Generation of polarization-entangled Bell states in monolithic photonic waveguides by leveraging intrinsic crystal properties

Advanced photonic quantum technologies—from quantum key distribution to quantum computing—require on-chip sources of entangled photons that are both efficient and readily scalable. This theoretical study demonstrates the generation of polarization-entangled Bell states in structurally simple waveguides by exploiting the intrinsic properties of nonlinear crystals. Elaborate phase-matching strategies that commonly involve the spatial modulation of a waveguide’s linear or nonlinear optical properties are thereby circumvented. This study begins with the derivation of general criteria for the second-order susceptibility tensor that enable the generation of cross-polarized photon pairs via spontaneous parametric down-conversion in single-material waveguides. Based on these criteria, all birefringent, non-centrosymmetric crystal classes are then categorized regarding their suitability. Finally, using coupled mode theory, a numerical analysis is performed of cuboid waveguides made from two materials that are highly relevant to integrated photonics: lithium niobate, a well-established platform, and barium titanate, an emerging alternative. Barium titanate is found to consistently outperform lithium niobate by providing a higher nonlinear efficiency and high concurrence over a significantly broader spectral range. These findings outline a practical route towards highly efficient, fabrication-friendly, and scalable sources of polarization-entangled photons for integrated quantum photonic circuits.