Fig. 5 shows a so-called DC SQUID. It consists of two superconductors connected by two weak links. The fluxoid quantization of a loop through the two bulk superconductors and the two weak links demands
If the self-inductance of the loop can be neglected the magnetic flux in the loop Φ is equal to the applied flux
with B the magnetic field, applied perpendicular to the surface, and A the surface area of the loop. The total supercurrent is given by
Substitution of Eq() in () gives
Using a well known geometrical formula we get
Since the sin-function can vary only between −1 and +1 a steady solution is only possible if the applied current is below a critical current given by
Note that the critical current is periodic in the applied flux with period . The dependence of the critical current on the applied flux is depicted in Fig. 6. It has a strong resemblance with the interference pattern generated by a laser beam behind a double slit. In practice the critical current is not zero at half integer values of the flux quantum of the applied flux. This is due to the fact that the self-inductance of the loop cannot be neglected.