  Here we describe how certain classes of two dimensional topological
insulators, including the CdTe$/$HgTe quantum wells, display a new type of
optical activity in two dimensions similar to the magneto-optical Kerr effect
in the quantum Hall effect. This optical activity is characterized by a genuine
Kerr angle and it is compatible with time reversal symmetry, being thus
fundamentally different to other known types of time reversal invariant optical
activity. The term responsible of such optical activity, having the form of
$(\mathbf{E}\cdot\partial\mathbf{B}/\partial
t-\mathbf{B}\cdot\partial\mathbf{E}/\partial t)$, can be considered a time
reversal invariant counterpart of the magneto-electric term
$\mathbf{E}\cdot\mathbf{B}$. The microscopical origin of this response is a
chiral non-minimal coupling between electrons and the external electromagnetic
field. This optical activity constitutes a proof of principle that there is
possible to find systems that are time reversal invariant displaying a genuine
Kerr effect.
