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

TL;DR

The paper addresses the challenge of achieving high-visibility interference between independent photons by enhancing the spectral purity of SPDC photon pairs. It directly compares two purity-enhancement strategies—pump-bandwidth engineering and narrowband interference filtering—using a type-0 PPLN waveguide, joint spectral intensity measurements with Schmidt decomposition, and Hong–Ou–Mandel interference under identical conditions. The findings show that both approaches can achieve roughly 80% HOM visibility, but pump-bandwidth tailoring preserves a higher three-fold coincidence rate, improving multi-photon generation efficiency. These results offer practical guidelines for optimizing the trade-off between spectral purity and source brightness, enabling high-fidelity, high-rate multi-photon time-bin GHZ states and scalable quantum networks.

Abstract

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.

Figures (3)

• Figure 1: Schematic diagram of the joint spectrum obtained by integrating the pump envelope function (PEF) and phase matching function (PMF), where the two axes show the corresponding wavelengths of the signal photons and idle photons. (a) PEF=PMF and $\theta_{PEF}=\theta_{PMF}$, Edamatsu:2011Jin:13 which shows a symmetric result. The Schmidt decomposition revealed high-purity SPDC photon pairs, where purity ($P$) is close to 1. (b) showed that when the PEF and PMF are not well matched, the joint spectrum is asymmetric and the purity is low. However, narrower interference filters can be used to recover high-purity photon pairs as shown in (c).
• Figure 2: (a) Experimental setup. The SPDC photon pair purity was varied using a pump bandwidth tunable filter. The 4-GHz FBGs were used to perform the JSI measurements. (b) and (c) are JSI measurement results with the pump bandwidths of 0.2 nm and 5 nm, respectively. The integration time for each coincidence measurement point was 1 s. SPF: Short-wave pass edge filters. LPF: Long-wave pass edge filter. SNSPD: superconducting nanowire single-photon detector.
• Figure 3: (a) Experimental setup for quantum interference measurement between independent light sources. (b), (c), and (d) show the HOM dip measurements performed by preparing different pump lights and different interference filter bandwidths. The upper figure shows that the JSI depends on the pump bandwidth and the interference filter bandwidth at each quantum interference measurement, and the lower figure shows the HOM dip interferometry measurement results.