Φ₀ = h/2e ≈ 2.07 × 10⁻¹⁵ Wb — magnetic flux in superconducting rings is quantized
In a superconducting ring, the macroscopic quantum wavefunction must be single-valued, forcing Φ = nΦ₀ where Φ₀ = h/2e ≈ 2.07 fWb. When external flux changes, the ring hosts persistent screening currents to maintain quantization. A SQUID (Superconducting QUantum Interference Device) uses Josephson junctions to detect flux changes smaller than 10⁻⁶Φ₀ — the world's most sensitive magnetometer.