  A decade ago, the first leptogenesis model based on inflation was proposed,
where the complex phase of the inflaton field carries lepton number. If the
inflaton field is an axion, it can couple to gravitational waves and gauge
fields via. Chern-Simons invariants. Due to these couplings, birefringent
gravitational and gauge primordial perturbations are created during inflation
to generate a lepton asymmetry, establishing a possible connection between
non-vanishing TB-parity violating polarization cross-correlations and
leptogenesis. We also discuss the prospect for a subset of these models can
directly source circular (V-mode) polarization in the CMB.
