Quantum Key Distribution (QKD) technology, based on the principles of quantum mechanics, provides theoretically unconditional security for information exchange, enabling effective detection and resistance against potential eavesdropping. QKD is categorized into discrete variable QKD (DVQKD) and continuous variable QKD (CVQKD), with this study focusing on theoretical analysis within the terahertz (THz) spectrum. By comparing the performance of the direct reconciliation homodyne detection protocol with the reverse reconciliation homodyne detection protocol, the secure key rate and transmission distance of the CVQKD system in the 0.1-1THz frequency range are investigated under both atmospheric absorption loss alone and the combined effects of atmospheric absorption loss and free-space path loss. The investigation further underscores the beneficial effects of increased antenna gain in mitigating losses, underscoring the critical role of judicious antenna design in maximizing the performance of QKD systems Furthermore, it reviews the latest international advancements in high-gain antennas within the 0.1-1THz band.
Quantum Key Distribution (QKD) technology, based on the principles of quantum mechanics, provides theoretically unconditional security for information exchange, enabling effective detection and resistance against potential eavesdropping. QKD is categorized into discrete variable QKD (DVQKD) and continuous variable QKD (CVQKD), with this study focusing on theoretical analysis within the terahertz (THz) spectrum. By comparing the performance of the direct reconciliation homodyne detection protocol with the reverse reconciliation homodyne detection protocol, the secure key rate and transmission distance of the CVQKD system in the 0.1-1THz frequency range are investigated under both atmospheric absorption loss alone and the combined effects of atmospheric absorption loss and free-space path loss. The investigation further underscores the beneficial effects of increased antenna gain in mitigating losses, underscoring the critical role of judicious antenna design in maximizing the performance of QKD systems Furthermore, it reviews the latest international advancements in high-gain antennas within the 0.1-1THz band.
We conducted a study to evaluate the performance of Terahertz Continuous Variable Quantum Key Distribution (THz CVQKD) systems in atmospheric conditions with the finite-size effect. Our research delved into the secure key rates and transmission distances of THz CVQKD for four protocols, all based on Gaussian coherent states, under the threat of collective Gaussian attacks. These protocols include reverse reconciliation with homodyne detection, direct reconciliation with homodyne detection, reverse reconciliation with heterodyne detection, and direct reconciliation with heterodyne detection. Our findings revealed distinctive key rates across various THz frequencies and protocols. However, the finite-size effect had a diminishing impact on the key rates of CVQKD in all scenarios. Fortunately, by harnessing a substantial number of keys exceeding 1012 and optimizing modulation variance, both the key rates and transmission distances of THz CVQKD can be significantly extended. This research contributes to advancing the practicality of THz CVQKD technology.
We propose a novel continuous-variable quantum key distribution (CVQKD) protocol that employs orthogonal frequency-division multiplexing (OFDM) in the terahertz (THz) band to enable high-throughput and secure quantum communication. By encoding quantum information across multiple subcarriers, the protocol enhances spectral efficiency and mitigates channel dispersion and atmospheric attenuation. We present a comprehensive security analysis under collective Gaussian attacks, considering both terrestrial free-space channels, accounting for humidity-induced absorption, and inter-satellite links, incorporating realistic intermodulation noise. Simulations show secret key rates (SKR) reaching ~72 bits per channel use in open-air conditions. While intermodulation noise imposes trade-offs, optimised modulation variance enables resilience and secure communication range. The maximum terrestrial quantum link extends up to 4.5 m due to atmospheric THz absorption, whereas inter-satellite links can support secure communication over distances exceeding 100 km, owing to minimal propagation channel losses in space. We evaluate the practical implementation of our protocol using recently developed on-chip coherent THz sources based on superconducting Josephson junctions. These compact, voltage-tunable emitters produce wideband coherent radiation, making them ideal candidates for integration in scalable quantum networks. By incorporating their characteristics into our simulations, we assess secure key generation under various environmental conditions. Our results show secure communication over distances up to 3 m in open air, and up to 26 km in cryogenic or vacuum environments. This work advances the prospect of compact, high-capacity CVQKD systems for both terrestrial and space-based THz quantum communication.
We have introduced, for the first time, a protocol for Continuous-Variable Measurement-Device-Independent Quantum Key Distribution (CV-MDI-QKD) in the terahertz (THz) frequency band. We have conducted a secret key rate analysis against collective attacks. The proposed THz CV-MDI-QKD is immune to all detector attacks, significantly enhancing the security assurance of practical THz CVQKD implementations. Furthermore, we investigated the impact of finite key length (the finite-size effect) and finite reconciliation efficiency on the performance of practical THz CV-MDI-QKD systems. Our findings reveal that by employing a large number of keys or signals and optimizing the modulation variance, the detrimental effects arising from the finite-size effect and suboptimal reconciliation efficiency can be notably mitigated. These insights play a crucial role in advancing the feasibility of THz CVQKD technology towards practical applications.
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.
Although multiple-input multiple-output (MIMO) terahertz (THz) continuous-variable quantum key distribution (CVQKD) is theoretically secure, practical vulnerabilities may arise due to detector imperfections. This paper explores a CV measurement-device-independent (MDI) QKD system operating at THz frequencies within a MIMO framework. In this system, measurement is delegated to an untrusted third party, Charlie, rather than the receiver, eliminating all detector attacks and significantly enhancing the system's practical security. Using transmit-receive beamforming techniques, the system transforms MIMO channels into multiple parallel lossy quantum channels, enabling robust key distribution between Alice and Bob. This study examines entanglement-based and prepare-and-measure protocols, deriving secret key rates for both asymptotic and finite code scenarios. Simulations reveal the critical role of multiple antenna configurations and efficient homodyne detection in mitigating free-space path loss and maximizing key rates. Results indicate that system performance is optimized at lower THz frequencies for long-range transmissions and higher frequencies for short-range applications. The proposed protocol offers a scalable solution for secure quantum communications in next-generation wireless networks, demonstrating potential for deployment in both indoor and outdoor environments.
Quantum key distribution (QKD) provides information-theoretic security by leveraging quantum mechanical principles, while Terahertz (THz) communications offer unprecedented data rates for next-generation networks. We sketch out an evolutionary pathway for bridging these domains by critically appraising THz-based continuous-variable QKD (CV-QKD) systems conceived for wireless environments like space-air-ground integrated networks (SAGINs) and vehicle-to-everything (V2X) scenarios, where time-varying frequency-selective fading poses critical challenges. To address these issues, we elaborate on the feasibility of THz based CV-QKD applications from a wireless perspective. Specifically, we commence by highlighting a typical CV-QKD protocol. Then present three sophisticated CV-QKD system designs.Firstly, we propose a codeword-based – rather than syndrome-based – reconciliation scheme capable of accommodating diverse forward error correction (FEC) codes, imposing a similar complexity on the transmitter and receiver. As a further improvement, we demonstrate that powerful irregular convolutional codes (IRCCs) attain a near-capacity performance, thus offering longer secure distance and/or improved secret key rate (SKR).Secondly, the secure boundaries derived for the optical and THz band – even down to microwave frequencies – demonstrate the feasibility of short-distance THz CV-QKD systems. Furthermore, multiple-input multiple-output (MIMO) schemes are harnessed for improving the SKR performance.Thirdly, an orthogonal time frequency space (OTFS) modem-based CV-QKD system is conceived for time-varying frequency-selective THz fading channels. Finally, we conclude by identifying several promising research directions for CV-QKD systems.