Exceptional-point-like Sensing near Hermitian Transmission Peak Degeneracies

Non-Hermitian sensing near exceptional points (EPs), can exhibit a sublinear response to perturbations and thus promises sensitivity beyond conventional Hermitian sensors operating at diabolic points (DPs). Recent studies have further shown that the square-root readout can be attained near transmission peak degeneracy (TPD), where the splitting of spectral extrema rather than the eigenvalues is observed. However, in non-Hermitian platforms, the available sensitivity enhancement is fundamentally constrained by the divergent Petermann factor because of non-Hermitian eigenvector degeneracy, and practically by the system complexity. Here, we experimentally demonstrate TPD-enhanced sensing of refractive-index change in a simple chiral Hermitian cavity. We show that the Hermitian transmission spectrum can also exhibit TPD-induced square-root splitting of spectral extrema while preserving eigenvector orthogonality. As a result, this Hermitian TPD-based sensor retains the response enhancement characteristic of non-Hermitian TPD sensing, yet remains free from the Petermann-factor limit intrinsic to non-Hermitian systems, thereby enabling enhanced sensitivity. This work establishes a Hermitian route toward ultrasensitive sensing that surpasses the conventional DP sensor and provides a sensitivity advantage over non-Hermitian implementations when Petermann-factor-amplified noise is relevant.