Josephson Junction Detectors

How can I measure a voltage of 10 V? Can I detect the magnetic field generated by the human brain?What is the most sensitive detector of far infrared signals reaching us from distant galaxies?The answers to these seemingly unconnected problems, and to others requiring very sensitive instrumentation, are being provided to an increasing extent by devices based on Josephson junctions (1).These devices offer unprecedented sensitivity in certain applications, and enable us to measure incredibly small signals that are totally inaccessible to other instruments.The Josephson effect is a superconducting phenomenon, and occurs only at temperatures below a few degrees Kelvin.Consequently, until recently operation of Josephson devices was restricted to the low-temperature physics laboratory.However, with the advent of robust and portable cryostats, these instruments are now being used in the field in a variety of applications.In this article, I will explain the principles of the Josephson effect, describe how practical devices have been made, and discuss some of the applications.I have divided the applications into two broad classes.The first involves the measurement of voltages, magnetic fields, magnetic field gradients, and magnetic susceptibilities at or below audio frequencies (2).In these areas, the Josephson devices have no competitors.The second class is concerned with the detection of high frequency (microwave and far infrared) electromagnetic radiation (2).In this application, the Josephson devices appear to be comparable in performance with other detectors, but improvements are likely to be made in the reasonably near future.In this short review, I have not been able to even mention a great