The following year saw the appearance of the SLUG (Superconducting Low-inductance Undulatory Galvanometer)_a blob of solder frozen around a length of niobium wire_that was used as a voltmeter with femtovolt resolution. Although primitive by today's standards, the SLUG was used successfully in a number of ultrasensitive experiments, including a comparison of the Josephson voltage-frequency relation in different superconducting materials and the detection of charge imbalance in superconductors.
A full theory of the dc SQUID (Superconducting QUantum Interference Device) appeared in 1977.
Today, the square washer dc SQUID with an integrated input coil finds a wide range of applications. SQUIDs are used in a variety of configurations_for example, magnetometers, gradiometers, low-frequency and microwave amplifiers, and susceptometers_in applications including magnetoencephalography, magnetocardiography, geophysics, nondestructive evaluation, standards, cosmology, reading out superconducting quantum bits, and a myriad of one-of-a-kind experiments in basic science. Experiments are described to hunt for the axion_a candidate for cold dark matter_ and to perform magnetic resonance imaging in microtesla magnetic fields.
The square washer dc SQUID with an integrated input coil finds a wide range of applications.