| Quantum cryptography is a new type of secure communication technologies that based on Heisenberg uncertainty principle and no-cloning principle of quantum mechanics. It has been proved to be absolutely safe in theory. Because of its absolute security, quantum cryptography has a wide range of applications in military, diplomatic, communications, e-commerce and other fields.It is shown that the light has both spin angular momentum (SAM) and orbital angular momentum (OAM), different from the SAM, the OAM state can contain any number of states.Based on BB84 quantum key distribution (QKD) protocol, we first apply OAM to QKD scheme and obtain a new multi-dimensional QKD protocol. The simulation results show that the protocol greatly improves the efficiency of key generation, and the states used in the transmission are invariant under rotations about the propagation direction, independent of the alignment between the reference frames of the sender and receiver. Hence the protocol appeals for free space communication. Secondly, we obtain two free-space QKD protocols by combining the concept of mutual unbiased basis and the entanglement. They are named random phase modulation QKD protocol and entanglement-based on OAM state QKD protocol. We describe their implementation process in detail in the article, and the two protocols'feasibilities are verified by the simulation and experimental. At last, since OAM states are the spatial wave function, they are inevitably impacted by atmospheric turbulence, so we present the simulation of atmospheric aberration, and analyze the impact of the atmospheric turbulence on the proposed QKD protocols. The article can be a reference for the study of quantum communications in free space and in interstellar communication. |
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