magnetic flux quantum
Magnetic fluxexact by definition
The magnetic flux quantum is the smallest flux that can thread a superconducting loop, exactly h/2e, or 2.067833848e-15 weber. It is exact by definition since 2019, both the Planck constant and the elementary charge being fixed. Flux through a superconducting ring is quantised in whole multiples of it, and each vortex in a type II superconductor carries exactly one.
Watch out: The symbol is written Φ0 here rather than Φ₀ because the subscript-zero glyph has no font metrics in this site's renderer. A SQUID magnetometer resolves small fractions of one flux quantum, which is the reason it can measure femtotesla fields.
| 1 Φ0 | 2.0678338e-15 Wb |
The factor of 2 in the denominator is the experimental fingerprint of Cooper pairing: the charge carrier in a superconductor is two electrons, not one. Deaver and Fairbank and, independently, Doll and Näbauer measured the quantum in 1961 and found half the value expected for single electrons, which confirmed BCS theory in a single number.