Towards a direct measurement of vacuum magnetic birefringence: PVLAS achievements

TL;DR

This paper reports the development of a highly sensitive small prototype ellipsometer capable of detecting vacuum magnetic birefringence, demonstrating the feasibility of measuring this nonlinear vacuum effect directly.

Contribution

The authors designed and characterized a new small ellipsometer with record sensitivity, paving the way for direct measurement of vacuum magnetic birefringence.

Findings

Achieved a sensitivity of 2x10^-8 1/√Hz with a finesse of 414000

Demonstrated the feasibility of detecting vacuum magnetic birefringence

Successfully isolated the system on a seismically isolated bench

Abstract

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.