Efficient entangled quantum states generation in silica nanofibers for QKD-based quantum cryptography protocols

Entangled quantum states generation is crucial for quantum cryptography, particularly in quantum key distribution (QKD). While optical fibers enable low-loss photon transmission, standard silica fibers exhibit weak nonlinearities for spontaneous parametric processes. This paper proposes an optimized scheme for entangled quantum states generation in subwavelength silica nanofibers, leveraging effective second-order nonlinearities χ⁽²⁾ induced by surface inversion symmetry breaking. Unlike bulk silica, tapered silica nanofibers exhibit significant second-order responses via surface dipole and bulk quadrupole nonlinearities, enabling spontaneous parametric down-conversion (SPDC). Our key innovation is a second-order nonlinear coating (PMMA/DR1 encapsulated with Teflon), enhancing $\chi _{eff}^{(2)}$ and boosting the spectral density efficiency of entangled quantum states generation by a factor of 10³. This approach optimizes phase matching while maintaining optical stability and minimizing scattering losses. Additionally, the proposed source inherently avoids spontaneous Raman scattering (SRS) noise, which is a major limitation in fiber-based SPDC systems, ensuring high-purity entangled quantum states generation. Our results demonstrate that silica nanofibers, with tailored nonlinear coatings, offer a scalable platform for integrated quantum light sources in fiber-based quantum networks.