  Twisted bilayer graphene is a chiral system which has been recently shown to
present circular dichroism. In this work we show that the origin of this
optical activity is the rotation of the Dirac fermions' helicities in the top
and bottom layer. Starting from the Kubo formula, we obtain a compact
expression for the Hall conductivity that takes into account the dephasing of
the electromagnetic field between the top and bottom layers and gathers all the
symmetries of the system. Our results are based in both a continuum and a
tight-binding model, and they can be generalized to any two-dimensional Dirac
material with a chiral stacking between layers.
