SQUIDs--Past, present, and future

The superconducting quantum interference device (SQUID) was the first superconducting electronic (SCE) circuit employing Josephson junctions. It has matured over approximately 15 years and is now the most widely used SCE device. Although its application is based on a fundamental periodic response, it has been developed into the most sensitive linear detection system for magnetic flux. As a result, SQUIDs are used for such diverse studies as flux quantization, superconducting properties, thermal noise and intrinsic fluctuations, geophysics, metrology, biomagnetism, susceptometry, nuclear magnetic resonance, oceanography, gravity waves, and relativity. Further improvement in performance is predicted. Additionally, micro-SQUIDs (configured in microcircuits) have a bright future as "ultimate" devices for parametric amplifiers, pulse counters, A/D converters, shift registers, digital logic, and memory. This paper reviews the history of this field, emphasizing recent developments, and projects the future direction of SQUID electronics.