FBG-Based Weak Coherent State and Entanglement-Assisted Multidimensional QKD

In this paper, we propose several fiber Bragg gratings (FBGs) based on both weak coherent state (WCS) and entanglement-assisted (EA) multidimensional QKD (MQKD) protocols with mutually unbiased bases (MUBs) derived from the complex Hadamard matrices. We also propose how to implement them by using FBGs with orthogonal impulse responses, derived from Slepian sequences. The proposed FBG-based protocols are capable of simultaneously achieving unprecedented spectral efficiency and secret-key rates in long-distance quantum communication. Regarding the WCS protocols, random base selection and FGB-MUB-based protocols are introduced. Regarding EA protocols, we introduce a generic class of FBG-based N-dimensional protocols and describe the corresponding FBG-based encoders and decoders. The security analysis of the proposed MQKD protocols is further performed by taking into account FBG fabrication imperfections, modeled by the L-neighbor model. It has been shown that the proposed MQKD protocols significantly outperform the conventional two-dimensional QKD protocols, and impose less stringent bandwidth requirements compared to multidimensional time-frequency and time-bin protocols.