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) 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 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.
Quantum Key Distribution (QKD) is a potential mitigation against quantum computation advances that threaten currently deployed public key cryptosystems. QKD is also a precursor to large-scale quantum communications and the quantum internet. Terrestrial direct transmission of quantum photonic signals is restricted by optical fibre exponential absorption, hence space-based systems have been proposed for intercontinental distribution of quantum keys with lower-loss free-space transmission through vacuum. However, challenges remain for the practical deployment of satellite QKD (SatQKD) such as weather, background light, and terminal deployment. In particular, the short transmission time inherent in low Earth orbit satellite QKD and finite statistical uncertainty can limit secure key generation. Here we illustrate the analysis of these effects and some practical engineering design and operational constraints on key length using the Satellite Quantum Modelling and Analysis (SatQuMA) software package being developed for satellite quantum communications research.
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.
Progress in realising quantum computers threatens to weaken existing public key encryption infrastructure. A global quantum key distribution (QKD) network can play a role in computational attack-resistant encryption. Such a network could use a constellation of high altitude platforms such as airships and satellites as trusted nodes to facilitate QKD between any two points on the globe on demand. This requires both space-to-ground and inter-platform links. However, the prohibitive cost of traditional satellite based development limits the experimental work demonstrating relevant technologies. To accelerate progress towards a global network, we use an emerging class of shoe-box sized spacecraft known as CubeSats. We have designed a polarization entangled photon pair source that can operate on board CubeSats. The robustness and miniature form factor of our entanglement source makes it especially suitable for performing pathfinder missions that studies QKD between two high altitude platforms. The technological outcomes of such mission would be the essential building blocks for a global QKD network.
Information security is a growing concern in our modern world, where almost everything can be done online. To protect security, classical encryption protocols, such as RSA, are used. These encryption protocols are almost always based on mathematical problems that are computational difficult. Therefore, the security is only valid under limited computational resources, and therefore do not provide provable security. An exception is the one-time pad protocol, which is provably secure but requires an existing shared key that is as long as the information it must encrypt. The exchange of such a key can be challenging, often requiring the two partied to physically meet to exchange the key or the use a trusted courier to physically carry the key on a hard drive. \n \nQuantum key distribution (QKD) offers a solution by utilizing quantum mechanics to grow a secure cryptographic key shared between two distant parties. The quantum nature of the particles used in the exchange ensure that any eavesdropper would leave signs of their presence, allowing the users to precisely quantify the security of the key that is produced. This allows secure generation of a random key which can be used for the one-time pad encryption protocol which, unlike most encryption protocols, does not rely on computational assumptions, and is secure regardless of available computational power. \n \nCurrent implementations of QKD are limited to a single link distance of ≈200 km, preventing implementation of QKD on a global scale, or even between distant cities within a single country, without some additional techniques. One promising solution is the use of orbiting low Earth orbit satellite platforms as trusted nodes in a quantum communications network. The main purpose of this work has been to show the viability of this solution, trough theoretical performance modeling and experimental demonstrations. \n \nThorough numerical simulations have been developed to evaluate the performance and challenges in implementing QKD using a low Earth orbit satellite platform and determine an optimal approach to its implementation. The simulations include a realistic satellite orbit analysis, all expected loss mechanisms, estimates of background contributions and realistic simulation of quantum optical processes. This work addresses the questions of optimal wavelength and beam waist, the effect of the telescope design and pointing error, and the impact of detector degradation due to exposure to radiation in the space environment. The simulation is used to determine the length of secure key for QKD and the performance of fundamental quantum optics experiments such as Bell tests and quantum teleportation. We compare the advantages and disadvantages of uplink and dowlink scenarios, and show that an uplink, despite having reduced performance compared to a downlink, offers more scientific freedom, by allowing changes to the quantum source, and benefits from a simpler satellite design with reduced pointing requirement. This work provides a theoretical foundation for ongoing design and development of quantum systems for satellite implementation of QKD. \n \nIn addition to theoretical analyses, experiments where developed and conducted corresponding to the challenges likely to be faced by a satellite uplink. Two main challenges where identified and experimentally overcome. First, the difficulty of operating in the high loss regime of a satellite uplink, which typically experience losses in the range of 40-50 dB when above 40° of elevation from the horizon, an elevation only reached by half of the satellite passes. This challenge was addressed by demonstrating full QKD protocols at losses exceeding 50 dB. Also, the ability to perform full QKD during the short duration of a satellite pass was shown by replicating the varying loss of archetypal passes and successfully extracting secure keys. Secondly, the difficulty of accurately tracking and pointing to a receiver platform traveling at the high angular speed of a satellite was overcome by successfully exchange quantum signals to a truck traveling at angular speeds exceeding that of a low Earth orbit satellite. This required the design, construction, and implementation of a transmitter system and a quantum receiver system capable of active pointing using a custom pointing system. \n \nAn additional experiment was also performed where we experimentally investigate the feasibility of performing QKD using light scattered by a diffusive screen. A system capable of doing so could be used to create QKD hot-spots, where a QKD source could be aimed at a diffusive screen to allow multiple users to simultaneously exchange secure keys with the source without the need for high precision pointing. \n \nThrough these analyses and experimental demonstrations we evaluated the performance of QKD using a satellite and have shown its technological readiness to be implemented. Implementation of such a system would allow QKD to be performed on a global scale, enabling communications security that is not based on computational assumptions without the need to physically transport the key from one party to the other.
Space-based quantum communication is at a point where we will soon see satellites be launched to demonstrate complete and usable Quantum Key Distribution (QKD) links between several optical ground-stations in an industrial live environment. Therefore, and given the specificities of Quantum Communication, researchers as well as industrials need tools and models to leverage their existing classical optics heritage to design their future QKD constellations. Based on our experience of QKD at Fraunhofer IOF, we have developed a centralizing system which capitalize on our QKD models and completes it with the simulation of constellations, the atmosphere, the scheduling of operations and pointing specifications. This simulation tool is particularly aimed at system engineers, in both research and industry, to assess the performance of their envisioned quantum communication space. This tool encompasses all major design factors of a constellation and quantum channels for the simulation of quantum communication channels which includes optical systems as well as atmospheric propagation. Results are providing important inputs for the design of QKD space missions and systems as well as the expected level of possible services in a real-life environment. Our QKD models not only incorporate all systems in the quantum channel, and their design parameters from the laser source on the emitting side to the final click on the detectors on the receiving side, but also the post-processing impact and dynamic optimization of the quantum parameters to maximize the final secret key rate. Furthermore, the evolutive and modular design of the simulation tool itself makes it possible to incorporate different models for all parts of the simulation, for instance the atmosphere, the constellation propagation or the QKD protocols. This architecture makes it possible to interface with other experts of any field of this space system and incorporate their models too. Moreover, this allows for the assessment and comparison of different models and protocols on the final performance of the space QKD system. Our current implementation uses an in-house BBM92 model and is planned to incorporate further models developed at IOF on BB84, Hyperentangled QKD and High-Dimensional QKD.