We propose a satellite-based Quantum Key Distribution (QKD) network that enables global-scale, end-to-end secure key exchange without relying on trusted intermediate nodes. The network is formed by a ring constellation of satellites that maintain persistent inter-satellite connectivity and support two configurations: a polar Type-I constellation providing global coverage, and an equatorial Type-II constellation offering continuous, terrestrial-like operation. End-to-end secrecy is achieved through the use of Twin-field Quantum Key Distribution (TFQKD) and a redundant XOR-based key-forwarding protocol, in which each forwarding step incorporates independently generated QKD keys from ground–satellite and inter-satellite links. As a result, the final secret key is never exposed to any intermediate satellite, eliminating the single-point vulnerabilities inherent in trusted-node networks. Scaling the network offers two benefits: improved security and higher key rates. Increasing the constellation size enhances security by forcing an adversary to compromise a larger number of nodes to break the protocol, while simultaneously improving link availability and key throughput. Using realistic uplink and Inter-Satellite Link (ISL) models, we compute finite-size secret-key lengths based on the Sending-or-not-sending (SNS)–TF-QKD protocol. Our results show that the achievable key rates scale favourably with constellation size, with Type-II constellations reaching operational continuity and generating multi-gigabit secret keys per day, demonstrating a practical route toward secure global quantum communication.
We propose a satellite-based Quantum Key Distribution (QKD) network that enables global-scale, end-to-end secure key exchange without relying on trusted intermediate nodes. The network is formed by a ring constellation of satellites that maintain persistent inter-satellite connectivity and support two configurations: a polar Type-I constellation providing global coverage, and an equatorial Type-II constellation offering continuous, terrestrial-like operation. End-to-end secrecy is achieved through the use of Twin-field Quantum Key Distribution (TFQKD) and a redundant XOR-based key-forwarding protocol, in which each forwarding step incorporates independently generated QKD keys from ground–satellite and inter-satellite links. As a result, the final secret key is never exposed to any intermediate satellite, eliminating the single-point vulnerabilities inherent in trusted-node networks. Scaling the network offers two benefits: improved security and higher key rates. Increasing the constellation size enhances security by forcing an adversary to compromise a larger number of nodes to break the protocol, while simultaneously improving link availability and key throughput. Using realistic uplink and Inter-Satellite Link (ISL) models, we compute finite-size secret-key lengths based on the Sending-or-not-sending (SNS)–TF-QKD protocol. Our results show that the achievable key rates scale favourably with constellation size, with Type-II constellations reaching operational continuity and generating multi-gigabit secret keys per day, demonstrating a practical route toward secure global quantum communication.
We propose a satellite-based Quantum Key Distribution (QKD) network that enables global-scale, end-to-end secure key exchange without relying on trusted intermediate nodes. The network is formed by a ring constellation of satellites that maintain persistent inter-satellite connectivity and support two configurations: a polar Type-I constellation providing global coverage, and an equatorial Type-II constellation offering continuous, terrestrial-like operation. End-to-end secrecy is achieved through the use of Twin-field Quantum Key Distribution (TF-QKD) and a redundant XOR-based key-forwarding protocol, in which each forwarding step incorporates independently generated QKD keys from ground-satellite and inter-satellite links. As a result, the final secret key is never exposed to any intermediate satellite, eliminating the single-point vulnerabilities inherent in trusted-node networks. Scaling the network offers two benefits: improved security and higher key rates. Increasing the constellation size enhances security by forcing an adversary to compromise a larger number of nodes to break the protocol, while simultaneously improving link availability and key throughput. Using realistic uplink and Inter-Satellite Link (ISL) models, we compute finite-size secret-key lengths based on the Sending-or-not-sending (SNS)-TF-QKD protocol. Our results show that the achievable key rates scale favourably with constellation size, with Type-II constellations reaching operational continuity and generating multi-gigabit secret keys per day, demonstrating a practical route toward secure global quantum communication.
This thesis aims to evaluate the feasibility of a global key distribution architecture without localizing the relay nodes entirely by investigating space geometry and a constraints-aware framework for satellite-based Twin-Field quantum key distribution (TF–QKD). The investigation concentrates on the inter-satellite link (ISL) and uplink (UL) segments in the context of realistic orbital dynamics. Under these conditions, the availability and quality of the link are jointly influenced by elevation, slant range, and line of sight. Geometric sizing of the Tx/Rx telescope apertures, Beer-Lambert atmospheric extinction, beam pointing instability, background scattering due to Sun/Moon radiation and synchronization penalties are all included in channel efficiency models, and they are all treated stochastically to reflect scalability with fidelity. XOR-based key forwarding is employed to evaluate end-to-end secure key generation on top of these physical layers. Path Routing and scheduling are guaranteed by graph based networks and numerical calculations. Various routing strategies are evaluated, such as key rate-aware optimization, distance weighed shortest path (KSP), and k-disjoint parallel chains. Performance across service-level metrics is quantified by simulation campaigns that span month-long orbital cycles, including delivered keys per pass, continuity curves, outage probability, time-to-key, throughput and k-disjoint availability. The findings indicate that the primary network design choices include uplink elevation and short, repeatable ISLs. By raising elevation masks and coordinating orbital phasing, narrow peaks in synchronous-key transmission are transformed into wide plateaus with a high secret key rate. The system maintained 12.3 Gbit/month of end-to-end keys under conservative XOR bottlenecks in the Sun-synchronous orbit (SSO) con stellation scenario that was demonstrated. It also accomplished median multi-path SKR in the tens of kbps with near-zero outages and provided three disjoint paths for virtually the entire run due to a calculated resource-intensive network. For a practical architectural candidate for global-scale quantum networks, these findings agree on satellite TF–QKD with XOR forwarding. The framework demonstrates how non-localized trust can be maintained without sacrificing continuity by utilizing orbital repeatability and impairment-aware scheduling. This work parallelize industry friendly post-quantum cryptography and advances the field of quantum communication.
Quantum Key Distribution (QKD) offers a fundamentally secure method for key exchange, grounded in the principles of quantum mechanics rather than computational complexity. As advancements in quantum computing threaten classical cryptographic systems (such as RSA) QKD presents itself as a crucial technology for future secure communications. Among the primary challenges for global-scale QKD is the limited range of optical fiber channels, making satellite-based free-space optical links an attractive alternative. QKD presents unique constraints related to the short and unreliable communication windows, due to atmospheric conditions and the presence of sunlight, which makes the real time operation of QKD constellations almost impossible. Traditional configurations offering instantaneous global coverage, such as Walker constellations, are not necessary for QKD networks and other constellation designs may be explored. In this work, we present as an alternative a two-height, single orbital plane constellation, which facilitates regular and extended inter-satellite visibility, enabling the constellation to function as a virtual trusted node and increasing network capacity and flexibility. The main technical contribution is the development of a simulation tool designed to evaluate and optimize satellite QKD constellations. The simulator allows researchers and mission planners to assess key generation rates and overall system performance, helping to reduce the risks, costs, and uncertainties involved in satellite deployment. Our approach provides a powerful framework for advancing the development of scalable and efficient global QKD networks.
Satellite-based optical communication enables quantum key distribution (QKD) on global scales, with realistic near-term architectures usually considering prepare-and-measure protocols that employ satellites in Low Earth Orbit (LEO) acting as trusted nodes. One major challenge is constituted by the high loss in the free-space link from the satellite to the optical ground station (OGS) due to the large distances involved. If the quantum signal is directly forwarded from the OGS to an end-user via a ground-based fiber connection, the additional losses limit the key generation to the vicinity of a few kilometers. For longer-distance links, instead, the OGS would have to act as another trusted node to relay the key, introducing a severe bottleneck in the practical security. Here we consider an alternative solution based on the use of Mode-Pairing (MP) QKD, employing the OGS as an untrusted intermediate node. Due to its improved scaling of the key rate with the channel transmission, MP QKD allows end-users up to hundreds of kilometers around the OGS to establish a secure key with the satellite, without having to trust the OGS provider. We discuss the effect of Doppler shifts on the performance of MP QKD and propose a possible implementation to mitigate its impact with sufficient precision.
Quantum Key Distribution (QKD) is a technology that enables the exchange of private encryption keys between two legitimate parties, using protocols that involve quantum mechanics principles. The rate at which secret keys can be exchanged depends on the attenuation that is experienced. Therefore, it is more convenient to replace many terrestrial fiber segments (and repeaters) by just few optical satellite links that would enable flexible global coverage. Then, the satellite nodes can take the role of trusted-relays, forwarding the secret keys from source to destination. However, since the rate at which secret keys can be generated in each quantum link is limited, it is very important to select the intermediate satellite nodes to inter-connect ground stations efficiently. This paper studies the most convenient allocation of resources in a QKD network that combines complementary connectivity services of GEO and LEO satellites. The aim of the centralized routing algorithm is to select the most convenient trusted-relays to forward the secret keys between pairs of ground stations, verifying the constraints that satellite-to-ground and inter-satellite quantum channels have.
Establishing information-theoretically secure communication between arbitrary locations on Earth is a central goal of quantum networking, for which space-based distribution of quantum entanglement is a leading approach. Modelling and analysis of the performance of satellite entanglement pair distribution is therefore important for the architecture and design of future constellations and space systems. However, entanglement-based quantum key distribution in the absence of quantum repeaters, is especially prone to finite-key effects due to low coincident count rates compared to trusted-node single-path links. Quantifying these effects under realistic orbital conditions is therefore essential for the design of practical satellite quantum communication systems. Here, we develop a high-fidelity model of pair distribution from a low-Earth-orbit satellite that captures orbital dynamics, elevation-dependent loss, background noise, and extraneous detector effects. Integrating this with a finite-key security framework for the BBM92 protocol, we optimise the secret key length across different overpass geometries, orbital altitudes, and optical ground station (OGS) separations. We show that secure key generation remains feasible in early clear daytime conditions using baseline parameters representative of current technologies, while showing that modest system improvements could further extend daytime operation. Finally, we evaluate the annual secret key performance for representative intercity-scale OGS pairings. Our results provide quantitative performance bounds and practical design guidelines for near-term entanglement-based satellite QKD, enabling informed trade-offs between satellite payload complexity, ground infrastructure, and achievable secure key throughput.