Squid concepts and systems

In this chapter I try to give an overview of the current state of the SQUID art. I cannot hope to describe all of the SQUIDs that have been made or, even less, all of the applications in which they have been successfully used. I begin, in Sec. II, with a brief review of the resistively-shunted Josephson junction, with particular emphasis on the effects of noise. Section III contains a description of the dc SQUID: how these devices are made and operated, and the limitations imposed by noise. Section IV contains a similar description of the properties of rf SQUIDs, but because there has been little development of these devices in the 1980's, I shall keep this section relatively brief. In Sec. V, I describe a selection of instruments based on SQUIDs and mention some of their applications. Section VI contains a discussion of the impact of high temperature superconductivity on SQUIDs, and of future prospects in this area, while Sec. VII contains a few concluding remarks. (orig./WL)