The PVLAS experiment: a 25 year effort to measure vacuum magnetic birefringence

This paper describes the 25 year effort to measure vacuum magnetic birefringence and dichroism with the PVLAS experiment. The experiment went through two main phases: the first using a rotating superconducting magnet and the second using two rotating permanent magnets. The experiment was not able to reach the predicted value from QED. Nonetheless the experiment set the current best limits on vacuum magnetic birefringence and dichroism for a field of $B_{\rm ext} = 2.5$ T, namely, $\Delta n^{\rm (PVLAS)} = (12\pm17)\times10^{-23}$ and $|\Delta\kappa|^{\rm (PVLAS)} = (10\pm28)\times10^{-23}$. The uncertainty on $\Delta n^{\rm (PVLAS)}$ is about a factor 7 above the predicted value of $\Delta n^{\rm (QED)} = 2.5\times10^{-23}$ @ 2.5 T.