By Brian David Josephson in 1962, the electrical current density through a weak electric contact between two superconductors depends on the phase difference Δφ of the two superconducting wave functions. Moreover, the time derivative of Δφ is correlated with the voltage across this weak contact. In a superconducting ring with one (so-called rf SQUID) or two (dc SQUID, fig. 1, blue) weak contacts, Δφ is additionally influenced by the magnetic flux Φ through this ring. Therefore, such a structure can be used to convert magnetic flux into an electrical voltage. This is the basic working principle of a SQUID magnetometer.