多光子糾纏作為量子通訊、分散式量子資訊處理與量子網路中的重要資源，並且，在通訊波段產生高品質的多光子糾纏態，不僅有利於與現有光纖通訊系統整合，也為長距離量子網路的發展提供重要的實驗基礎。然而，在實驗上實現高保真度多光子糾纏源仍受到多種物理條件與限制，其中，來自獨立光源之光子間的不可區分性直接影響多光子干涉與最終糾纏態的品質。本研究以通訊波長偏振糾纏光源為基礎，系統性研究透過光子干涉所產生的四光子糾纏態，並著重探討光子的頻譜特性與頻譜重疊如何影響多光子干涉品質。透過理論分析與實驗驗證，進一步在實驗上改善光子頻譜不可區分性的方法，以提升四光子干涉品質與最終糾纏態的保真度，並評估改善後的多光子糾纏源應用於不同量子網路任務的能力。
在本研究中我們利用兩組獨立的薩格納克干涉儀分別產生中心波長約為1550nm的偏振糾纏光子對，並將來自兩個獨立糾纏光源的光子進行干涉與融合，以產生四光子糾纏態。在改善前，四光子態於相干基底下僅呈現約0.31的量測對比度。因此，我們首先探討自發參數下轉換過程中的多光子對噪音在量測結果的影響，並透過量測資料的後處理方式改善對比度。然而，實驗結果顯示此類改善仍相對有限，表示多光子對噪音並不足以完整解釋所觀察到低干涉對比度的現象。因此，我們進一步回到光子干涉所需的基本不可區分條件，並特別針對干涉光子之間的頻譜特性進行分析。透過本研究所提出的頻譜改善方法，樹立理論基礎，並在實驗上使光子在干涉前通過更窄的帶通濾波片，以及調控幫浦光的帶寬，使相干基底對比度提升至約0.64，並獲得0.81 的四光子態保真度。最後，我們進一步利用不同多體糾纏判據評估改善後的四光子態，結果顯示可滿足不同量子網路架構與任務對應的糾纏需求。

Multipartite photonic entanglement is an important resource for quantum communication, distributed quantum information processing, and quantum networks. The generation of high-quality multi-photon entangled states at telecommunication wavelengths not only facilitates integration with existing optical-fiber communication infrastructure, but also provides an important experimental foundation for the development of long-distance quantum networks. However, the experimental realization of high-fidelity multi-photon entangled sources remains subject to various physical limitations. In particular, the indistinguishability of photons originating from independent sources directly affects the quality of multiphoton interference and, consequently, the fidelity of the generated entangled state. In this work, we investigate four-photon entangled states generated through photon interference us ing polarization-entangled photon sources at telecommunication wavelengths, with how the spectral properties and spectral overlap of the interfering photons affect the multi-photon interference quality. Through theoretical analysis and experimental verification, we further improve the spectral indistinguishability of the photons in the experiment, thereby enhancing the four-photon interference quality and the fidelity of the generated entangled state. The capability of the improved multi-photon entangled source for different quantum-network tasks is also evaluated.
In this study, two independent Sagnac interferometers are employed to generate polarization-entangled photon pairs with central wavelengths of approximately 1550 nm. Photons originating from the two independent entangled-photon sources are subsequently interfered and fused to generate a four-photon entangled state. Before optimization, the four-photon state exhibited a measured visibility of only approximately 0.31 in the coherence basis. We therefore first investigated the influence of multi-pair emission noise arising from the spontaneous parametric down-conversion process and applied a post-processing procedure to suppress its contribution to the measured data. However, the resulting improvement in visibility was relatively limited, indicating that multi-pair emission noise alone cannot fully account for the observed degradation in the interference quality.
We therefore returned to the fundamental indistinguishability conditions required for photon interference and focused specifically on the spectral properties of the interfering photons. A theoretical framework was developed to analyze how the spectral properties of the SPDC photon pairs affect the indistinguishability of photons generated from independent sources and, consequently, the four-photon interference visibility. Guided by this analysis, the experimental conditions were further optimized by introducing narrower band-pass filters before photon fusion and by adjusting the spectral bandwidth of the pump laser. Through these spectral optimization methods, the measured coherence-basis visibility was increased from approximately 0.31 to approximately 0.64, and a four-photon-state fidelity of approximately 0.81 was achieved.
Finally, the optimized four-photon state was further evaluated using different multipartite entanglement criteria. The results show that the improved state satisfies the entanglement requirements associated with different quantum-network architectures and tasks. Overall, this work establishes a theoretical and experimental framework for understanding and improving the spectral indistinguishability of independently generated photons, providing practical guidance for the generation of high-quality four-photon entangled states.

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