ABSTRACT Quantum key distribution (QKD) allows the generation of cryptographic keys beyond the computational hardness paradigm and is befitting for secure data transmission requiring long‐term security. The communication distance of fiber‐based QKD, however, is limited to a few hundred kilometers due to the exponential scaling of signal attenuation. Satellite QKD (SatQKD) can instead leverage free‐space optical links to establish long‐range connections and enable global‐scale QKD. In this work, we review the manifold of design choices that concur to form the set of possible SatQKD architectures. These include the choice of the QKD protocol and its physical implementation, but also the satellite orbit, the optical link direction, and whether or not to use trusted‐node relays. The possible SatQKD architectures are then evaluated in terms of key generation throughput, latency and maximum reachable communication distance, but also system‐level security and implementation complexity. Given the technical challenges of realizing SatQKD systems, it is paramount, for near‐future satellite missions, to adhere to the simplest possible architecture that still allows to deliver the QKD service. We thus identify as advisable options the use of low‐Earth orbit satellites as trusted nodes for prepare‐and‐measure discrete‐variable QKD downlinks with weak laser pulses. The decoy‐state version of BB84 is found to be the most promising QKD protocols due to the maturity of the security proofs, the high key generation rate, and low system complexity. These findings are confirmed by the multitude of current and planned SatQKD missions that are adopting these architectural choices.
ABSTRACT Quantum key distribution (QKD) allows the generation of cryptographic keys beyond the computational hardness paradigm and is befitting for secure data transmission requiring long‐term security. The communication distance of fiber‐based QKD, however, is limited to a few hundred kilometers due to the exponential scaling of signal attenuation. Satellite QKD (SatQKD) can instead leverage free‐space optical links to establish long‐range connections and enable global‐scale QKD. In this work, we review the manifold of design choices that concur to form the set of possible SatQKD architectures. These include the choice of the QKD protocol and its physical implementation, but also the satellite orbit, the optical link direction, and whether or not to use trusted‐node relays. The possible SatQKD architectures are then evaluated in terms of key generation throughput, latency and maximum reachable communication distance, but also system‐level security and implementation complexity. Given the technical challenges of realizing SatQKD systems, it is paramount, for near‐future satellite missions, to adhere to the simplest possible architecture that still allows to deliver the QKD service. We thus identify as advisable options the use of low‐Earth orbit satellites as trusted nodes for prepare‐and‐measure discrete‐variable QKD downlinks with weak laser pulses. The decoy‐state version of BB84 is found to be the most promising QKD protocols due to the maturity of the security proofs, the high key generation rate, and low system complexity. These findings are confirmed by the multitude of current and planned SatQKD missions that are adopting these architectural choices.
Quantum key distribution (QKD) is a cryptographic method enabling two parties to establish a private encryption key. The range of communication of ground-based QKD is limited to an order of 100km, due to in-fibre attenuations and atmospheric losses, and the development of quantum repeaters remains technologically challenging. While trusted-node links make communication over large distances possible, satellite-QKD is required for communication over global distances. By using satellites equipped with high-quality optical links, satellite-QKD can achieve ultra-long-distance quantum communication in the 1000-km range. The significant potential of satellite-QKD for the creation of global quantum networks thus makes it a particularly interesting field of research. In this analysis, we begin with an overview of the technical parameters of performing satellite-QKD, including infrastructure and protocols. We continue with a high-level summary of advancements in satellite-QKD by analysing past, present and proposed satellite-QKD missions and initiatives around the world. We conclude by discussing the technical challenges currently faced in satellite-QKD, which can be tackled through future research in this area.
ABSTRACT This review presents the state of the art in quantum key distribution (QKD) for satellite‐to‐ground free‐space optical links. It first surveys continuous‐variable, discrete‐variable, entanglement‐based, measurement‐device‐independent, and twin‐field protocols, and discusses their feasibility under the losses and time‐varying conditions of orbital passes. It also distinguishes protocol security from practical implementation security. The paper then reviews enabling technologies, including single‐photon sources and detectors, followed by atmospheric impairments, time‐evolving turbulence, aperture averaging, and adaptive optics. Finally, physical channel limits and the trust assumptions of satellite‐network architectures are discussed. Overall, the literature indicates that discrete‐variable decoy‐state BB84 in LEO downlink configurations is the most mature near‐term SatQKD option, including for daylight operation when appropriate filtering and pointing/turbulence mitigation are employed.
Abstract Global-scale quantum communication networks will require efficient long-distance distribution of quantum signals. While optical fibre communications are range-limited due to exponential losses in the absence of quantum memories and repeaters, satellites enable intercontinental quantum communications. However, the design of satellite quantum key distribution (SatQKD) systems has unique challenges over terrestrial networks. The typical approach to modelling SatQKD has been to estimate performances with a fully optimised protocol parameter space and with few payload and platform resource limitations. Here, we analyse how practical constraints affect the performance of SatQKD for the Bennett-Brassard 1984 (BB84) weak coherent pulse decoy state protocol with finite key size effects. We consider engineering limitations and trade-offs in mission design including limited in-orbit tunability, quantum random number generation rates and storage, and source intensity uncertainty. We quantify practical SatQKD performance limits to determine the long-term key generation capacity and provide performance benchmarks to support the design of upcoming missions.
Quantum key distribution (QKD) enables tap-proof exchange of cryptographic keys guaranteed by the very laws of physics. One of the last remaining roadblocks on the way towards widespread deployment of QKD is the high loss experienced during terrestrial distribution of photons, which limits the distance between the communicating parties. A viable solution to this problem is to avoid the terrestrial distribution of photons via optical fibers altogether and instead transmit them via satellite links, where the loss is dominated by diffraction instead of absorption and scattering. First dedicated satellite missions have demonstrated the feasibility of this approach, albeit with relatively low secure key rates. In order for QKD to become commercially viable, the design of future satellite missions must be focused on achieving higher key rates at lower system costs. Current satellite missions are already operating at almost optimal system parameters, which leaves little room for enhancing the key rates with currently deployed technology. Instead, fundamentally new techniques are required to drastically reduce the costs per secret bit shared between two distant parties. Entanglement-based protocols provide the highest level of security and offer several pathways for increasing the key rate by exploiting the underlying quantum correlations. In this contribution, we review the most relevant advances in entanglement-based QKD which are implementable over free-space links and thus enable distribution of secure keys from orbit. The development of satellite missions is notoriously lengthy. Possible candidates for a new generation of quantum payloads should therefore be scrutinized as early as possible in order to advance the development of quantum technologies for space applications.
Satellite quantum key distribution (SatQKD) is widely regarded as a promising approach for extending quantum-secure key establishment beyond the distance limitations of terrestrial optical fiber links. However, practical assessment of SatQKD remains difficult because published results rely on heterogeneous assumptions regarding channel loss, orbital visibility, atmospheric conditions, detector behavior, finite-key analysis, and classical post-processing efficiency. As a result, comparisons across protocols, architectures, and mission concepts are often difficult to interpret from an operational engineering perspective. This paper presents an engineering-oriented review of SatQKD in the context of hybrid optical satellite communication systems. The review synthesizes prior results reported across protocol-level, link-budget, orbital, finite-key, and network-level studies. Within this scope, service-level notions such as pass-integrated and availability-limited key delivery are used as an interpretive lens for discussing practical relevance under orbital intermittency, atmospheric attenuation, receiver non-idealities, finite-key constraints, and classical post-processing limitations. The contribution of this review is a cross-layer engineering synthesis of prior SatQKD results reported under heterogeneous assumptions, aimed at supporting more consistent operational interpretation across protocols, architectures, and mission concepts.
Quantum repeaters and satellite-based free-space optical links are complementary technological approaches slated to overcome the exponential photon loss occurring in optical fibers and thus allow Quantum Key Distribution (QKD) at global scales. We analyse architectures where these approaches are combined and satellites are used as quantum repeater nodes to distribute entanglement to far-away optical ground stations. Here, we simulate dynamical, three-dimensional satellite-ground-station passes, going beyond previous investigations which typically consider static satellite links. Therefore, we numerically solve the equations of motion of the dynamic system consisting of three, low-Earth-orbit satellites separated in the along-track direction. The optical link model includes atmospheric attenuation, single mode fiber coupling, beam wandering and broadening, as well as adaptive optics effects. Analytical expressions for the Bell state measurement and associated error rates are derived for quantum memory assisted communications, accounting for exponentially decreasing retrieval efficiency and state coherence. We consider uplink and downlink architectures for continental and inter-continental connections and assess the influence of varying orbital elements on the expected. We also investigate alternative solutions for realising practically secure QKD networks in the near-term, whereby trusted nodes are only located in space and relatively inaccessible. By exploiting the newly introduced mode-pairing QKD protocol, we show that users located hundreds of kilometres from a ground station could establish a secure key with a satellite using well-established technologies and without requiring intermediate trusted nodes.