A Multichannel Metasurface for Multiprotocol Quantum Key Distributions.

Multiprotocol quantum key distribution (mQKD) enables users to flexibly select protocols for secure quantum communication, though achieving mQKD introduces considerable system complexity and resource demands. Here, we report the first realization of mQKD using a metasurface, which generates multiple hybrid states of photonic spin angular momentum (SAM) and orbital angular momentum (OAM) and distributes them to different users. The incident polarization-entangled photon pair interacts with the metasurface, producing four SAM-OAM hybrid states with high fidelity through spin-orbit conversion. Among these hybrid states, two execute the BB84 protocol, while the other two perform the BBM92 protocol, all demonstrating high secret key rates and low quantum bit error rates. This approach provides a robust, compact solution for generating and distributing SAM-OAM hybrid states and stands out for the remarkable capability of a metasurface in secured information processing.