Satellite Quantum Key Distribution: Performance Limits and System Design

Quantum key distribution (QKD) offers the potential for unconditionally secure communication, relying on the principles of quantum mechanics rather than computational complexity. While terrestrial implementations are maturing, extending this security to global scales necessitates satellite-based systems, presenting considerable engineering challenges. Photon loss due to atmospheric turbulence, beam divergence and imperfect pointing are significant impediments to establishing secure links over vast distances. A new analysis, detailed in the article ‘Physical Limits of Entanglement-Based Quantum Key Distribution over Long-Distance Satellite Links’, investigates these physical constraints on entanglement-based QKD protocols, specifically focusing on inter-satellite communication. Mohammad Taghi Dabiri, alongside Mazen Hasna, Saif Al-Kuwari and Khalid Qaraqe, all affiliated with the IEEE, present a comprehensive model evaluating signal detection probabilities, background noise and bit error rates, offering insights into optimising system parameters for reliable, long-distance satellite quantum communication.

The prioritisation of practical implementation defines the current trajectory of quantum key distribution (QKD), notably through satellite deployment and free-space optical (FSO) communication. Researchers consistently address the challenges inherent in establishing secure quantum networks over extended distances, moving beyond purely theoretical frameworks. Current investigations focus on mitigating photon loss and maintaining acceptable bit error rates, quantified as quantum bit error rate (QBER), while a pronounced trend emerges towards modelling and analysing the physical layer limitations of these systems. Protocols such as E91 and BBM92, which leverage quantum entanglement to establish secure keys, receive attention for their potential in satellite-to-satellite communication. Modelling efforts account for critical impairments including beam divergence, pointing errors, and background noise, all of which significantly impact key generation rates.

A substantial portion of research employs FSO communication as the transmission medium, necessitating detailed channel modelling. Studies develop and refine models to accurately predict performance under atmospheric turbulence, scattering, and other impairments. Unmanned aerial vehicles (UAVs) receive considerable attention as potential relays or nodes within these FSO systems, offering increased flexibility and range, and their integration into QKD networks represents a developing area of exploration. This suggests a move towards hybrid architectures combining the global coverage of satellites with the agility and cost-effectiveness of UAV-based communication.

The collected research unequivocally demonstrates a strong and growing focus on the practical realisation of satellite QKD and broader space-based quantum communication networks, prioritising system implementation and addressing the inherent challenges of long-distance FSO links. A significant portion of recent publications, particularly those from 2023 to 2025, centres on finite key performance analysis, realistic channel modelling, and comprehensive architectural assessments for satellite QKD systems. Accurate channel modelling consistently emerges as a critical area of investigation, reflecting the need to understand and mitigate the effects of atmospheric turbulence, pointing errors, and other impairments on optical communication links.

The bibliography highlights the interdisciplinary nature of this field, drawing upon expertise from quantum physics, optical engineering, communication theory, and networking. This convergence of disciplines is essential for overcoming the complex technical hurdles associated with building practical quantum communication systems. The analytical work presented focuses on modelling signal detection probabilities, background photon influence, and bit error rates, incorporating key parameters such as link distance and transmitter tracking jitter. These models provide actionable design insights for optimising system performance and ensuring reliable key generation.

Future research should concentrate on developing robust techniques for mitigating the effects of atmospheric turbulence and pointing errors. Adaptive optics and advanced signal processing techniques offer promising avenues for improvement. Researchers investigate novel modulation and coding schemes to enhance the resilience of quantum signals against noise and channel impairments, and explore the use of quantum repeaters to extend the range of quantum communication beyond the limitations imposed by signal loss. They are also developing advanced security protocols to protect QKD systems against potential attacks. The field of satellite QKD is rapidly evolving, driven by the demand for secure communication in a world increasingly reliant on digital technologies. Ongoing research and development efforts promise to unlock the full potential of this transformative technology, with profound implications for government, finance, healthcare, and critical infrastructure.