  In the presence of strong magnetic fields, the vacuum becomes a birefringent
medium. We show that this QED effect decouples the polarization modes of
photons leaving the NS surface. Both the total intensity and the intensity in
each of the two modes is preserved along a ray's path through the neutron-star
magnetosphere. We analyze the consequences that this effect has on aligning the
observed polarization vectors across the image of the stellar surface to
generate large net polarizations. Counter to previous predictions, we show that
the thermal radiation of NSs should be highly polarized even in the optical.
When detected, this polarization will be the first demonstration of vacuum
birefringence. It could be used as a tool to prove the high magnetic field
nature of AXPs and it could also be used to constrain physical NS parameters,
such as $R/M$, to which the net polarization is sensitive.
