Orthogonal Frequency-Division Multiplexing Continuous-Variable Terahertz QKD for Large-Scale Wireless Quantum Communication

We introduce a continuous-variable quantum key distribution (CVQKD) protocol that combines orthogonal frequency-division multiplexing (OFDM) with terahertz (THz) carriers to deliver high-throughput and hardware-compatible quantum communication. By distributing quantum states across multiple subcarriers, our approach achieves a noticeable increase in spectral efficiency while mitigating dispersion and atmospheric losses that limit the performance of existing optical and microwave CVQKD systems. A full security analysis under collective Gaussian attacks is presented, incorporating realistic noise models for both terrestrial and inter-satellite channels. In terrestrial free-space links, our simulations show secret key rates approaching 72 bits per channel use, with secure distances up to 4.5 m under strong humidity-induced absorption. For inter satellite links, where propagation losses are minimal, secure transmission is sustained over >100 km. A distinctive feature of our work is its direct link to practical hardware. We evaluate implementation using emerging on-chip coherent THz sources based on superconducting Josephson junctions. These compact, voltage-tunable emitters provide wideband coherent radiation ideally matched to our protocol, enabling scalable, chip integrated quantum networks. By incorporating their measured characteristics into our models, we demonstrate secure communication up to 3 m in ambient conditions and ∼26 km under cryogenic or vacuum conditions. By uniting advanced protocol design, rigorous security modelling, and hardware driven performance analysis, this work establishes THz CVQKD as a viable and scalable route to next-generation terrestrial and space-based quantum communication.