Most Subradiant Bound Photon Pairs from Chirality-Mediated Dispersion Softening

We study the subradiant bound states (BSs) in a two-level atom array chirally coupled to a one-dimensional waveguide. We demonstrate that the chiral interaction can drive BSs to become the most subradiant two-excitation states across a wide spacing range. This phenomenon is rooted in a mechanism of chirality-mediated dispersion softening, where the BS band distortion suppresses the band curvature $\mathrm{\mid }{\alpha }_2\mathrm{\mid }$ at an extremum point. We rigorously prove that the BS decay rate follows the scaling $\mathrm{\Gamma }\sim \mathrm{\mid }{\alpha }_2\mathrm{\mid }\mathrm{/}{N}^3$, revealing that the reduction of $\mathrm{\mid }{\alpha }_2\mathrm{\mid }$ is key to suppressing emission and enhancing subradiance. We also show the existence of chiral BSs in a realistic nanofiber interface.