Multiple-input multiple-output CV-MDI-QKD system for terahertz channels based on orthogonal time frequency space

Continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) offers robust protection against side-channel and detector manipulation attacks by outsourcing the detection process to a third party. In this work, we propose a multiple-input multiple-output (MIMO) CV-MDI-QKD system for time-varying and frequency-selective fading terahertz (THz) channels based on orthogonal time frequency space. The information bit mapping symbols are mapped into a two-dimensional plane in the 'delay-Doppler domain,' enhancing resistance to multi-path and Doppler effects, without relying on trusted measurement devices. Moreover, we increase the secret key rate and transmission distance by leveraging MIMO beamforming to mitigate free-space path loss at high THz frequencies. We calculate the secret key rate under a framework of strict composable security, including finite-size effects. Simulation results verify that by integrating OTFS modulation with MIMO beamforming, the proposed system achieves higher secret key rates and longer secure transmission distances, while maintaining strong security against optimal collective Gaussian attacks. This work is of great significance in driving the advancement of wireless communication technology in the 5G and post-5G eras.