We analyze the theoretically optimal operation of an optical key distribution (OKD) link based on fine intensity modulation of an optical signal transmitted over an attenuating channel to a direct detection receiver. With suitable digital postprocessing, the users may generate a secret key that will be unknown to an unauthorized party collecting passively a fraction of the signal that escapes detection by the legitimate recipient. The security is ensured by the presence of the shot noise that inevitably accompanies an eavesdropper’s attempt to detect the collected signal. It is shown that the key amount depends on a ratio that compares the legitimate recipient’s and eavesdropper’s capabilities to detect the signal, including noise contributed by their respective detectors. A simple proportionality relation is derived in the strong eavesdropping regime, and closed expressions for the optimal depth of binary intensity modulation as well as the discrimination thresholds for hard-decoded direct detection are given. The presented results substantially simplify the design of practical OKD systems operating under changing external conditions, e.g. variable atmospheric absorption in the case of free-space optical links.
We analyze the theoretically optimal operation of an optical key distribution (OKD) link based on fine intensity modulation of an optical signal transmitted over an attenuating channel to a direct detection receiver. With suitable digital postprocessing, the users may generate a secret key that will be unknown to an unauthorized party collecting passively a fraction of the signal that escapes detection by the legitimate recipient. The security is ensured by the presence of the shot noise that inevitably accompanies an eavesdropper’s attempt to detect the collected signal. It is shown that the key amount depends on a ratio that compares the legitimate recipient’s and eavesdropper’s capabilities to detect the signal, including noise contributed by their respective detectors. A simple proportionality relation is derived in the strong eavesdropping regime, and closed expressions for the optimal depth of binary intensity modulation as well as the discrimination thresholds for hard-decoded direct detection are given. The presented results substantially simplify the design of practical OKD systems operating under changing external conditions, e.g. variable atmospheric absorption in the case of free-space optical links.
Intensity modulation/direct detection (IM/DD) optical key distribution (OKD) is a method to generate a secret key whose security against passive eavesdropping is guaranteed by the shot noise inherent to the photodetection process. Here the effects of intensity-dependent symbol shape distortion on the IM/DD OKD security are investigated assuming that the eavesdropper can implement temporal mode demultiplexing using e.g. the quantum pulse gating technique. The quantitative analysis includes key generation based on either hard- or soft-decoding of the detected signal as well as the impact of excess detection noise. A simple, rule-of-thumb relation between the severity of the symbol shape distortion and the signal strength required to ensure key security is presented.
Optical key distribution (OKD) protects the physical layer of communication links by taking advantage of the inherent noise present in the photodetection process. It allows for efficient generation of a shared random key between two distant users that is secure against passive eavesdropping and can be subsequently used for cryptographic purposes. Moreover, it can be straightforwardly implemented over standard intensity modulation and direct detection links, making it an attractive alternative to quantum key distribution. Here we present a comprehensive security analysis against more powerful eavesdroppers possessing the ability to perform either coherent detection, or even quantum-optimal measurements on the intercepted transmission.
We report the experimental distribution of a cryptographic key based on binary intensity-modulation/direct-detection (IM/DD) transmission of an optical signal. The implemented secret key agreement protocol ensures security against passive wiretap eavesdropping, notably in the regime when the eavesdropper has an advantage in the intercepted signal strength over the legitimate recipient. Operating at a 1 GBaud symbol rate, the experimental fiber optic testbed yielded secret key rates between 43.5 Mbps at 61.1 km and 640 kbps at 81.6 km of the effective link length, with a 9% wiretap installed at the transmitter. The protocol performance was analyzed using a theoretical model incorporating transmitter noise and non-ideal key reconciliation efficiency. The presented results validate IM/DD optical key distribution as an efficient low-complexity solution for protecting optical communication systems against the predominant physical-layer threat of wiretap eavesdropping.
In this study, we analyze the secret key capacity of intensity modulation/direct detection optical key distribution (IM/DD OKD) for a free-space optical (FSO) link between a low-Earth orbit satellite and an optical ground station. Focusing on downlink communication, we account for atmospheric turbulence, which causes random variations in the transmittance of the FSO channel. We implement an atmospheric channel model that accounts for absorption and scattering, geometric losses, pointing errors, and intensity fluctuations. The secret key capacity is quantified under different noise scenarios and reconciliation code efficiencies, assuming a hard decoding scheme. The performance of the IM/DD OKD protocol is compared under direct and reverse reconciliation regimes. Additionally, we examine the impact of weak and strong wind on the strength of atmospheric turbulence, leading to different results of the secret key capacity. Furthermore, we analyze the characteristics of error distributions that arise from protocol optimization. Our results provide insights into optimizing IM/DD OKD protocols for varying atmospheric conditions.
This paper proposes a novel two-way free-space quantum key distribution (QKD) protocol, which can be implemented using standard free-space optical (FSO) systems with subcarrier intensity-modulation (SIM) binary phase-shift-keying (BPSK) and direct-detection (DD) receiver. Different eavesdropping threats, including unauthorized receiver, beam-splitting, and intercept-resend attacks, are considered in the security analysis of the proposed protocol. Under the constraints on security requirements, we analytically investigate the design criteria for transmitter and receiver, in particular, the intensity modulation depth at the transmitter and the dual-threshold setting of the receiver in two-way free-space QKD systems. Quantum bit error rate (QBER), the ergodic secret-key rate, and the probability of detecting eavesdropper of the proposed system are analytically derived in closed-form expressions, considering the atmospheric loss, turbulence, and receiver noises.
Quantum key distribution (QKD) enables secret-key establishment through measurements of quantum states, but practical performance depends strongly on the optical receiver, modulation format, decision process, and reconciliation strategy. This thesis develops three receiver-oriented approaches for improving the information advantage of legitimate users under explicitly stated security models. First, a dual-branch photon-number-resolving (DB-PNR) receiver is developed for coherent-state hybrid QKD. Complementary displacement branches and four photon-count outputs reduce the sign ambiguity of single-branch reception and recover information associated with both optical quadratures. Under the adopted pure-loss models, DB-PNR reception improves mutual information and secret-key-generation rate relative to homodyne, heterodyne, and single-branch PNR references. Second, a four-level multi-threshold intensity-modulation and direct-detection (MT-IM–DD) protocol is formulated. Optical intensity levels, five decision thresholds, and two erasure regions are jointly optimized under Gaussian classical, quantum, and thermal receiver-noise terms. The analysis separates conditional key rate from acceptance-weighted secret throughput and evaluates forward and reverse reconciliation under beam-splitting and intercept–resend attacks. Third, binary IM–DD is operated directly in the low-photon-number quantum regime using photon-number-resolving direct detection. Integer Poisson observations are processed with a likelihood-symmetric dual-threshold rule. The closed-form coupling between the two boundaries reduces the threshold optimization from two independent variables to one while preserving most of the benefit of free two-threshold optimization. Across all three studies, improved receiver measurements and deliberate post-selection increase performance under the stated assumptions. The results are comparative asymptotic bounds, not universal composable-security proofs. Finite-key effects, detector nonidealities, soft-information eavesdropping, general quantum attacks, and experimental validation remain necessary extensions.