Quantum Key Distribution (QKD) is a cryptographic solution that leverages the properties of quantum mechanics to be resistant and secure even against an attacker with unlimited computational power. Satellite-based links are important in QKD because they can reach distances that the best fiber systems cannot. However, links between satellites in low Earth orbit and ground stations have a duration of only a few minutes, resulting in the generation of a small amount of secure keys. In this context, we investigate the optimization of the information reconciliation step of the QKD postprocessing in order to generate as much secure key as possible. As a first step, we build an accurate model of the downlink signal and Quantum Bit Error Rate (QBER) during a complete satellite pass, which are time-varying due to three effects: 1 the varying link geometry over time; 2) the scintillation effect; and 3) the different signal intensities adopted in the Decoy-State protocol. Leveraging the a priori information on the instantaneous QBER, we improve the efficiency of information reconciliation (i.e., the error correction phase) in the Decoy-State BB84 protocol, resulting in a secure key that is almost 3% longer for realistic scenarios, with no computational or hardware complexity overhead.
Quantum Key Distribution (QKD) is a cryptographic solution that leverages the properties of quantum mechanics to be resistant and secure even against an attacker with unlimited computational power. Satellite-based links are important in QKD because they can reach distances that the best fiber systems cannot. However, links between satellites in low Earth orbit and ground stations have a duration of only a few minutes, resulting in the generation of a small amount of secure keys. In this context, we investigate the optimization of the information reconciliation step of the QKD postprocessing in order to generate as much secure key as possible. As a first step, we build an accurate model of the downlink signal and Quantum Bit Error Rate (QBER) during a complete satellite pass, which are time-varying due to three effects: 1 the varying link geometry over time; 2) the scintillation effect; and 3) the different signal intensities adopted in the Decoy-State protocol. Leveraging the a priori information on the instantaneous QBER, we improve the efficiency of information reconciliation (i.e., the error correction phase) in the Decoy-State BB84 protocol, resulting in a secure key that is almost 3% longer for realistic scenarios, with no computational or hardware complexity overhead.
Quantum key distribution (QKD) is a cryptographic solution that leverages the properties of quantum mechanics to be resistant and secure even against an attacker with unlimited computational power. Satellite-based links are important in QKD because they can reach distances that the best fiber systems cannot. However, links between satellites in low Earth orbit (LEO) and ground stations have a duration of only a few minutes, resulting in the generation of a small amount of secure keys. In this context, we investigate the optimization of the information reconciliation step of the QKD post-processing in order to generate as much secure key as possible. As a first step, we build an accurate model of the downlink signal and quantum bit error rate (QBER) during a complete satellite pass, which are time-varying due to three effects: (i) the varying link geometry over time, (ii) the scintillation effect, and (iii) the different signal intensities adopted in the Decoy-State protocol. Leveraging the a-priori information on the instantaneous QBER, we improve the efficiency of information reconciliation (IR) (i.e., the error correction phase) in the Decoy-State BB84 protocol, resulting in a secure key that is almost 3\% longer for realistic scenarios.
In satellite-based quantum-key-distribution (QKD), the number of secret bits that can be generated in a single satellite pass over the ground station is severely restricted by the pass duration and the free-space optical channel loss. High channel loss may decrease the signal-to-noise ratio due to background noise, reduce the number of generated raw key bits, and increase the quantum bit error rate (QBER), all of which have detrimental effects on the output secret key length. Under finite-size security analysis, a higher QBER increases the minimum raw key length necessary for nonzero secret-key-length extraction due to less efficient reconciliation and postprocessing overheads. We show that recent developments in finite-key analysis allow three different small-satellite-based QKD projects, CQT-Sat, the United Kingdom QUARC-ROKS, and QEYSSat, to produce secret keys even under conditions of very high loss, improving on estimates based on previous finite-key bounds. This suggests that satellites in low Earth orbit can satisfy finite-size security requirements but that this remains challenging for satellites further from Earth. We analyze the performance of each mission to provide an informed route toward improving the performance of small-satellite QKD missions. We highlight the short- and long-term perspectives on the challenges and potential future developments in small-satellite-based QKD and quantum networks. In particular, we discuss some of the experimental and theoretical bottlenecks and the improvements necessary to achieve QKD and wider quantum networking capabilities in daylight and at different altitudes. Published by the American Physical Society 2024
In this paper we analyze the feasibility of performing Quantum Key Distribution (QKD), in Earth-satellite up and downlinks and in intersatellite links, with two quantum cryptography protocols: BB84 and SARG04, and with two implementation options: with and without decoy states. As real measurements in these scenarios are not possible yet, the objective is to obtain results as realistic as possible to support the design of future satellite missions performing QKD. Therefore, we use realistic values for the optical hardware and take into account usual atmospheric conditions. In the same line, we assume specific types of attacks, namely the photon number splitting and the intercept-resend with unambiguous discrimination attacks, which could likely be the main threat to the first satellite-based QKD applications. A lower bound on the key generation rate of SARG04 with two decoy states is presented. The optimum signal- and decoy-states mean photon numbers for each protocol and each distance are also computed. The resulting values for the signal-state are larger than those often employed. We show that it may be possible to establish QKD with LEO (Low Earth Orbit) and, under certain circumstances, with MEO (Medium Earth Orbit) satellites, but not with GEO (Geostationary) ones. Furthermore, we obtain that the optimum signal-state mean photon number for SARG04 with two decoy states is almost independent of the link distance, which greatly facilitates its use in a real scenario.
We have analyzed the feasibility of performing Quantum Key Distribution (QKD), in earth-satellite and intersatellite links, with four quantum cryptography protocols: BB84 and SARG04 with and without decoy states. In order to facilitate the protocols comparison we have computed the rates using the optimal mean photon number for each protocol and for each distance. Furthermore, we have formulate a lower bound on the key generation rate of SARG04 with a finite number of decoy states, and we have obtained all numerical results using realistic values for optical hardware and accounting for realistic atmospheric effects. The results of the analysis indicate that the maximum rate and transmit distance are obtained when the proposed method is applied, and it is possible to establish QKD with LEO (Low Earth Orbit) and MEO (Medium Earth Orbit) satellites.
Satellite-based free-space optics (FSO) quantum key distribution (QKD) systems have recently attracted significant research interest due to their potential to enable globally secured applications. However, the inherent uncertainty of FSO channels, caused by weather conditions and satellite mobility, induces severe fluctuations in quantum bit-error rate (QBER) between legitimate users. This makes designing an efficient key reconciliation, an essential step in the QKD post-processing, particularly challenging. In this work, we propose a key reconciliation scheme that combines protograph rate-compatible (RC) low-density parity-check (LDPC) codes with a syndrome-based error estimation method. The proposed error estimation method reduces the number of communication rounds without requiring additional information disclosure. Furthermore, to our best knowledge, an analytical framework is first developed to evaluate end-to-end secret-key throughput (SKT), accounting for the impact of imperfect error estimation and dynamic FSO channel conditions. Numerical results demonstrate that the proposed scheme consistently outperforms conventional blind reconciliation under diverse FSO channel conditions and provide practical guidelines for system parameter selection. Finally, we validate the proposed framework through a case study that incorporates data from a Starlink low-Earth orbit (LEO) satellite and moving ground vehicles.
Abstract Information reconciliation (IR) in quantum key distribution (QKD) is usually treated as leaking the total amount of publicly disclosed reconciliation information. In weak-coherent-pulse decoy-state QKD, this can be overly conservative under the Gottesman–Lo–Lütkenhaus–Preskill (GLLP) tagging framework, because disclosures determined by tagged multi-photon bits that are already assumed known to an eavesdropper reveal no additional information and therefore need not be counted again as IR leakage. Here, we experimentally demonstrate a tighter IR-leakage estimation model that excludes this redundant contribution and bounds the additional information the eavesdropper acquires during reconciliation. The model is incorporated into asymptotic and composable finite-key analyses of decoy-state BB84 while leaving the constant privacy-amplification contribution and the underlying GLLP security assumptions unchanged. We experimentally evaluate the model using 2.5 GHz silicon-photonic chips with polarization-encoded decoy-state BB84 protocol under controlled channel loss and over a 55.41 km deployed metropolitan fiber link. The experimental results show a secure key rate increment of up to 78.82%, while the corresponding finite-key analysis indicates ~ 2.35 dB of additional tolerable system loss. These results experimentally demonstrate that tighter reconciliation-leakage accounting can improve the secure key rate performance of practical chip-based QKD systems without modifying the quantum hardware or the public communication required for reconciliation. Consequently, this methodology holds significant potential across diverse real-world quantum-secure communication scenarios, including high-demand metropolitan and intercity networks, satellite-to-ground QKD links, and next-generation quantum internet nodes.