In this paper, a multiple-input multiple-output (MIMO) wireless system incorporating a reconfigurable intelligent surface (RIS) to efficiently operate at terahertz (THz) frequencies is considered. The transmitter, Alice, employs continuous-variable quantum key distribution (CV-QKD) to communicate secret keys to the receiver, Bob, which utilizes either homodyne or heterodyne detection. The latter node applies the least-squared approach to estimate the effective MIMO channel gain matrix prior to receiving the secret key, and this estimation is made available to Alice via an error-free feedback channel. An eavesdropper, Eve, is assumed to employ a collective Gaussian entanglement attack on the feedback channel to avail the estimated channel state information. We present a novel closed-form expression for the secret key rate (SKR) performance of the proposed RIS-assisted THz CV-QKD system. The effect of various system parameters, such as the number of RIS elements and their phase configurations, the channel estimation error, and the detector noise, on the SKR performance are studied via numerical evaluation of the derived formula. It is demonstrated that the RIS contributes to larger SKR for larger link distances, and that heterodyne detection is preferable over homodyne at lower pilot symbol powers.

翻译：本文研究了一种集成可重构智能表面（RIS）的多输入多输出（MIMO）无线系统，旨在实现太赫兹（THz）频段的高效运行。发送方Alice采用连续变量量子密钥分发（CV-QKD）协议向接收方Bob传输秘密密钥，Bob采用零差或外差检测方案。接收节点在接收密钥前采用最小二乘方法估计等效MIMO信道增益矩阵，并通过无误差反馈信道将该估计值告知Alice。假设窃听者Eve在反馈信道上实施集体高斯纠缠攻击以获取估计的信道状态信息。我们为所提出的RIS辅助太赫兹CV-QKD系统建立了新颖的密钥生成率（SKR）闭式表达式。通过对推导公式的数值计算，研究了RIS单元数量及其相位配置、信道估计误差、探测器噪声等多种系统参数对SKR性能的影响。研究结果表明：在较长链路距离下RIS能显著提升SKR性能，且在较低导频符号功率条件下外差检测方案优于零差检测方案。