Photon–Photon Interaction Mediated by an Inverted <i>Y</i> ‐type Four‐Level Atom

ABSTRACT We investigate the transport properties of two photons in a system consisting of an inverted Y –type four‐level atom chirally coupled to a 1D dual‐waveguide structure, focusing on the two‐photon antibunching effect. By analyzing the two‐photon scattering states, we identified a range of nonlinear effects, including spatial attraction and repulsion between photons, background fluorescence, and the emergence of two distinct two‐photon bound states. The key parameters for controlling photon antibunching are systematically identified. In both symmetric and chiral coupling scenarios between the atom and the waveguides, suitably designed two‐photon inputs can eliminate correlations between transmitted photons. This mechanism suppresses the fluorescence quenching through destructive interference between the two bound states, thereby enabling precise control of two‐photon antibunching.