Low-frequency applications of superconducting quantum interference devices

The applications of Josephson junctions to the measurement of low-frequency magnetic fields and voltages are reviewed. The relevant ideas of flux quantization and Josephson tunneling are very briefly reviewed, and the various methods of making Josephson junctions mentioned. The two basic types of magnetic sensor, the dc superconducting quantum interference device (SQUID) and the RF SQUID, are described in some detail. Their theory of operation, noise limitations, and application to practical devices are discussed. The resolution of SQUID's commonly used in an analog mode is 10 -4 to 10 -3 Φ 0 /√Hz, where Φ 0 is the flux quantum. The basic sensor may be used in conjunction with a flux transformer to measure magnetic fields (with a resolution of 10 -14 T/√Hz), magnetic-field gradients (with a resolution of 10 -12 T/m/√Hz), and magnetic susceptibilities. The SQUID may also be used as a voltmeter. The resolution is limited by the Johnson noise developed in the resistance of the low-temperature circuit, provided that this resistance is not too large: the upper limit in the liquid-He 4 temperature range appears to be a few tens of ohms.