A multiple-input multiple-output (MIMO) system operating at terahertz (THz) frequencies and consisting of a transmitter, Alice, that encodes secret keys using Gaussian-modulated coherent states, which are communicated to a legitimate receiver, Bob, under the assistance of a reconfigurable intelligent surface (RIS) is considered in this paper. The composite wireless channel comprising the direct Alice-to-Bob signal propagation path and the RIS-enabled reflected one is modeled as a passive linear Gaussian quantum channel, allowing for a unitary dilation that preserves the canonical commutation relations. The security of the considered RIS-empowered MIMO system is analyzed under collective Gaussian entangling attacks, according to which an eavesdropper, Eve, is assumed to have access to environmental modes associated with specific propagation segments. We also study, as a benchmark, the case where Eve has access to the purification of the overall channel. The legitimate receiver, Bob, is designed to deploy homodyne detection and reverse reconciliation for key extraction. Novel expressions for the achievable secret key rate (SKR) of the system are derived for both the considered eavesdropping scenarios. Furthermore, an optimization framework is developed to determine the optimal RIS phase configuration matrix that maximizes the SKR performance. The resulting optimization problem is efficiently solved using particle swarm optimization. Numerical results are presented to demonstrate the system's performance with respect to various free parameters. It is showcased that the considered RIS plays a crucial role in enhancing the SKR of the system as well as in extending the secure communication range. This establishes RIS-assisted THz MIMO CV-QKD as a promising solution for next generation secure wireless networks.

翻译：本文考虑一个工作在太赫兹（THz）频段的多输入多输出（MIMO）系统，该系统包含发送方Alice，她使用高斯调制的相干态编码密钥，并在可重构智能表面（RIS）的辅助下，将密钥传输给合法接收方Bob。由Alice到Bob的直接信号传播路径和RIS辅助的反射路径组成的复合无线信道被建模为一个无源线性高斯量子信道，该信道允许进行保持正则对易关系的酉扩张。在集体高斯纠缠攻击下分析了所考虑的RIS赋能MIMO系统的安全性，根据这种攻击，窃听者Eve假设能够访问与特定传播段相关的环境模式。作为基准，我们还研究了Eve能够访问整个信道纯化的情况。合法接收方Bob采用零差检测和逆向协商进行密钥提取。针对所考虑的两种窃听场景，推导了系统可实现密钥率（SKR）的新型表达式。此外，开发了一个优化框架来确定最大化SKR性能的最优RIS相位配置矩阵。该优化问题通过粒子群优化算法高效求解。给出了数值结果以展示系统性能与各种自由参数的关系。结果表明，所考虑的RIS在提升系统的SKR以及扩展安全通信距离方面起着关键作用。这确立了RIS辅助太赫兹MIMO连续变量量子密钥分发作为下一代安全无线网络的一种有前景的解决方案。