A 25-year effort to measure vacuum magnetic birefringence and dichroism, run in two phases: first with a rotating superconducting magnet, then with two rotating permanent magnets.
The experiment did not reach the QED-predicted value but set the current best limits at 2.5 T: Δn = (12±17)×10⁻²³ and |Δκ| = (10±28)×10⁻²³.
The uncertainty on Δn remains about a factor of 7 above the predicted value.