Demonstration of quantum cipher communication using quadrature amplitude modulation technologies over 100 km optical fiber

We report a hybrid secure communication of continuous-variable quantum key distribution (CV-QKD) and quadrature amplitude modulation/quantum noise stream cipher (QAM/QNSC). In QAM/QNSC a coherent multi-level optical signal is hidden in quantum noise. We demonstrated 70 Gbit/s on-line QAM/QNSC transmission over 100 km using an FPGA-based transmission and receiver, where secure keys are delivered by a CV-QKD system. In the security analysis, we adopt a realistic assumption that an eavesdropper does not have a lossless fiber. This hybrid system is highly compatible with commercial optical communication system, will contribute to realize high capacity and secure communication.