Quantum technologies, including computing, communication, and sensing, offer advantages over classical counterparts. Quantum networking enhances these functionalities. Quantum key distribution (QKD), a primary application of quantum communication, guarantees secure key sharing using the laws of physics, not just computational power. While QKD has seen significant technological advancements, including hand-held devices, integrated optics, and photon detectors, the main limitation remains the transmission distance due to exponential loss in optical fibers. Current ground-based systems, even with advanced techniques like twin-field QKD, are limited to around 500 km. Conventional repeaters are unsuitable for QKD due to the no-cloning theorem. Current long-distance links use trusted nodes, which are potential security weaknesses. Untrusted operation, achievable with quantum repeaters (QRs) using quantum memories (QMs), is necessary for long-range entanglement distribution and overcomes terrestrial limits. However, even fibre-based QRs struggle beyond 4000km. Satellites offer a potential solution, but line-of-sight limitations restrict their range unless they act as trusted nodes. This paper proposes a novel approach using satellites equipped with QMs to create free-space optical repeater links, connecting ground stations via MA-QKD protocols. This approach offers higher rates and device-independent security, providing a quantitative analysis of space-based QMs and a substantial improvement over existing methods. The authors benchmark entanglement distribution rates, analyze the impact of memory characteristics and losses, and propose several QM platforms suitable for space applications.

The authors review existing quantum repeater architectures, categorizing them based on error correction mechanisms. First-generation QRs rely on post-selected entanglement, while improved generations utilize active error correction codes. The paper focuses on first-generation architectures using ensemble-based QMs, highlighting the DLCZ protocol and its modifications. The limitations of ground-based repeaters and hybrid satellite-ground architectures are discussed, emphasizing the challenges posed by atmospheric losses and the need for good weather conditions at multiple ground stations. The paper also reviews previous satellite QKD (SatQKD) work, noting its limitations in reaching global distances without trusted nodes.

The authors present results for two QR protocols for global entanglement distribution: the DLCZ protocol and a QND-QR protocol. They utilize entanglement distribution time (Ttot) as a benchmark, comparing their proposed fully space-based architecture with a hybrid ground-space architecture. The DLCZ protocol's time is calculated using Equation (1), considering parameters such as nesting level (n), segment length (L0), detection efficiency (ηd), transmission efficiency (ηt), and memory efficiency (ηm). The QND-QR protocol's time is calculated using Equation (2), incorporating additional parameters like QND detection efficiency (ηg), source repetition rate (Rs), and average two-photon transmission (p0). The authors analyze entanglement distribution times as functions of total ground distance, beam divergence, and memory efficiency. The impact of atmospheric losses and weather conditions are also investigated. For MA-QKD, the authors adapt existing protocols to a space-based scenario, benchmarking key rates against ent-QKD protocols. They analyze uplink and downlink configurations, considering factors such as atmospheric diffraction, memory storage time, and memory efficiency. The model considers parameters including orbital height, telescope aperture radii, beam divergence, storage time, memory pairs, and detector efficiency. The calculations incorporate models for diffraction losses (Equation 3), atmospheric losses (Equation 5 and 6), pointing losses (Equation 7), dark counts, and stray light (Equation 8). Secret key rate is calculated using Equation (9), incorporating QBER and error correction inefficiency. The authors also compare their key rate results with previous experimental demonstrations, specifically mentioning the MICIUS experiment.

The fully space-based QND protocol significantly outperforms the hybrid ground-space and DLCZ protocols in terms of entanglement distribution time, achieving a three-order-of-magnitude speedup for global distances. The space-based approach’s robustness against atmospheric loss is highlighted. The analysis reveals a strong dependence of entanglement distribution time on beam divergence and memory efficiency. Imperfect beams significantly reduce the speed of the QND protocols. Highly efficient memories are crucial for practical implementation. In MA-QKD schemes, the uplink configuration offers high operating rates but suffers from significant atmospheric losses. The downlink configuration, while inherently slower due to the speed-of-light limitation, can achieve comparable key rates with temporally multimode QMs and multiple memory pairs. The authors provide detailed performance maps showing the impact of memory parameters on key rates. The uplink MA-QKD protocol achieves a key rate speedup over the no-memory protocol but has a limited range. The downlink MA-QKD protocol requires longer storage times but benefits from temporal multiplexing and multiple memory pairs. The optimal choice depends on the specific application and technical constraints.

The findings address the central challenge of achieving global-scale quantum communication by demonstrating the significant advantages of integrating QMs into space-based quantum communication systems. The results highlight the feasibility of a global untrusted quantum network using near-term technologies. The superior performance of the space-based QND protocol, compared to hybrid systems, is attributed to reduced atmospheric losses and improved robustness against weather conditions. The impact of memory characteristics is significant, emphasizing the need for high efficiency, long lifetimes, and multimode capabilities. The MA-QKD results offer insights into the trade-offs between uplink and downlink configurations. The work addresses some limitations of existing satellite QKD protocols and paves the way for practical global quantum networks. The analysis provides crucial benchmarks for assessing the performance of various tasks and guides the selection of appropriate QM platforms. The research is relevant to diverse applications including global quantum communication, navigation, positioning, and sensing.

This paper provides a theoretical analysis demonstrating the significant advantages of using space-borne quantum memories for global quantum networking. The proposed fully space-based QND protocol significantly outperforms existing protocols for entanglement distribution and MA-QKD. The authors identify key performance requirements for quantum memories and suggest potential platforms. Future research directions include exploring the effects of orbital dynamics, constellation designs, and the impact of finite block sizes on key rates. Optimizing satellite network topologies for efficient entanglement distribution and addressing practical implementation challenges are also essential for realizing a global untrusted quantum network.

The study primarily focuses on theoretical analysis, and experimental verification is needed. The models used incorporate certain simplifying assumptions, such as ideal Gaussian beams and perfect detectors, which might not fully reflect real-world conditions. Further investigation into the impact of orbital dynamics, constellation designs, and finite block sizes on performance is warranted. The availability of highly efficient and multimode quantum memories with long coherence times is crucial for practical implementation.