Quantum communication rates in terrestrial quantum networks are fundamentally limited by fiber loss, even in the presence of quantum repeaters. Satellites offer a solution for long-distance communication, with the most commonly explored scenario involving prepare-and-measure protocols connecting from orbit to a trusted-node ground station via free-space down-links. In contrast, up-link scenarios allow for entanglement to be distributed between a satellite and remote end-users in terrestrial networks, eliminating any trust requirement on the ground station. Here, we demonstrate an ultra-bright source of far-non-degenerate entangled photons and perform quantum key distribution in emulated high-loss satellite scenarios. With a loss profile corresponding to that of one of the pioneering Micius up-link experiments, and a terrestrial end-user separated by 10 km of telecom fiber, we achieve secure key bit accumulation of 5.2 kbit in a single emulated overpass in the asymptotic limit. Our results confirm the viability of upcoming low-Earth orbit receiver satellite missions.
Quantum communication rates in terrestrial quantum networks are fundamentally limited by fiber loss, even in the presence of quantum repeaters. Satellites offer a solution for long-distance communication, with the most commonly explored scenario involving prepare-and-measure protocols connecting from orbit to a trusted-node ground station via free-space down-links. In contrast, up-link scenarios allow for entanglement to be distributed between a satellite and remote end-users in terrestrial networks, eliminating any trust requirement on the ground station. Here, we demonstrate an ultra-bright source of far-non-degenerate entangled photons and perform quantum key distribution in emulated high-loss satellite scenarios. With a loss profile corresponding to that of one of the pioneering Micius up-link experiments, and a terrestrial end-user separated by 10 km of telecom fiber, we achieve secure key bit accumulation of 5.2 kbit in a single emulated overpass in the asymptotic limit. Our results confirm the viability of upcoming low-Earth orbit receiver satellite missions.
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.
The realization of the quantum internet as an interconnected network of quantum hardware requires the distribution of entanglement over long distances. Due to the exponential photon loss in optical fibers, the necessary communication rates for end-user applications cannot be achieved if direct links over several hundred kilometers are required. Two possible solutions to increase the link distance are the implementation of quantum repeaters and the use of free-space optical satellite links. A natural extension would be to combine both approaches, by using satellites as quantum repeater nodes and take advantage of the low losses in free-space. However, due to the higher technological requirements for space-based technologies and the already low communications losses in free-space, it is not yet clear whether satellite- based quantum repeaters can lead to an additional improvement in the communication rate. Here we show for which operating scenarios a satellite-based quantum repeater outperforms a single satellite for entanglement distribution. To this end, we developed a satellite simulation software that includes a model of the optical link model and of entanglement swapping in order to analyze entanglement distribution rates of the satellite-based quantum repeater and single satellite. The simulation captures important effects such as the single-mode fiber coupling efficiency, adaptive optics corrections, decoherence and decay of information in the quantum memory as well as its retrieval efficiency. Based on these effects, we show advantages and challenges of satellite constellations for quantum communications in low, medium and geosynchronous orbits.
We report on entanglement-based quantum key distribution between a low-Earth-orbit satellite equipped with a space borne entangled-photon source and a ground observatory. One of the entangled photons is measured locally at the satellite, and the other one is sent via a down link to the receiver in the Delingha ground station. The link attenuation is measured to vary from 29 dB at 530 km to 36 dB at 1000 km. We observe that the two-photon entanglement survives after being distributed between the satellite and the ground, with a measured state fidelity of ≥0.86. We then perform the entanglement-based quantum key distribution protocol and obtain an average final key rate of 3.5 bits/s at the distance range of 530-1000 km.
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.
Satellite quantum key distribution (SatQKD) intermediated by a trusted satellite in a low-Earth orbit to ground stations along the satellite's path allows remote users to connect securely. To establish a secure connection, a SatQKD session must be conducted to each user over a dynamically changing free-space link, all within just a few hundred seconds. Because of the short time and large losses under which the QKD protocol will be implemented, it has not yet been possible to form a complete key by transmitting all the relevant information required within a single overpass of the satellite. Here, we demonstrate a real-time QKD system that is capable of forming a 4.58-megabit secure key between two nodes within an emulated satellite overpass. We anticipate that our system will set the stage for practical implementations of intercontinental quantum secure communications that can operate over large networks of nodes and enable the secure transmission of data globally.
Long-distance, entanglement distribution over thousands of kilometers is necessary to build a global-scale quantum network. Directly transferring entangled, telecom photons through fiber is relatively straight forward in local networks where the separation between nodes is below 100 km, corresponding to a 1% transmission. However, the exponentially greater loss at larger distances (e.g. only 10−6 transmission at 300 km) leads to unacceptably low transmission rates, which drop below the noise rates of typical photon detectors. Alternatively, the Earth’s rarefied atmosphere at high altitudes reduces the absorption and scattering of infrared photons. Therefore, free-space links between satellite and ground stations have been shown to enable entangled photon transmission over 2000 km. While this demonstration showed the viability of satellite-enhanced entanglement distribution, the source used in this demonstration produced distinguishable photons, which cannot be used for many fundamental protocols like teleportation. We are building a testbed to demonstrate the feasibility of distributing entangled, indistinguishable photons over a space-to-ground link using entanglement swapping.