Improving quantum interference visibility between independent sources by enhancing the purity of correlated photon pairs

High-visibility quantum interference between independent photons is essential for demonstrating multi-photon quantum information processing, and it is closely linked to the spectral purity of correlated photon pairs. In this study, we investigate two approaches to enhance the purity of photon pairs generated from a type-0 PPLN waveguide by systematically varying both the pump bandwidth and the interference-filter bandwidth, and we directly compare their performance under identical experimental conditions. The spectral purity is evaluated from measured joint spectral intensities using Schmidt decomposition. Both methods significantly improve the Hong-Ou-Mandel interference visibility to approximately 80%. However, the former approach also yields a higher three-fold coincidence rate, which is advantageous for our ongoing efforts to increase the state fidelity and generation rate of multi-photon time-bin Greenberger-Horne-Zeilinger (GHZ) states.