SQUIDs and Detectors
The Superconducting QUantum Interference Device (SQUID) combines the phenomena of flux quantization and Josephson tunneling. First predicted by F. London1, flux quantization was observed experimentally by Deaver and Fairbank2 and Doll and Na¨bauer3 in 1961. They showed that the flux contained in a closed superconducting loop is quantized in units of the flux quantum Φ0 ≡ h/2e≈ 2.07×10−15 Wb, where, h≡ 2pih¯ is Planck’s constant, and 2e is the charge on the Cooper pair-the cornerstone of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity4. Flux quantization originates in the fact that the macroscopic wave function Ψ(~r, t) = Ψ0(~r, t)exp[iφ(~r, t)] must be single-valued in going once around a superconducting loop. In the absence of applied fields
or currents, the phase φ(~r, t) takes the same value throughout the superconductor for all Cooper pairs. In the case of a loop threaded by a magnetic flux, however, the phase around the loop changes by 2pi ·n, where n is the number of enclosed flux quanta. In the year preceding the observation of flux quantization, Giaever5 demonstrated the tunneling of single electrons between a superconductor (S) and a normal metal (N) separated by a thin insulating (I) layer. Subsequently, he observed the tunneling of single electrons through SIS junctions6. Single particle tunneling between superconductors was explained in terms of a tunneling Hamiltonian by Cohen, Falicov and Phillips7.