Subcarrier multiplexing multiple-input multiple-output quantum key distribution scheme with orthogonal quantum states

Quantum key distribution (QKD) system is presently being developed for providing high-security transmission in future free-space optical communication links. However, currentQKDtechnique restricts quantum secure communication to a lowbit rate. To improve theQKDbit rate, we propose a subcarrier multiplexing multiple-input multiple-output quantum key distribution (SCM-MQKD) scheme with orthogonal quantum states. Specifically, we firstly present SCM-MQKD system model and drive symmetrical SCM-MQKD system into decoherence-free subspaces. We then utilize bipartite Werner and isotropic states to construct multiple parallel single photon with orthogonal quantum states that are invariant for unitary operations. Finally, we derive the density matrix and the capacity of SCM-MQKD system, respectively. Theoretical analysis and numerical results show that the capacity of SCM-MQKD system will increase log(2)(N-2 + 1) times than that of single-photon QKD system.