  We report on recent results obtained by analyzing axion--photon mixing in the
framework of quantum field theory. We obtain corrections to the oscillation
formulae and we reveal a new effect of the vacuum polarization due to the
non-zero value of the vacuum energy for the component of the photon
polarization mixed with the axion. The study of axion--photon mixing in curved
space is also presented. Numerical analysis show that some quantum field theory
effect of axion--photon mixing, in principle, could be detected experimentally.
