SQUID (Superconducting Quantum Interference Device) is a very sensitive magnetic flux sensor based on magnetic flux quantization and the Josephson effect. This paper describes the principles of operation of SQUID and gives historical details of some RF SQUID prototypes. A general model for a SQUID magnetometer comprising a SQUID sensor, RF amplifier and phase-locked loop is described. Its flux resolution limit, maximum flux slew rate and input coil current sensitivity can be estimated, as described in the paper. Various instruments which apply SQUID are now under development…SQUID can be used to measure voltage, current, noise temperature, magnetic susceptibility, and the magnetic fields of the earth and the human body. The paper shows a magnetocardiogram obtained with a SQUID magnetometer; this magnetometer was developed at the authors' company. The paper concludes by listing references which describe some of the many possible applications of SQUID.
The national voltage standard has recently been maintained by using the Josephson effect in more than ten countries. The voltage standard had been maintained by the standard cell for a long time. The stability of the voltage standard has been improved by the introduction of the Josephson voltage standard. The Josephson voltage standard is based on the constancy and universality of the fundamental physical constant 2e/h. The adopted value of 2e/h in ETL is 483 594GHz/V. The accuracy of the Josephson voltage standard is 10-7 to 10-8 in the practical use. The further improvement of the accuracy would be expected. The commercial Josephson voltage standard system will be widely used in near future. In this article, the above profile of the Josephson voltage standard is described.
In this report, Josephson junction detectors for millimeter and submillimeter waves are surveyed. Emphasis is placed on heterodyne detectors (mixers). Others are briefly referred to, such as video detectors, bolometers, quasiparticle mixers and parametric amplifiers. Also, three kinds of Josephson junctions for detector use are mentioned and brief comments are given on them. These are tunnel junctions, thin film microbridges and point contacts. Thermally cyclable oxide barrier tunnel junctions have been obtained and used as low noise quasiparticle mixers at 115GHz. They have lower noise temperatures compared to Josephson effect mixers, but the receiver noise temperatures are not lowered owing to their lower conversion efficiencies. Thin film microbridges suffer from low junction resistances and also from heating effects caused by d.c. bias supply and r.f. incident power. There seems to exist many problems to be overcome in order to make good submillimeter wave detectors by tunnel junctions or microbridges. Point contacts are now widely used as sensitive detectors for millimeter and submillimeter waves. Although there exist difficulties for making rugged and thermally cyclable point contacts, the techniques for making them have made considerable progress, and a new construction of detector mount has appeared, which brought a low noise mixer at 115GHz thermally cyclable. It is shown that the sensitivity of video detectors decreases as proportional to the inverse square of frequency. So, for submillimeter waves, video detectors are inferior to superconducting bolometers. Recent progress in heterodyne detectors (fundamental and harmonic mixers) are touched on. They are superior to Shottky diode mixers in that they have much smaller NEP's and much higher conversion efficiencies (especially in harmonic mixers), and also in that they operate with much smaller LO power. Experimental curves of IF output power v.s. bias voltage are given for harmonic mixers with harmonic number N=7 and N=10. It is noted that harmonic mixers with even harmonic number are preferable, because they can operate at zero bias voltage. Among them, low even numbers 2 or 4 are recommended in regards to good conversion efficiency. By using Josephson junctions, a peculiar parametric operation is possible, namely doubly degenerate mode. This mode operates under zero d.c. bias by choosing pump frequency as 2ωp=ωs+ωi, ωp-ωs-ωi. Zero bias operation allows the adoption of microbridge arrays. A parametric amplifier with 160 microbridge arrays was built at 33GHz, and showed amplifier gain of 15dB and noise temperature of 20K.