Principles and applications of SQUIDs

An overview of the current status of DC and RF SQUIDs (superconducting quantum interference devices) is presented. Using Nb-based thin-film technology, a number of groups can now routinely fabricate DC SQUIDS with input circuits integrated on the same chip. At 4.2 K, the DC SQUID, which operates with a constant bias current, can detect signals with an equivalent energy per unit bandwidth of a few hundred h2 pi . RF SQUIDs, which operate with an RF flux bias, are usually machined from bulk Nb, and have a typical energy sensitivity per unit bandwidth of 10/sup 5/ to 10/sup 6/h2 pi . Both types of SQUID can be used in a wide variety of applications, including magnetometers, gradiometers, susceptometers, voltmeters, RF amplifiers, gravity-wave antennas, and gravity gradiometers. Both DC and RF SQUIDs have been fabricated from high-temperature superconductors and operated at temperatures of up to 77 K. These devices exhibit high levels of 1/f noise, which generally arises from the motion of trapped magnetic flux.<
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