The PVLAS experiment and its results: probing the quantum vacuum with polarized light and magnetic fields

The experiment PVLAS studies the optical properties of the vacuum, that behaves much like a material medium when it is permeated by an external (electric or magnetic) field. Using a strong superconducting magnet and a very sensitive ellipsometer we have searched for modifications of the index of refraction of the vacuum due to the presence of a magnetic field. A birefringence is predicted to arise because of the vacuum fluctuations of the electromagnetic field, and a similar effect (dichroism) could be due to the presence of yet undiscovered low mass particles interacting with two photons. At present PVLAS has set the best existing limits on such processes, its sensitivity being limited by external noise sources which have now been accounted for. A completely redesigned prototype apparatus is now under construction: it is based on rotating permanent magnets and an ellipsometer employing an ultra stable Fabry-Perot resonator. A 50-fold improvement in the sensitivity of the ellipsometer has now been achieved, and we hope to improve our best limits when the magnets system will be installed.