Relationship of time-reversal symmetry breaking to optical Kerr rotation

We prove an instance of the reciprocity theorem that demonstrates that Kerr rotation, also known as the magneto-optical Kerr effect, may only arise in materials that break microscopic time-reversal symmetry. This argument applies in the linear-response regime and only fails for nonlinear effects. Recent measurements with a modified Sagnac interferometer have found finite Kerr rotation in a variety of superconductors. The Sagnac interferometer is a probe for nonreciprocity, so it must be that time-reversal symmetry is broken in these materials.