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.
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.
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.
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.
Abstract Previous research has aimed to precisely estimate information leakage to improve the secure key rate (SKR) and maximum transmission distance in quantum key distribution (QKD). However, existing methods repeatedly considered the information of the multi-photon pulses known to Eve before and after information reconciliation, resulting in an overestimation of the leakage amount. We propose a novel approach that considers the quantum part's effect on post-processing, providing a more accurate estimation of information reconciliation leakage to improve the SKR. Theoretical analysis shows that our method more accurately estimates the information reconciliation leakage, significantly improving the SKR at any distance as well as the maximum transmission distance. It is worth mentioning that previous studies treat information leakage of the error correction as the Shannon bound Nh(e), and our method can estimate it more tightly. Simulation results for decoy-BB84 and measurement-device-independent (MDI) protocols using Cascade are consistent with the theoretical analysis. The farther the transmission distance, the greater the growth rate of SKR. When the error rate is 33%, compared with the original method, the SKR growth rate of decoy-BB84 at 100KM is 100.4%, and the transmission distance of MDI increases by 22KM.
Quantum Key Distribution (QKD) enables the sharing of cryptographic keys secured by quantum mechanics. The BB84 protocol assumes single-photon sources, but practical systems rely on weak coherent pulses vulnerable to Photon-Number-Splitting (PNS) attacks. The Gottesman-Lo-Lütkenhaus-Preskill (GLLP) framework addresses these imperfections, deriving secure key rate bounds under limited PNS scenarios. The decoy-state protocol further improves performance by refining single-photon yield estimates, but still considers multi-photon states as insecure, thereby limiting intensities and constraining key rate and distance. More recently, finite-key security bounds for decoy-state QKD have been extended to address general attacks, ensuring security against adversaries capable of exploiting arbitrary strategies. In this work, we focus on a specific class of attacks, the generalized PNS attack, and demonstrate that higher pulse intensities can be securely used by employing Bayesian inference to estimate key parameters directly from observed data. By raising the pulse intensity to 10 photons, we achieve a 50-fold increase in key rate and a 62.2% increase in operational range (about 200 km) compared to the decoy-state protocol. Furthermore, we accurately model after-pulsing using a Hidden Markov Model (HMM) and reveal inaccuracies in decoy-state calculations that may produce erroneous key-rate estimates. While this methodology does not address all possible attacks, it provides a new approach to security proofs in QKD by shifting from worst-case assumption analysis to observation-dependent inference, advancing the reach and efficiency of discrete-variable QKD protocols.
The rate at which a secure key can be generated in a quantum key distribution (QKD) protocol is limited by the channel loss and the quantum bit-error rate (QBER). Increases to the QBER can stem from detector noise, channel noise, or the presence of an eavesdropper, Eve. Eve is capable of obtaining information of the unsecure key by performing an attack on the quantum channel or by listening to all discussion performed via a noiseless public channel. Conventionally a QKD protocol will perform the information reconciliation over the authenticated public channel, revealing the parity bits used to correct for any quantum bit errors. In this invited paper, the possibility of limiting the information revealed to Eve during the information reconciliation is considered. Using a covert communication channel for the transmission of the parity bits, secure key rates are possible at much higher QBERs. This is demonstrated through the simulation of a polarization based QKD system implementing the BB84 protocol, showing significant improvement of the SKRs over the conventional QKD protocols.
Quantum Key Distribution (QKD) promises information-theoretically secure communication, but practical implementations face significant security and efficiency challenges. The vulnerability of realistic photon sources to photon-number-splitting (PNS) attacks necessitates advanced protocols like the Decoy-State Assisted BB84 (DA-BB84). Concurrently, the classical post-processing phase, particularly information reconciliation, can become a bottleneck for high-rate systems. This paper presents a comprehensive simulation of an end-to-end DA-BB84 QKD system that addresses both challenges. We model a realistic quantum channel with photon statistics, channel loss, and detector imperfections. Security against PNS attacks is established by using decoy states to accurately estimate the single-photon yield (Y1) and error rate (e1). Furthermore, we introduce and analyze a "Tag-Aware" optimization for the classical error correction routine. This hybrid method uses signal quality metadata to accelerate reconciliation for unambiguous errors while falling back on a robust, multi-pass protocol for complex cases. Our simulation results demonstrate the successful generation of a positive, secure key rate under realistic channel conditions (4.0 dB loss), and validate that the tag-aware reconciliation protocol can successfully correct all errors, thereby balancing provable security with post-processing efficiency.