Flux Quantization and Time-Reversal Degeneracy

It is shown on the basis of very general considerations that the observed quantization of the magnetic flux enclosed by a ring superconductor in units of $\frac{\pi \hslash c}{e}$ implies and is implied by time-reversal degeneracy of the states of the superconductor. This degeneracy results in vanishing electric current density at thermodynamic equilibrium, which is required for consistency with the Meissner effect. The quantization of flux is also used to shed some light on the applicability of the Bardeen-Cooper-Schrieffer approximation in the presence of an enclosed magnetic field.