  Extremely strong magnetic fields change the vacuum index of refraction.
Although this polarization dependent effect is small for typical neutron stars,
it is large enough to decouple the polarization states of photons traveling
within the field. The photon states evolve adiabatically and follow the
changing magnetic field direction. The combination of a rotating magnetosphere
and a frequency dependent state decoupling predicts polarization phase lags
between different wave bands, if the emission process takes place well within
the light cylinder. This QED effect may allow observations to distinguish
between different pulsar emission mechanisms and to reconstruct the structure
of the magnetosphere.
