The same phase effect is responsible for the quantized-flux requirement in superconducting loops. This quantization occurs because the superconducting wave function must be single valued: its phase difference Δ Δ φ φ around a closed loop must be an integer multiple of 2 π π (with the charge q = 2 e for the electron Cooper pairs), and thus the flux must be a multiple of h / 2 e . The superconducting flux quantum was actually predicted prior to Aharonov and Bohm, by F. London in 1948 using a phenomenological model.