In a strong magnetic field, vacuum becomes birefringent: linearly polarized light acquires an ellipticity, an effect predicted by the Euler–Heisenberg Lagrangian.
Detection uses high-finesse Fabry–Perot cavities (finesse above 2×10⁵) crossed by strong magnetic fields.
The method is also sensitive to hypothetical new physics: axion-like particles and millicharged fermions or bosons.