PVLAS: probing vacuum with polarized light
  The PVLAS experiment operates an ellipsometer which embraces a superconducting dipole magnet and can measure ellipticity and rotation induced by the magnetic field onto linearly polarized light. The sensitivity of the instrument is about 10-7rad Hz-1/2. With a residual pressure less than 10-7mbar the apparatus gives both ellipticity and rotation signals at the 10-7rad level with more than 8 sigma sob ratio in runs that last about 1000sec. These signals can be interpreted as beeing generated largely by vacuum ellipticity and dichroism induced by the transverse magnetic field.If this interpretation is correct, a tool has become available to characterize physical properties of vacuum as if it were an ordinary transparent medium. A microscopic effect responsible for this induced dichroism could be the existence of ultralight spin zero bosons with mass of the order of 10-3eV, that would couple to two photons and would be created in the experiment by interaction of photons of the laser beam with virtual photons of the magnetic field. The inverse of the coupling constant to two photons would correspond to a mass M of the order of 10+6GeV. 
PVLAS Collaboration: E.Zavattini, et al