SQUID devices

C. Hilbert and J. Clarke, SQUID 85, Proceedings of the International Conference on SQUID Devices and Their Applications, H. D. Hahlbohm and H. Lubbit, eds., Walter de Gruyter, Berlin 1985. 

Simple SQUID devices in a few forms have already been demonstrated to operate in the liquid nitrogen temperature range. Koch et al. (48) successfully operated at 68 K a one-layer thin film dc SQUID with long, granular, weak-link bridges, while Iguchi et al. (49) and Zimmerman et al.(50) successfully operated rf- and dc-SQUIDs, respectively, based on bulk ceramic loops... 

Fig. 6.50. A loop containing two Josephson junctions for a dc-SQUID device (schematic).

A further enhancement is provided by alternative detection methods. Pines and co-workers " " have coupled the use of HP xenon with a SQUID, which provides very high-sensitivity detection of magnetic fluxes at low resonance frequencies. The authors showed that the use of optically pumped xenon and a high-Tc SQUID device could be coupled to allow the detection of laser-polarized xenon gas at only 27 kHz, corresponding to a magnetic field of 2.3 mT. MRI of xenon adsorbed on a piece of silica aerogel was also shown using this approach (see Fig. 27). " SQUID... 

Well, first of all, there are the ingenious experiments designed to show us that quantum effects can, indeed, appear in the macroscopic realm. In the form of SQUID devices, with their superimposed superconducting currents, such macroscopic quantum devices can become practical measuring instruments. And there are such delicate laboratory experiments such as the preparation of macroscopic collections of atoms in a single degenerate Bose-Einstein state. But now the claim might be that such quantum effects, if not limited to the microscopic, are, perhaps, limited to special, technically prepared situations of scientific artifacts. 

To realize an automatic evaluation system, it would be desirable to also suppress geometrically caused signals as well, so that only the actual defect signals are obtained. Several approaches have already been made which are also to be implemented as part of a SQUID research project (SQUID = Super Conducting Quantum Interference Device). 

The modern approach to measuring magnetic properties is to use a superconducting quantum interference device (a SQUID), which is highly sensitive to small magnetic fields and can make very precise measurements on small samples. 

Magnetic field detectors (superconducting quantum interference devices or SQUIDS). 

Stoichiometric reaction of 5 with phenylsilane produced the iron(O) bis(silane) c-complex 18, which was confirmed by the single-crystal X-ray analysis as well as SQUID (Superconducting QUantum Interference Device) magnetometry (Scheme 19). Complex 18 as a precatalyst showed high activity for the hydrosilylation of 1-hexene. 

The main hardware types offered by physics are mentioned, namely trapped ions (or trapped atoms), quantum dots, quantum optical cavities, rf superconducting quantum interference devices (SQUIDs) and nitrogen-vacancy (NV) defects on diamond. Some are important simply as a benchmark to evaluate the quality of the implementations offered by chemistry, whereas others might be combined with lanthanide complexes to produce heterogeneous quantum information processors which combine the advantages of different hardware types. 

The superconducting quantum interference device (SQUID) is formed from a superconducting loop containing at least one Josephson junction. Basically, a SQUID amplifier converts an input current to an output voltage with a transresistance of the order of 107 V/A. The input noise is of the order of 10-11 A/(Hz)1/2. The bandwidth of the SQUID amplifier can be up to 80kHz. The dynamic range in 1 Hz bandwidth can be 150dB. 

The SQUID obviously has several potential advantages for on-line applications. It can operate at very low inhomogeneous magnetic fields, avoiding the need for large expensive shimmed magnets. It also does not require tuning and has very low power requirements. However there are also severe technical limitations to be overcome before it can be used in on-line commercial operations. The whole SQUID needs to be immersed in liquid helium and a conventional RF coil is still needed to excite the NMR resonance. It remains to be seen whether such devices will find future on-line application in horticulture. 

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