Mid‐Infrared and Telecom Band Two‐Color Entanglement Source for Quantum Communication

Due to the high noise caused by solar background radiation, the existing satellite‐based free‐space quantum communication experiments are mainly carried out at night, hindering the establishment of a practical, all‐day, real‐time global‐scale quantum network. Given that the 2–5 µm mid‐infrared (MIR) band has extremely low solar background radiation and strong scattering resistance, it is one of the ideal bands for free‐space quantum communication. Here, first, on the preparation of a high‐quality MIR (3370 nm) and telecom band (1555 nm) two‐color polarization‐entangled photon source is reported, and the effective detection of MIR photons through a frequency upconversion process. Then, the source is used to realize a principle quantum key distribution based on free‐space and fiber hybrid channels in a laboratory. This work represents a significant step toward developing all‐day global‐scale quantum communication networks.