ABSTRACT This thesis explores the photon-number characteristics and quantum communication po tential of superposed squeezed vacuum states, emphasizing their relevance to Quantum Key Distribution (QKD). The analysis investigates how statistical indicators of the light field—such as the average photon number and its fluctuations—reveal nonclassical be havior, including reduced quadrature noise and altered photon-number distributions that support efficient quantum encoding. Secure communication performance is evaluated through conditional probability methods and mutual information calculations, while Q function phase-space representations illustrate interference patterns and deformation in amplitude distributions arising from squeezing and photon correlations. A Gaussian se curity model is applied to estimate the Holevo bound and quantify possible information gained by an eavesdropper. MATLAB simulations demonstrate that tuning the squeez ing strength and superposition parameters enhances the achievable key rate, showing that superposed squeezed states exhibit increased resilience to channel loss and common quantum attacks, making them strong candidates for secure, low-photon, long-distance QKDapplications. Keywords:-Photon number variation,Q-function analysis, Quadrature squeezing, Quantum Key Distribution (QKD),Secret key rate analysis,Superposed squeezed vac uum states i
ABSTRACT This thesis explores the photon-number characteristics and quantum communication po tential of superposed squeezed vacuum states, emphasizing their relevance to Quantum Key Distribution (QKD). The analysis investigates how statistical indicators of the light field—such as the average photon number and its fluctuations—reveal nonclassical be havior, including reduced quadrature noise and altered photon-number distributions that support efficient quantum encoding. Secure communication performance is evaluated through conditional probability methods and mutual information calculations, while Q function phase-space representations illustrate interference patterns and deformation in amplitude distributions arising from squeezing and photon correlations. A Gaussian se curity model is applied to estimate the Holevo bound and quantify possible information gained by an eavesdropper. MATLAB simulations demonstrate that tuning the squeez ing strength and superposition parameters enhances the achievable key rate, showing that superposed squeezed states exhibit increased resilience to channel loss and common quantum attacks, making them strong candidates for secure, low-photon, long-distance QKDapplications. Keywords:-Photon number variation,Q-function analysis, Quadrature squeezing, Quantum Key Distribution (QKD),Secret key rate analysis,Superposed squeezed vac uum states i
ABSTRACT This thesis explores the photon-number characteristics and quantum communication po tential of superposed squeezed vacuum states, emphasizing their relevance to Quantum Key Distribution (QKD). The analysis investigates how statistical indicators of the light field—such as the average photon number and its fluctuations—reveal nonclassical be havior, including reduced quadrature noise and altered photon-number distributions that support efficient quantum encoding. Secure communication performance is evaluated through conditional probability methods and mutual information calculations, while Q function phase-space representations illustrate interference patterns and deformation in amplitude distributions arising from squeezing and photon correlations. A Gaussian se curity model is applied to estimate the Holevo bound and quantify possible information gained by an eavesdropper. MATLAB simulations demonstrate that tuning the squeez ing strength and superposition parameters enhances the achievable key rate, showing that superposed squeezed states exhibit increased resilience to channel loss and common quantum attacks, making them strong candidates for secure, low-photon, long-distance QKDapplications. Keywords:-Photon number variation,Q-function analysis, Quadrature squeezing, Quantum Key Distribution (QKD),Secret key rate analysis,Superposed squeezed vac uum states i
Abstract Recently, photon subtraction from the single-mode squeezed vacuum state (SVS) has been explored as a technique to enhance its squeezing strength (SS). Notably, the enhancement is observed only when an even number of photons is subtracted, and this effect is limited to a specific range of the squeezing parameter r. In this paper, we introduce a novel class of single-mode nonclassical states, termed as qubit superposed SVSs (qSSVSs), via the coherent superposition of a SVS and a single-photon-subtracted SVS. The primary objective of this work is to enhance the SS for any r > 0 through the creation of these superposed states. Our analysis reveals that the qSSVSs not only improve the SS but also exhibit enhanced nonclassical features such as the negativity of Wigner function, single-mode antibunching effect, photon-number distribution, and sub-Poissonian photon statistics. Furthermore, we demonstrate that the qSSVSs provide superior phase sensitivity in a quantum Mach–Zehnder interferometer compared to the original SVS. Importantly, the generation of qSSVSs is feasible using readily available optical devices, including a beam splitter, a photodetector, and an amplitude displacement. These states hold significant potential for practical applications in quantum information science, particularly for tasks such as information encoding and processing in quantum computing systems.
In the last ten years, quantum computing has made substantial progress, presenting a strong challenge to traditional encryption methods that depend on solving discrete algorithmic problems. With the advent of the quantum era, conventional encryption methods are becoming more susceptible to attacks. This paper investigates possible attacks that exploit vulnerabilities in quantum cryptography, specifically investigating weaknesses in quantum key distribution (QKD) systems. The text thoroughly explores advanced attack strategies of photon number splitting highlight the existing vulnerabilities. The goal is to highlight the need for strong safeguards to protect against these threats, which requires continuous research and development to improve the robustness of quantum cryptography algorithms. The experimental investigation includes analyzing crucial parameters, including quantum channel length, loss, QKD protocol, quantum bit error rate (QBER), signal-to-noise ratio (SNR), as well as attack parameters such as probability of photon number splitting (PNS), eavesdropper distance, and detection efficiency. The evaluation includes performance indicators such as secure key rate, quantum bit error rate (QBER) vs distance, vulnerability to photon number splitting (PNS) attacks, and the influence on the pace of key generation.
Quantum key distribution (QKD) protocols allow two trusted parties to distribute a cryptographic key which they can further use for the unconditionally secure classical communication. During the last decades the field has grown mature with the commercial prototypes being available. They mostly rely on the faint laser pulses implementing the protocols on the basis of single qubits and qubit pairs. Alternatively, the continuous-variable (CV) protocols based on the multi-photon states of light were developed recently. They are typically built using the quadrature modulation of the coherent or squeezed states and subsequent homodyne detection. In the case of Gaussian modulation the security of the protocols is based on the extremality of Gaussian states, which allows estimating bounds on the leaked information. The Gaussian CV QKD protocols were shown secure for any degree of channel attenuation, but are restricted by the channel noise. Moreover, the applicability of Gaussian CV QKD protocols is limited by the effectiveness of classical post-processing algorithms, used by the trusted parties to process the measured data and distill the cryptographic key. On the other hand, the squeezedstate CV QKD protocols were previously considered ineffective under limited degrees of squeezing, while strong squeezing remains experimentally challenging. In the present work we distinguish between the classical and quantum resources in the Gaussian CV QKD and address the role of nonclassicality in the Gaussian protocols. We demonstrate, that by properly combining squeezed resource and coherent modulation, trusted parties are able to decouple eavesdropper from the channel, thus being able to establish the secure key from any amount of the classical mutual information. Our result shows a very promising path towards the long-distance CV QKD.
Quantum communications enable secure key transmission and entanglement distribution, but their rates and maximum transmission distances are limited due to the losses and noise introduced by quantum channels. Various techniques have been developed to address these limitations, tailored to the platform and the nature of the decoherence. In this thesis, we explore ways to enhance the performance of quantum communications, with a particular focus on increasing the key rates and transmission distances in quantum key distribution (QKD), a method that leverages quantum mechanics to securely share keys between trusted parties. Additionally, we aim to improve the purity of entangled states shared between two parties, contributing to broader advancements in quantum communications. We begin by introducing a software-enhanced continuous-variable QKD (CV-QKD) protocol, where keys are encoded onto the phase and amplitude degrees of the optical field. This protocol enhances both transmission distance and key rates through a post-selection method employing arbitrary filters, experimentally demonstrated on two different platforms. Its adaptability makes it highly promising, as it can be implemented on existing CV-QKD platforms and dynamically optimises key rates for rapidly changing channels. We then present a protocol for the pure-loss channel leveraging quantum repeaters to mitigate the channel losses by segmenting the total communication distance into smaller, more manageable intervals. This repeater-like protocol introduces a more efficient entanglement-swapping measurement at the repeater station than the state-of-the-art twin-field protocols. Instead of polarisation encoding, our protocol relies on encoding keys in the photon-number basis, optimising the states to achieve the highest key rates for each distance. Notably, this protocol shows strong potential for improving long-distance quantum communication by surpassing the capacity of the direct communication channel at a shorter distance than the existing repeater protocols such as twin-field QKD while achieving an overall higher transmission distance. Finally, in this thesis, we demonstrate that for the Pauli-dephasing channel, placing a node in the middle of the quantum channel to perform a Bell-state measurement as an entanglement-swapping operation does not result in improved key rates, even with reduced decoherence. As a result, we propose an entanglement purification protocol that refines a larger set of noisy entangled states into a smaller set of high-fidelity ones using local operations and classical communication. This purification protocol builds upon earlier purification methods, extending their application from two Bell pairs to any number of Bell pairs and thereby achieving the capacity of the dephasing channel. Through numerous recursive iterations, it produces near-perfect Bell pairs, making it well-suited for quantum repeaters and correcting dephasing errors in quantum computers.
Abstract Quantum secure direct communication (QSDC) promises unconditional security for direct information transmission without pre-shared keys. However, practical implementations using weak coherent sources face critical challenges from device imperfections, such as emitting vacuum states and multiphoton states. Here, we introduce a heralded QSDC protocol leveraging squeezed-state superposition in a cross-Kerr nonlinear medium, where single-mode squeezed states interfere with coherent states to generate superposition states via post-measurement selection. We have considered three detector models to analyze the heralding performance of light sources under different conditions, including the ideal photon number detector, the practical photon number detector, and the practical photon detector. Our proposed protocol dramatically reduces the vacuum state ratio compared to weak coherent pulse sources, and the multiphoton ratio is still maintained at a low level, significantly enhancing the security information capacity and the maximum secure communication distance. We perform numerical simulations and compare our protocol with the optical single-photon-based QSDC protocol. At 7.5 km with an average photon number of 0.01, the security information capacity improves by 13.42 times, 12.20 times, and 8.64 times for three detector models. Extending to 17.5 km, these gains will be to 277.59 times, 251.58 times, and 172.91 times. Our work provides a robust framework for high-capacity, long-distance, secure quantum communication systems. PACS numbers: 03.67.Pp, 03.67.Hk, 03.65.Ud