  Axions mix with neutral pions after the QCD phase transition through their
common coupling to the radiation bath via a Chern-Simons term, as a consequence
of the $U(1)$ anomaly. The non-equilibrium effective action that describes this
mixing phenomenon is obtained to second order in the coupling of neutral pions
and axions to photons. We show that a misaligned axion condensate induces a
neutral pion condensate after the QCD phase transition. The dynamics of the
pion condensate displays long and short time scales and decays on the longer
time scale exhibiting a phenomenon akin to the ``purification'' in a Kaon beam.
On the intermediate time scales the macroscopic pion condensate is proportional
to a condensate of the abelian Chern-Simons term induced by the axion. We argue
that the coupling to the common bath also induces kinetic mixing. We obtain the
axion and pion populations, and these exhibit thermalization with the bath. The
mutual coupling to the bath induces long-lived axion-neutral pion coherence
independent of initial conditions. The framework of the effective action and
many of the consequences are more broadly general and applicable to scalar or
pseudoscalar particles mixing in a medium.
