Upper optical bench view. It is located on the top of the granite tower which support the PVLAS optical system. More precisely in the presence of a massive dark matter candidate that interacts with the photon, like the axion, the virtual dark matter particles will be preferentially created by photons whose polarization is parallel to the external field. These virtual particles travel less then the speed of light, and so the effective speed of the photons with a parallel polarization is reduced. Thus the vacuum is birefringent, as the index of refraction depends on the polarization. This flattening is characteristic of the propagation of light in a birefringent medium.