Magnetic flux quantum

Magnetic flux through a superconducting ring cannot take any value it likes. The ring's electrons share a single quantum wavefunction that must close on itself, and that forces the enclosed flux into integer multiples of h/2e ≈ 2.07 fWb — a fantastically small amount, roughly the flux of Earth's field through a square 6 μm on a side. The factor of 2 in the denominator is the charge of a Cooper pair, and its experimental confirmation in 1961 was direct evidence that superconductivity is carried by paired electrons, exactly as BCS theory had claimed three years earlier.

Flux quantisation is the working principle of the SQUID, the most sensitive magnetometer ever built: a loop interrupted by Josephson junctions counts flux quanta, letting it resolve femtotesla fields for magnetoencephalography and geophysical survey. Type-II superconductors admit magnetic field as a lattice of vortices, each carrying exactly one Φ₀, and pinning those vortices is what lets an MRI magnet hold hundreds of amperes without loss.