Vacuum Birefringence in a Rotating Magnetic Field
  We calculate the vacuum polarization-induced ellipticity acquired by a linearly polarized laser beam of angular frequency $\bar \omega$ on traversing a region containing a transverse magnetic field rotating with a small angular velocity $\Omega$ around the beam axis. The transmitted beam contains the fundamental frequency $\bar \omega$ and weak sidebands of frequency $\bar \omega \pm 2 \Omega$, but no other sidebands. To first order in small quantities, the ellipticity acquired by the transmitted beam is independent of $\Omega$, and is the same as would be calculated in the approximation of regarding the magnetic field as fixed at its instantaneous angular orientation, using the standard vacuum birefringence formulas for a static magnetic field. Also to first order, there is no rotation of the polarization plane of the transmitted beam. Analogous statements hold when the magnetic field strength is slowly varying in time. 
Stephen L. Adler