How does a squid work? A squid consists in one superconducting ring with two superconducting-insulating-superconducting junctions. An entirely superconducting ring only allows certain magnetic flux values in its centre : the magnetic field (penetration of the field in the ring) is quantized and must be an integer multiple of the flux quantum f0. In the squid, the situation is subtler because of the junctions. Indeed, each junction allows a small electric current to go through by Josephson effect, and that current is also very sensitive to the magnetic field in the ring.
The voltage of the whole squid hence varies when the magnetic flux is applied, and goes back to the same value whenever an additional flux quantum goes through the ring. In other words, the squid’s voltage is periodic when a magnetic field is progressively applied to the ring, and the period of this effect is the flux quantum f0. If you measure this variation, you can determine how many f0 flow through the ring when it comes close to a magnetic field. Thanks to a type of electronics very sensitive to these variations, variations of a millionth of a f0 can be detected. The weak value of the flux quantum (2,07 10-15 Tm2) explains why squids are the most sensitive measurement systems for magnetic fields, sensitive to the femtotesla.
