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

TL;DR

This paper presents a theoretical method to generate polarization-entangled Bell states in simple monolithic waveguides by exploiting intrinsic crystal properties, avoiding complex phase-matching techniques, and comparing materials for optimal performance.

Contribution

It introduces general criteria for nonlinear crystal susceptibility tensors enabling entangled photon generation in single-material waveguides, and systematically analyzes suitable crystal classes and materials.

Findings

Barium titanate outperforms lithium niobate in efficiency and spectral range.

The approach simplifies fabrication of entangled photon sources.

High concurrence and efficiency are achievable over broad spectra.

Abstract

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. In this theoretical study, we demonstrate the generation of polarization-entangled Bell states in structurally simple waveguides by exploiting the intrinsic properties of nonlinear crystals. We thereby circumvent elaborate phase-matching strategies that commonly involve the spatial modulation of a waveguide's linear or nonlinear optical properties. We derive 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, we systematically categorize all birefringent, non-centrosymmetric crystal classes in terms of their suitability. Using coupled mode theory, we then numerically analyze 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. We find that barium titanate consistently outperforms lithium niobate by providing a higher nonlinear efficiency and high concurrence over a significantly broader spectral range. These findings outline a practical route toward highly efficient, fabrication-friendly, and scalable sources of polarization-entangled photons for integrated quantum photonic circuits.