PVLAS (acronym for Polarizzazione del Vuoto con LASer) is an apparatus dedicated to the detection of dark matter. It is located at the Legnaro National Laboratory of National Institute of Nuclear Physics near Padova, Italy.[1] Experiments with the apparatus began in 2001.
As dark matter is a form of matter that does not emit or reflect enough electromagnetic radiation to be detected directly, the PVLAS is used to detect the small flattening acquired by a linearly polarised laser beam after it passes through a vacuum space where an intense magnetic field has been applied.[2]
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 slower than 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. Notice however that uncharged dark matter candidates will not interact with the photon and so will not be detectable by PVLAS. The birefrigence of the vacuum of quantum electrodynamics in the presence of an external field is generally credited to Stephen L. Adler, who presented the first general derivation in Photon splitting and photon dispersion in a strong magnetic field in 1971.
The PVLAS experiment investigates the vacuum after being perturbed by external magnetic fields. [3]