Revealing the Origins of Two-Photon Components in Solid-State Single-Photon Filters

Solid-state single-photon filters are pivotal for quantum networks, yet multiphoton contamination fundamentally limits their fidelity. While prior works attributed degradation primarily to residual laser reflection in bare cavities, we employ low-reflectivity micropillar cavities to minimize this background without postselection. Crucially, through dynamic analysis of the two-photon processes, we reveal that multiexcitation events and stimulated emission─rather than the residual laser light─dominate the multiphoton generation. Specifically, when 68 ps coherent light wavepackets with an average photon number of 1 are incident, the output field exhibits a single-photon purity of 0.8. Further analysis identifies that 61.9% of the two-photon component originates from multiple excitations, 37.6% from stimulated emission, and only 0.5% from the residual laser background. Our findings provide critical insights into the atom–photon interface, laying the groundwork for new design strategies toward high-purity quantum light sources.