The Ubiquitous SQUID: History and Applications

One-half century after the discovery of superconductivity, superconducting electronics was born in a remarkably short time: 1961 -- 1964. The essential steps were the observation of the quantization of magnetic flux in a superconducting loop in units of the flux quantum, the prediction and subsequent observation of Josephson tunneling, and the invention of the SQUID---Superconducting QUantum Interference Device. The SQUID is an ultrasensitive detector of changes in magnetic flux that one can configure to measure many other physical properties, for example current, voltage, magnetic field gradient and high frequency electromagnetic fields. SQUIDs have a wide range of applications. A widely used commercial system enables the automated measurement of a great variety of magnetic and other physical properties, ranging from high temperature superconductors to blood samples. A SQUID-based geophysical survey technique has discovered the world's largest deposit of silver. In magnetoencephalography (MEG), helmets containing typically 300 SQUIDs detect signals emanating from the human brain. This technique is used in presurgical mapping of brain tumors and locating and presurgical mapping of epileptic centers, as well as in many research topics. Ultralow field magnetic resonance imaging (ULFMRI) at fields comparable with the Earth's field offers high tissue contrast, for example, in imaging tumors. The combination of MEG and ULFMRI using the same SQUID array offers intriguing new clinical possibilities. Astronomical telescopes containing many thousands of SQUIDs reading out superconducting transition edge sensors---with single photon sensitivity---have made major advances in studies of the cosmic microwave background. The Axion Dark Matter eXperiment (ADMX), enabled by a high frequency SQUID amplifier, is the world's most sensitive detector in the search for the axion, a candidate particle for the cold dark matter that is the dominant form of mass in the Universe.