Recent analysis has confirmed earlier general arguments that the Kerr response vanishes in any time-reversal invariant system which satisfies the Onsager relations. Thus, the widely cited relation between natural optical activity (gyrotropy) and the
Kerr response, employed in Hosur textit{et al}, Phys. Rev. B textbf{87}, 115116 (2013), is incorrect. However, there is increasingly clear experimental evidence that, as argued in our paper, the onset of an observable Kerr-signal in the cuprates reflects point-group symmetry rather than time-reversal symmetry breaking.
The Kerr effect can arise in a time-reversal invariant dissipative medium that is gyrotropic, i.e. one that breaks inversion ($mathcal I$) and all mirror symmetries. Examples of such systems include electron analogs of cholesteric liquid crystals, an
d their descendants, such as systems with chiral charge ordering. We present arguments that the striking Kerr onset seen in the pseudogap phase of a large number of cuprate high temperature superconductors is evidence of chiral charge ordering. We discuss additional experimental consequences of a phase transition to a gyrotropic state, including the appearance of a zero field Nernst effect.
