  We study the mixing of different kind of fields (scalar in 0+1D, scalar in
3+1D, fermion in 3+1D) treating the mixing term as an interaction. To this aim,
we employ the usual perturbative series in the interaction picture. We find
that expression for flavor changing probability exhibits corrections with
respect to the usual quantum mechanical (e.g. neutrino) oscillation formula, in
agreement with the result previously obtained in the non-perturbative flavor
Fock space approach.
