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.
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.
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.
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 propose and analyze numerically concurrent intensity modulation/direct detection (IM/DD) optical key distribution (OKD) combined with conventional data transmission over a LEO-to-ground optical link. The idea is validated by numerical simulations which indicate that in a realistic scenario it is possible to generate simultaneously up to 200 Mbits of a secret key and downlink transmit up to 140 Gbits of data during a single nearly-zenithal pass of a LEO satellite. Such lengths of the secret key are three orders of magnitude higher in comparison to typical quantum key distribution systems, although they are generated under different security assumptions. The simplicity and robustness of IM/DD OKD protocols could make them an attractive and cost-effective option to provide security in the physical layer of space communication systems.
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.
Quantum key distribution (QKD) has been studied for achieving perfectly secure cryptography based on quantum mechanics. This paper presents a novel QKD scheme that is based on an intensity-modulation and direct-detection system. Two slightly intensity-modulated pulses are sent from a transmitter, and a receiver determines key bits from the directly detected intensity. We analyzed the system performance for two typical eavesdropping methods, a beam splitting attack and an intercept-resend attack, with an assumption that the transmitting and receiving devices are fully trusted. Our brief analysis showed that short- or middle-range QKD systems are achievable with a simple setup.
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.