  We analyze axion--photon mixing in the framework of quantum field theory. The
condensate structure of the vacuum for mixed fields induces corrections to the
oscillation formulae and leads to non-zero energy of the vacuum for the
component of the photon mixed with the axion. This energy generates a new
effect of the vacuum polarization and it has the state equation of the
cosmological constant, $w = -1$. This result holds for any homogeneous and
isotropic curved space-time, as well as for diagonal metrics. Numerical
estimates of the corrections to the oscillation formulae are presented by
considering the intensity of the magnetic field available in the laboratory.
Moreover, we estimate the vacuum energy density induced by axion--photon mixing
in the Minkowski space-time. A value compatible with that of the energy density
of the universe can be obtained for axions with a mass of $(10^{-3}-10^{-2})
eV$ in the presence of the strong magnetic fields that characterize
astrophysical objects such as pulsars or neutron stars. In addition, a value of
the energy density less than that of the Casimir effect is obtained for
magnetic fields used in experiments such as PVLAS. The vacuum polarization
induced by this energy could be detected in next experiments and it might
provide an indirect proof of the existence of the axion--photon mixing. The
quantum field theory effects presented in this work may lead to new methods for
studying axion-like particles.
