Quantum Key Distribution (QKD) provides an efficient means to exchangeinformation in an unconditionally secure way. Historically, QKD protocols havebeen based on binary signal formats, such as two polarisation states, and thetransmitted information efficiency of the quantum key is intrinsically limitedto 1 bit/photon. Here we propose and experimentally demonstrate, for the firsttime, a high-dimensional QKD protocol based on space division multiplexing inmulticore fiber using silicon photonic integrated lightwave circuits. Wesuccessfully realized three mutually unbiased bases in a four-dimensionalHilbert space, and achieved low and stable quantum bit error rate well belowboth coherent attack and individual attack limits. Compared to previousdemonstrations, the use of a multicore fiber in our protocol provides a muchmore efficient way to create high-dimensional quantum states, and enablesbreaking the information efficiency limit of traditional QKD protocols. Inaddition, the silicon photonic circuits used in our work integrate variableoptical attenuators, highly efficient multicore fiber couplers, andMach-Zehnder interferometers, enabling manipulating high-dimensional quantumstates in a compact and stable means. Our demonstration pave the way to utilizestate-of-the-art multicore fibers for long distance high-dimensional QKD, andboost silicon photonics for high information efficiency quantum communications.
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