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) 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.
We put forward a hybrid quantum key distribution protocol based on coherent states, Gaussian modulation, and photon-number-resolving (PNR) detectors, and show that it may enhance the secret key generation rate (KGR) compared to homodyne-based schemes. Improvement in the KGR may be traced back to the dependence of the two-dimensional discrete output variable on both the input quadratures, thus overcoming the limitations of the original protocol. When reverse reconciliation is considered, the scheme based on PNR detectors outperforms the homodyne one both for individual and collective attacks. In the presence of direct reconciliation, the PNR strategy is still the best one against individual attacks, but for the collective ones the homodyne-based scheme is still to be preferred as the channel transmissivity decreases.
What we believe to be a new architecture based on dual-branch photon-number-resolving (DB-PNR) detectors for the receiver in the hybrid quantum key distribution (QKD) is introduced. DB-PNR scheme empowers the receiver to detect both quadratures of the coherent state simultaneously. Our results show that using DB-PNR detectors, the key generation rate (KGR) is almost doubled as compared to homodyne detection which detects only one quadrature at a time. The mutual information is enhanced by around 0.85 bits/symbol using DB-PNR as compared to heterodyne detection. DB-PNR is also found to outperform the heterodyne detection against Eve's individual and collective attacks. PNR-based detection schemes require less local oscillator (LO) power compared to homodyne/heterodyne detection schemes and thereby, the power efficiency of the receiver is enhanced.
This paper proposes a novel design and analyzes security performance of quantum key distribution (QKD) protocol over free-space optics (FSO). Unlike conventional QKD protocols based on physical characteristics of quantum mechanics, the proposed QKD protocol can be implemented on standard FSO systems using subcarrier intensity modulation binary phase shift keying and direct detection with a dual-threshold receiver. Under security constraints, the design criteria for FSO transmitter and receiver, in particular, the modulation depth and the selection of dual-threshold detection, respectively, is analytically investigated. For the security analysis, quantum bit error rate, ergodic secret-key rate, and final key-creation rate are concisely derived in novel closed-form expressions in terms of finite power series, taking into account the channel loss, atmospheric turbulence-induced fading, and receiver noises. Furthermore, Monte-Carlo simulations are performed to verify analytical results and the feasibility of the proposed QKD protocol.
The focus of this research activity is to work on pragmatic information reconciliation applied to QKD schemes based on single photon or weak pulse laser (WPL) sources, so as to use feed-forward techniques which minimize the interaction between transmitter and receiver. The core ideas of the thesis are employing Forward Error Correction (FEC) coding as opposed to two-way communication for information reconciliation in QKD schemes, exploiting all the available information for data processing at the receiver including information available from the quantum channel, since optimized use of this information can lead to significant performance improvement, and providing a security versus secret-key rate trade-off to the end-user within the context of QKD systems. Moreover, as shown by accurate experimental studies, the communication channel used for quantum key exchange is not able to reach high levels of reliability (the Quantum Bit Error Rate -QBER may have a high value), both because of the inherent characteristics of the system, and of the presence of a possible attacker. In order to obtain acceptable residual error rates, it is necessary to use a parallel classical and public channel, characterized by high transmission rates and low error rates, on which to transmit only the redundancy bits of systematic channel codes with performance possibly close to the capacity limit. Furthermore, since the more redundancy is added by the channel code, the more the corresponding information can be used to decipher the private message itself, it becomes necessary to design high-rate codes obtained by puncturing a low-rate mother code, possibly achieving a redundancy such that elements of the secret message cannot be uniquely determined from the redundancy itself, so for that purpose we designed high rate LDPC codes. Using high rate codes increases the security with trade-off to performance. Other low photon number applications have also been considered, such as weak-laser pulses (WLP) communication. For that purpose, a low-complexity photon-counting receiver has been considered which may be employed in long-distance amplification-free classical optical communication schemes, and which is typically modeled as an equivalent Binary Symmetric Channel (BSC). We have developed a time varying Binary Input-Multiple Output (BIMO) channel model for this low-complexity photon-counting receiver, and analyzed its performance in presence of soft-metric based capacity approaching iteratively decoded error correcting codes, such as soft-metric based Low Density Parity Check (LDPC) codes and polar codes. We show that the classical channel capacity of the suggested BIMO model is higher than the capacity of the BSC model, and that the use of the BIMO model allows to feed the channel decoder with soft information, in the form of Log-Likelihood Ratios (LLRs), achieving a significant reduction in Bit Error Rate (BER) and Frame Error Rate (FER) with respect to classical hard-metric-based schemes which should be used in conjunction with a BSC channel model.
Quantum Key Distribution (QKD) provides a method for securely sharing a secret key between two distant parties, Alice and Bob, by leveraging the principles of quantum mechanics. This thesis focuses on Continuous-Variable (CV) QKD, a variant of QKD where information is encoded in the quadrature components of coherent light states, allowing for efficient and cost-effective detection using homodyne or heterodyne receivers. The thesis is divided into three main parts: an introduction to quantum cryptography concepts, the presentation of a CV-QKD protocol, and a detailed security proof of the protocol against different types of attacks. The security proof considers both finite and infinite symbol transmission and tackles various eavesdropping attacks, including individual, collective, and coherent attacks. The use of Gaussian states and operations provides a promising approach to enhance QKD security, particularly in the context of practical quantum cryptography. The final sections provide a mathematical framework for the optimization of secure key rates and strategies for addressing the photon-number cutoff assumption in practical implementations.
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.