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

ABSTRACT 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, SnP ₂ S ₆ . 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 SnP ₂ S ₆ film. Moreover, the fiber‐integrated SnP ₂ S ₆ 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.