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.
Logic and memory application is one of the most attractive application fields for Josephson devices. Recent studies on this field are outlined and future prospects are discussed. A variety of high-sensitivity Josephson switching devices have been progressively investigated for implementing a highspeed, low-power integrated logic circuit. Attention is paid to a nonlinear current in jection interferometer logic scheme, in which logical delay as fast as 13ps has been experimentally obtained with speed-power product less than 30aJ. In memory application, high-density integration has been predominantly investigated. A 16K bit RAM model circuit as well as a 64bit fully-decoded high speed RAM is successfully fabricated. Access time as fast as 2ns for 64bit RAM and 7ns for 16K bit RAM is obtained experimentally with extremely low power dissipation. Also, fabrication technology has been dramatically advanced, especially for Pb-alloy Josephson devices. Essential problems on reproducibility, production yield, and reliability for large scale integration are almost solved today. Hereafter, large scale integration, high-density packaging, high-efficiency cryo-cooling etc. for ultrafast Josephson computer assembly will be studied and developed. Functional logic device scheme and material research beside Pb-alloy are also interesting in the future. Much work in this respect will be expecsed.
Some technical problems are described concerning the field measurements of the geomagnetic field by using SQUID magnetometer. In the natural circumstances, the atmospheric temperature change, wind, rain and even the thunder activities bring several disturbances into the measurements of the geomagnetic field using SQUID. Among them, some of the troubles are attributed to the lower slew rate of the SQUID electronic circuits compared to that of the phenomena. Others come from the temperature effect of, also, the circuits, in particular, from the amplitude change of the RF oscillator due to the temperature change. Several examples observed at Mt. ASO (lower magnetic noise site) are shown. A comparison of small geomagnetic changes at Mt. ASO with that at Syowa base, Antarctica, is made, and the coincidental events are found between the two sites separated more than 10, 000km.
Biomedical Magnetic Measurement with SQUID Magnetometer SQUID magnetometer has made it possible to measure extremely weak magnetic field produced by biological electromotive force or other mechanisms. Biomedical magnetic informations are divided into two categories. One is the fluctuating magnetic field evoked by ion current of excitative tissues, i.e. cardiac muscle, brain, skeletal muscle, etc. These magnetic informations are called magnetocardiogram (MCG), magneto-encephalogram (MEG) and magnetomyogram (MMG) respectively. The other is the steady magnetic field induced by the magnetic particles taken into the lung or the gastointestinal tract. Levels of these biomagnetic informations are very low, generally less than 10-6 gauss. Taking into the consideration of background magnetic noise in urban environment, which is about 10-3 gauss, it is very difficult to detect these informations in good S/N ratio. To solve this problem, usually two methods are taken. One is the magnetic shielding method and the other is to use nosie canceling flux transformer. The latter method is rather practical and effective. Especially, a flux transformer of second derivative type is able to detect MCG or other biomagnetic informations without magnetically shielded room. As a medical-use system, materials and configuration of dewar containing liquid-He should be further studied. From a clinical standpoint, where and how to lead MCG or MEG is a important problem. So far, useful and standard method has not been established. MCG, which has the highest level in the biomagnetic informations, has been investigated from the early stage, and even now, is the main theme in this area. Usually, normal components of magnetic B vector to the chest wall are recorded as MCG and their patterns are similar to electrocardiograms (ECG). But differences between them are clear and interpretation and meaning of them are to be studied according to various cardiac patients. A significance of measuring biomagnetic informations has been discussed conceptually or theoretically. From these theoretical points of view, a meaning of magnetic measurement is definite. It is now very important to find practical usefulness based on lots of clinical data. Another interesting method using SQUID magnetometer is a “magnetic tracer”. This method is to detect and follow the dynamics of innocent magnetic particles like magnetite inhaled artificially in the lung. A time-course of magnetic diminishing indicates “clearance function” of the lung. It has been reported that asbestos miners, arc welders or heavy smokers impair this function. Magnetic tracer method, like radioisotope, has a great possibility to apply to various medical fields. A research on biomagnetic information is now only on the start line. The SQUID magnetometer is a strong and unique tool for measurement these informations. Development of reasonable, handy and stable meadical-use SQUID system is expected.
Requirements of small closed-cycle refrigeration systems to cryoelectronic devices have been increased. Most of the small superconducting devices including the applications of SQUID need the cooling temperature near 4K. For this temperature region, high efficiency and reliability is difficult to obain by using a single refrigerator. This paper reviews small closed-cycle refrigerators as follows: (1) Requirements for cryocooler. (2) Introductions of refrigeration cycles of interest. (3) Refrigeration systems for small superconducting devices. Most of the informations were obtained from the proceedings of a meeting held at NBS in Oct. 1977.