Generation of Tunable Three‐Photon Entanglement in Cubic Nonlinear Coupled Waveguides

Abstract The generation of three‐photon states with spatial entanglement in cubic nonlinear coupled waveguides is theoretically investigated using third‐order spontaneous parametric down‐conversion and quantum walks. This approach involves independently pumping two coupled waveguides to generate a path‐encoded three‐photon Greenberger–Horne–Zeilinger (GHZ) state, which then evolves with complex spatial dynamics governed by coupling coefficients and phase mismatch. By appropriate parameter tuning, the generation of robust heralded Bell states is demonstrated, uniform states, and GHZ‐like states at the chip output. This work demonstrates an integrated source of three‐photon spatial entanglement on a simple chip, offering additional reconfigurability for advanced multiphoton quantum applications.