Three-photon polarization entanglement of green light

Recently, great progress has been made in the entanglement of multiple photons at various wavelengths and with different degrees of freedom for optical quantum information applied in diverse scenarios. However, multiphoton entanglement in the transmission window of green light under water has not yet been reported. Here, by combining femtosecond-laser-based multiphoton entanglement and entanglement-maintaining frequency up-conversion techniques, we successfully generate a green two-photon polarization-entangled Bell state and a green three-photon Greenberger-Horne-Zeilinger state, whose state fidelities are $0.893\pm 0.002$ and $0.595\pm 0.023$, respectively. Our result provides a scalable method to prepare green multiphoton entanglement, which may have wide applications in underwater quantum information.