Fundamental Physics with squids
The best energy sensitivity expected for a squid operating conventionally in the linear detector mode is believed, on both experimental and theoretical grounds, to be ~ℏ/2 J Hz−1 (see section 4.3.1). As we have already stressed, in almost all real applications environmental noise actually limits the sensitivity at a level several orders of magnitude greater than this. However there are a number of fundamental experiments being carried on at the time of writing which are able to use the full capabilities of a quantum noise-limited squid. These investigations involve tests of general relativity, searches for fundamental particles and high-precision tests of some basic physical laws. In this section a variety of different experiments will be discussed in some detail, both because of the challenging nature of the effects which they are intended to investigate and because they provide classic examples of the type of high-precision measurement which is only possible using the unique properties of superconducting electronics.