A reconfigurable intelligent surface (RIS)-aided multiple-input multiple-output (MIMO) wireless communication system is considered in this paper wherein the transmitter, Alice modulates secret keys, by using a continuous variable quantum key distribution technique to be transmitted to the receiver, Bob, which employs homodyne detection for data decoding. The data is transmitted over two paths, namely a direct path between Alice and Bob and the wireless path between them via the RIS. Transmit and receive beamsplitters are employed in the system to transform the MIMO terahertz channels into parallel single-input single-output channels. Considering an eavesdropper, Eve, to attack all the three wireless channels in the system (i.e., the direct channel, the channel between Alice and RIS, and between the RIS and Bob) but having restricted quantum memory limiting it to store the ancilla modes from either of these three wireless channels, novel expressions for the secret key rate (SKR) of the system are derived. Numerical results are presented to demonstrate the dependency of the system's performance on various system parameters. It is observed that the RIS plays a key role in increasing the SKR of the system and the transmission distance, ensuring secure communications between Alice and Bob. The significance of employing RIS is observed specifically for the case when Eve measures the ancilla modes of the channel between the RIS and Bob. Furthermore, for all such measurement scenarios, optimal angles are obtained for the phase shifts of the RIS elements to maximize the SKR for various MIMO configurations and transmission distance between Alice and Bob.

翻译：本文研究了一种可重构智能表面辅助的多输入多输出无线通信系统，其中发送方Alice采用连续变量量子密钥分发技术调制密钥，并通过零差检测进行数据解码的接收方Bob传输。数据传输通过两条路径进行：Alice与Bob之间的直接路径，以及经由RIS的无线路径。系统中采用发射和接收波束成形器，将MIMO太赫兹信道转换为并行的单输入单输出信道。考虑窃听者Eve能够攻击系统中的全部三条无线信道（即直接信道、Alice至RIS信道以及RIS至Bob信道），但其量子存储器受限，仅能存储来自任意一条无线信道的辅助模式。本文推导了系统密钥速率的新表达式，并通过数值结果展示了系统性能对各种参数的依赖性。研究发现，RIS在提升系统密钥速率和传输距离方面发挥关键作用，从而确保Alice与Bob之间的安全通信。当Eve测量RIS至Bob信道的辅助模式时，采用RIS的重要性尤为显著。此外，针对所有此类测量场景，本文获得了RIS单元相移的最优角度，以在不同MIMO配置及Alice与Bob间传输距离下最大化密钥速率。