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 .