Towards a direct measurement of vacuum magnetic birefringence: PVLAS achievements

Nonlinear effects in vacuum have been predicted but never observed yet directly. The PVLAS collaboration has long been working on an apparatus aimed at detecting such effects by measuring vacuum magnetic birefringence. Unfortunately the sensitivity has been affected by unaccounted noise and systematics since the beginning. A new small prototype ellipsometer has been designed and characterized at the Department of Physics of the University of Ferrara, Italy entirely mounted on a single seismically isolated optical bench. With a finesse F = 414000 and a cavity length L = 0.5 m we have reached the predicted sensitivity of psi = 2x10^-8 1/sqrt(Hz) given the laser power at the output of the ellipsomenter of P = 24 mW. This record result demonstrates the feasibility of reaching such sensitivities and opens the way to designing a dedicated apparatus for a first detection of vacuum magnetic birefringence.