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
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.
Squeezed states of light have found importance in quantum cryptography due to the no-cloning theorem which prevents two states from being identical to each other. The quantum state with quadrature operators X1 and X2 can be visualized as a point in phase space with the center being l angle X1 r angle, l angle X2 r angle surrounded by an error region which satisfies the minimum uncertainty product l angle Delta X [1 over 2] r angle l angle Delta X [2 over2] r angle=1/16. These states are intrinsically secure since one needs to know which quadrature the measurement is to be made and any attempt to measure the wrong quadratures with arbitrary accuracy would disturb the message. Of course, the eavesdropper cannot simultaneously measure both quadratures with infinite precision for each. This thesis describes a method that not only encodes information in the amplitudes of the quadratures alone but also in the uncertainty of those states. One example of squeezed light is the number-phase squeezed state which satisfying the uncertainity relation l angle Delta n [superscript] 2 r angle l angle Delta phi [superscript] 2 r angle=1/4. An implementation is demonstrated where the information is encoded only in the photon number uncertainity and the phase variable is ignored.The barrier regulation mechanisms such as macroscopic coulomb blockade in semiconductor junction diodes are responsible for generating photon fluxes with penetration below the standard quantum limit(shot noise level). The thesis describes a comprehensive quantum mechanical Langevin model which details the various mechanisms responsible for producing photon number squeezing from the thermionic emission to the diffusion current limits. Quantities such as the pump fluctuations and cross correlation spectral densities are studied under constant current and constant voltage conditions. The research investigates the generation of photon number squeezed light from high efficiency light emitting diodes. A measurement setup for subshot noise is constructed and each stage is properly calibrated. Experiments were performed to determine the squeezing spectra and Fanofactors for the L2656 and the L9337 high efficiency LEDs. The L9337 produces a squeezing of 1.5dB below the shot noise level over a bandwidth of 25Mhz, the largest known penetration at room temperature. The quantum stochastic communicator is also demonstrated. The research shows that the switching elements used in the modulation of the electrical bias which in turn affect the regulation mechanisms do not affect the statistics of the emitted light under certain conditions. The decoding of the time varying variances is achieved by using time frequency analysis with the aid of the spectrum analyzer.
Abstract We address the crucial role of noisy squeezing in security and performance of continuous-variable (CV) quantum key distribution (QKD) protocols. Squeezing has long been recognized for its numerous advantages in CV QKD, such as enhanced robustness against channel noise and loss, and improved secret key rates. However, the excess noise of the squeezed states, that unavoidably originates already from optical loss as well as other imperfections in the source, raises concerns about its potential exploitation by an eavesdropper. For the widely adopted trust assumption on the excess noise in the signal states, we confirm the stability of the protocol against the noisy squeezing in both purely attenuating as well as noisy channels in the asymptotic limit, which implies perfect parameter estimation. In the finite-size regime we show that this stability largely holds at up to 107 data points using optimal biased homodyne detection for key distribution and parameter estimation. Untrusted assumption on the noisy squeezing, on the other hand, introduces additional security bounds on the squeezing excess noise already in the asymptotic regime, which is further enforced by the finite-size effects. Additionally, we show the critical negative role of noisy squeezing in the case of atmospheric free-space channels, already in the trusted-noise and asymptotic assumptions, which emphasizes the importance of squeezing purity in the free-space quantum channels. Our results pave the way towards practical realization of squeezed-state CV QKD protocols in both fibre and free-space channels.