Electronic circuits can be built from Josephson junctions, especially digital logic circuitry. Many researchers are working on building ultrafast computers using Josephson logic. Josephson junctions can also be fashioned into circuits called SQUIDs--an acronym for superconducting quantum interference device. These devices are extremely sensitive and very useful in constructing extremely sensitive magnetometers and voltmeters. For example, one can make a voltmeter that can measure picovolts. That's about 1,000 times more sensitive than other available voltmeters.

A SQUID consists of a loop with two Josephson junctions interrupting the loop. A SQUID is extremely sensitive to the total amount of magnetic field that penetrates the area of the loop--the voltage that you measure across the device is very strongly correlated to the total magnetic field around the loop.

SQUIDs are being used for research in a variety of areas. Since the brain operates electrically, one can, by sensing the magnetic fields created by neurological currents, monitor the activity of the brain--or the heart. You can also use a SQUID magnetometer for geological research, detecting remnants of past geophysical changes of the earth's field in rocks.

Similarly, changes in the ambient magnetic field are created by submarines passing below the surface of the ocean, and the U.S. Navy is very interested in SQUIDs for submarine detection. SQUIDs are also of considerable use in the research laboratory in specially designed voltmeters, in magnetometers and susceptometers and in scanning SQUID microscopes. In this last instrument, a SQUID is scanned across the surface of a sample, and changes in magnetism at the surface of the sample produce an image.

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