Abstract In pursuit of a global quantum key distribution (QKD) network, a service based on untrusted nodes on geostationary satellites could offer wide coverage, continuous operation, and enhanced security compared to the trusted node alternative. Although this scenario has been studied for entanglement-based protocols, such an approach would require large-area telescopes both on the ground and in space. In this work, we analyze the performance of two QKD protocols well adapted to this scenario, namely twin-field (TF) and mode-pairing (MP) QKD, which exhibit high resilience to high-loss channels. Leveraging an in-depth simulation of communication channels corrected with adaptive optics, we assess the expected secret key rates for both protocols in a configuration involving two 50 cm telescopes on board the satellite and ground-based telescopes ranging from 20 cm to 1 m in aperture. Our results show that, in the best case and considering realistic detectors, it is possible to achieve secret key rates on the order of a few 100 bit s −1 for both TF and MP-QKD. We show, notably, that secret key generation is potentially feasible even with 20 cm ground telescopes, highlighting the high scalability potential of such a configuration.
Abstract In pursuit of a global quantum key distribution (QKD) network, a service based on untrusted nodes on geostationary satellites could offer wide coverage, continuous operation, and enhanced security compared to the trusted node alternative. Although this scenario has been studied for entanglement-based protocols, such an approach would require large-area telescopes both on the ground and in space. In this work, we analyze the performance of two QKD protocols well adapted to this scenario, namely twin-field (TF) and mode-pairing (MP) QKD, which exhibit high resilience to high-loss channels. Leveraging an in-depth simulation of communication channels corrected with adaptive optics, we assess the expected secret key rates for both protocols in a configuration involving two 50 cm telescopes on board the satellite and ground-based telescopes ranging from 20 cm to 1 m in aperture. Our results show that, in the best case and considering realistic detectors, it is possible to achieve secret key rates on the order of a few 100 bit s −1 for both TF and MP-QKD. We show, notably, that secret key generation is potentially feasible even with 20 cm ground telescopes, highlighting the high scalability potential of such a configuration.
Due to the distance limitation of quantum communication via ground-based fibre networks, space-based quantum key distribution (QKD) is a viable solution to extend such networks over continental and, ultimately, over global distances. Compared to LEO, QKD from GEO would offer substantial advantages, i.e. large coverage, continuous link to ground stations (cloud cover limited), 24/24h operation (background limited), no tracking required, however, coming with large link losses seen the space-ground distance. TNO, together with Eutelsat and CGI-NL, performed a detailed study on the feasibility of QKD from GEO, including a high level system design of payload and ground segment. We conclude that QKD from GEO is technically feasible, and a favourable solution if the satellite needs to act as an untrusted node (that is, no security assumptions required for the space segment). However, the optimal solution, generating a higher value-for-money, is to use a hybrid system, implementing an untrusted and trusted mode BBM92 QKD protocol. In order to arrive at a minimum required secure bit rate of ~1 bit/s in untrusted mode, a 2x ~0.5m diameter telescope in the space segment is required with <0.25µrad pointing accuracy, a <1GHz entangled photon source, in combination with ~2m diameter telescopes on ground. Details on our assumptions and results and drawings of the high level system design will be presented, as well as the development roadmap describing the required technology improvements and building blocks for the overall hybrid approach, which is applicable to non-GEO applications as well.
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 cryptography promises unconditionally secure communication. Today, there exists a vast array of different QKD protocols that claim to offer security. However, looking in more detail, many subtleties lead to different security levels, or in the worst-case to no security at all. Here, we introduce the most crucial QKD security properties, ranging from different attack schemes to actual implementation security considerations. We present three different QKD use-cases with different network topologies: one trusted-node-based scenario (BB84 decoy) and two trusted-node-free (entanglement-based BBM92 and twin-field QKD). Using our, in cooperation with the European Space Agency (ESA), in-house developed simulations package, we simulate all relevant performance parameters, including the expected finite secret-key-rate. Furthermore, we assess all simulated QKD protocols’ applicability to satellite-based QKD networks and identify essential technologies. Interestingly, not the single-photon detection modules are the key drivers in terms of secure-key-rate performance, but the optical sending and receiving telescopes.
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.
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.
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.