Free-space measurement-device-independent quantum-key-distribution protocol using decoy states with orbital angular momentum
Wang Lea), Zhao Sheng-Mei†a),b), Gong Long-Yanc), Cheng Wei-Wena)
       
A schematic diagram of the decoy-state MDI-QKD protocol using OAM states. SLM: spatial light modulator, to convert a Gaussian beam from a laser to a designed OAM state beam by a specific hologram. Attenuator: to produce weak coherent states, including decoy states or signal states. Aperture: the diameter of the receiver aperture is D . BS: beam splitter with 50:50. L0, L1, R0, and R1: detectors for single photon. OAM sorter: to sort OAM states, which employ two static optical elements (R1 and R2) and a lens (L) to efficiently differentiate the different OAM states.