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.
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.
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.
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.
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.
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.
Quantum key distribution (QKD) is critical for future proofed secure communication. Satellites will be necessary to mediate QKD on a global scale. The limitations of the existing quantum memory and repeater technology mean that twin-field QKD (TF-QKD) provides the most feasible near-term solution to perform QKD with an untrusted satellite. However, the TF-QKD requires links between ground stations and satellites to be phase stable. We show that phase stabilization of the links to LEO and MEO satellites is feasible in spite of phase noise due to atmospheric turbulence, laser instability, and path length asymmetry while only incurring a quantum bit error rate (QBER) penalty of less than 1.5%. These results are also applicable to future untrusted satellite networks employing precisely synchronized quantum memories or quantum repeaters.
Satellite-based QKD is currently being developed to revolutionize global cryptographic key exchange by facilitating secure communication among remote parties at a global scale. By overcoming the exponential loss of fiber transmission, satellite-to-Earth communication can seamlessly interconnect vast distances as the link budget of such links is sufficient to support QKD links. In terms of this direction, DV-QKD implementations seems to be technologically ahead since key exchange has been experimentally demonstrated to perform much more efficiently by providing key rates that are orders of magnitude higher compared to entanglement-based key exchange. However, the specific requirements to support effectively functional DV-QKD satellite-to-ground links are yet to be defined. This work attempts to define the satellite and ground segment system requirements needed in order to achieve functional QKD service for various satellites orbits (LEO, MEO, and GEO). Finite key size effects are being considered to determine the minimum block sizes that are required for secure key generation between a satellite node and a ground terminal for a single satellite pass. The atmospheric link channel is modeled with consideration of the most important degradation effects such as turbulence and atmospheric and pointing loss. Critical Tx and Rx system parameters, such as the source’s intrinsic Quantum Bit Error Rate (iQBER), the Rx telescope aperture size, and detection efficiency, were investigated in order to define the minimum requirements to establish an operation satellite-to-ground QKD link under specific assumptions. The performance of each downlink scenario was evaluated for the wavelength of 1550 nm in terms of link availability, link budget, and in the distilling of secure key volumes over time. Finally, the feasibility and requirements for distributing the collected space photons via terrestrial telecom fibers was also studied and discussed, leading to the proposal of a more futuristic WDM-enabled satellite QKD architecture. This comprehensive analysis aims to contribute to the advancement and implementation of effective satellite-based QKD systems, which can further exploit the ground fiber segment to realize converged space/terrestrial QKD networks.