  Nonlinear QED interactions induce different polarization properties on a
given probe beam. We consider the polarization effects caused by the
photon-photon interaction in laser experiments, when a laser beam propagates
through a constant magnetic field or collides with another laser beam. We solve
the quantum Boltzmann equation within the framework of the Euler-Heisenberg
Lagrangian for both time-dependent and constant background field to explore the
time evolution of the Stokes parameters Q, U, and V describing polarization.
Assuming an initially linearly polarized probe laser beam, we also calculate
the induced ellipticity and rotation of the polarization plane.
