Magnetic flux quantum

The magnetic flux quantum, Φ₀ = h/2e, is the fundamental unit of magnetic flux in superconductors. Each superconducting vortex carries one quantum of flux.

Fluxoid quantization: The quantization of magnetic flux combined with the circulating supercurrent in a superconducting loop. It explains why superconducting loops quantize flux in units set by the flux quantum.
SQUID: A superconducting interference device that uses Josephson junctions to detect extremely small magnetic fields. Its flux-periodic response is calibrated directly in units of Φ₀.
Fritz London: A theoretical physicist who developed an early electrodynamic theory of superconductivity with his brother Heinz. London’s theory connected superconducting currents with quantized magnetic flux.
h/e flux period: A magnetic-flux periodicity of h/e associated with single-electron quantum interference. It is twice Φ₀, distinguishing single-electron interference from Cooper-pair flux quantization.
Cooper pair: A bound pair of electrons that behaves as a charge-2e boson in a superconductor. The pair’s charge of 2e sets the denominator in Φ₀ = h/2e.
Josephson voltage standard: A voltage reference based on quantized voltage steps in arrays of Josephson junctions. The voltage step per frequency is fixed by the Josephson constant, 2e/h = 1/Φ₀.
Brian Josephson: A physicist who predicted quantum tunneling effects between superconductors in 1962. His predicted effect made the flux quantum experimentally accessible through voltage-frequency measurements.
Half-quantum vortex: A superconducting vortex carrying half the conventional flux quantum in certain multicomponent superconductors. It is a special state in which internal order-parameter structure permits Φ₀/2 flux.
Planck constant: The fundamental constant setting the scale of quantum action. It supplies the quantum of action in the numerator of the flux quantum.
Flux pinning: The immobilization of magnetic vortices by defects or engineered structures in a superconductor. Pinned flux quanta determine how much current a practical superconductor can carry.

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