Quantum of circulation

The name comes from superfluids and superconductors, where circulation around a vortex is quantised in units of h/m — h/2m for Cooper pairs, which is why the magnetic flux quantum carries a 2e rather than an e. Onsager and Feynman predicted quantised circulation in liquid helium in the 1950s, and Vinen measured it in 1961.

Its modern importance is metrological. The quantity h/m for caesium or rubidium is what an atom-recoil interferometer measures: a photon absorbed by an atom gives it a recoil velocity ħk/m, and counting recoils against a frequency comb turns h/m into one of the two best routes to the fine-structure constant, via α² = (2R∞/c)(m/m_e)(h/m). It is also exactly half of h/m_e = 7.273 895 093 × 10⁻⁴ m²/s, a distinction worth checking before you use a tabulated value.