Realization of a linear-birefringence exceptional point in a high-finesse Fabry-Pérot cavity

Supermirrors are widely used in interferometric experiments and, when configured as Fabry-Pérot (FP) cavities, play a central role in cavity quantum electrodynamics (QED) by enabling strong coupling with only a small number of atoms. However, these mirrors inherently exhibit linear birefringence, which poses challenges for precision measurements. In this work, we theoretically investigate and experimentally demonstrate an exceptional point (EP) in an FP cavity formed by supermirrors, achieved by combining intrinsic birefringence with an engineered polarization-dependent loss. At the EP, the eigenvalues and eigenstates of two orthogonal linearly polarized modes coalesce into a single circularly polarized mode. Polarization-dependent loss is controlled using a knife-edge diffraction technique, while cavity ringdown spectroscopy is employed to characterize birefringence and dichroism. The observed non-Hermitian degeneracy, or the EP, is compatible with a cavity quality factor as high as 4.8 × 10⁹, highlighting its potential for future experiments that combine EP physics with cavity-QED phenomena.