Efficient Polarization-Entangled Photon-Pair Generation by a Fiber-In-Line van der Waals Material

The recent emergence of van der Waals (vdW) materials with exceptional optical nonlinearity has driven advances in next-generation nonlinear photonic devices beyond classical optics. While integrating these materials into fiber-optic platforms offers unprecedented functionalities in quantum systems, realizing telecom-band in-line photon-pair sources based on highly nonlinear vdW materials remains challenging. Here, we report photon-pair generation centered at telecom wavelength via spontaneous parametric down-conversion in a fiber-integrated device incorporating a ferroelectric vdW crystal, SnP₂S₆. Benefiting from its giant optical nonlinearity and relaxed phase-matching across near-visible to telecom-band wavelengths, a remarkable nonlinear conversion efficiency of 0.07% is achieved in a few-micrometer-thick SnP₂S₆ film. Moreover, the fiber-integrated SnP₂S₆ device delivers high-quality photon pairs at telecom wavelengths, achieving a detected coincidence rate of 102 counts/s. The coincidence-to-accidental ratio reached up to 55 662, which is much larger than that of previous vdW-material-based sources. Furthermore, the device generates polarization-entangled photon pairs, which are verified via quantum state tomography to exhibit fidelities of 0.97 and concurrences of 0.95. This demonstration establishes a scalable, robust fiber-integrated platform that combines the versatility of vdW materials, paving the way toward low-loss, functionalized quantum photonic systems for networking, sensing, and computing.