Every business and household rely on internet in this day and age. We are using electronic devices connected to the Internet. These devices are generating a considerable amount of data, which is usually transmitted using public/insecure communication channels. On the one hand, the technological advancement of universal connectivity brought so much ease for humans’ race in business, shopping, and financial transactions. The rapid pace of this technological advancement also introduced several concerns in terms of the security and secrecy of data. Security researchers developed several encryption algorithms that are in use to ensure the safety and confidentiality of data. The mathematical difficulty of prime factorisation is the fundamental element of modern encryption algorithms, and they require a considerable amount of processing power to reverse engineer (or break) these algorithms. Scientists and government agencies are trying to build quantum computers to solve some complex problems. These problems include prime factorisation of large numbers, a critical factor in the field of cryptography. Quantum computers are much more potent because of their nature. It processes information by using laws of quantum. The successful development of quantum computers will pit the security and secrecy of our data at risk because it is trivial for the quantum computer to break the currently used encryption algorithms. Bearing this in mind, Research have started working on systems that will provide secure communications in the age of quantum computing. Considering the importance of quantum physics-based communication systems, we have some working examples of these systems, which are called quantum key distribution systems (QKD). These system uses quantum physics to transmit quantum states from one party to another. In case of the presence of Eavesdropping, the whole system will be disturbed, letting both parties know the existence of eve. QKD systems have some success and have different protocols, but until now, they have a very long way to go. When these systems are mature enough, they will require to work with current internet infrastructure, which is very costly and brings so much complexity to the network that it will not be feasible to implement. This thesis proposes a Multiple Access QKD Network integrated with Internet infrastructure to addresses these issues of Secure Communication. The system proposed in this thesis takes existing protocols of data communication, QKD, along with hardware architecture of communication devices. A QKD based client and network switch have been designed and developed along with its operating system to enable multi-access communication in the LAN environment. A simulation model of the model proposed in this thesis has been by using OMNet++ simulation framework to test and evaluate the viability of this model. The proposed QKD mechanism will reduce the complexity for network administrators, reduce the cost of implementation for businesses, and ensure the secrecy and security of the data even in the age of quantum computing.

To address key challenges for both quantum communication and quantum computing applications in a simultaneous manner, we propose to employ the photon angular momentum approach by invoking the well-known fact that photons carry both the spin angular momentum (SAM) and the orbital angular momentum (OAM). SAM is associated with polarization, while OAM is associated with azimuthal phase dependence of the complex electric field. Given that OAM eigenstates are mutually orthogonal, in principle, an arbitrary number of bits per single photon can be transmitted. The ability to generate/analyze states with different photon angular momentum, by using either holographic or interferometric methods, allows the realization of quantum states in multidimensional Hilbert space. Because OAM states provide an infinite basis state, while SAM states are 2-D only, the OAM can also be used to increase the security for quantum key distribution (QKD) applications and improve computational power for quantum computing applications. The goal of this paper is to describe photon angular momentum based deterministic universal quantum qudit gates, namely, {generalized-X, generalized-Z, generalized-CNOT} qudit gates, and different quantum modules of importance for various applications, including (fault-tolerant) quantum computing, teleportation, QKD, and quantum error correction. For instance, the basic quantum modules for quantum teleportation applications include the generalized-Bell-state generation module and the QFT-module. The basic quantum module for quantum error correction and fault-tolerant computing is the nonbinary syndrome calculator module. The basic module for entanglement assisted QKD is either the generalized-Bell-state generation module or the Weyl-operator-module. The possibility of implementing all these modules in integrated optics is discussed as well. Finally, we provide security analysis of entanglement assisted multidimensional QKD protocols, employing the proposed qudit modules, by taking into account the imperfect generation of OAM modes.

Yes / Satellite QKD/FSO systems, which facilitate quantum key distribution (QKD) over free-space optical (FSO) links between satellites and ground stations, present a promising pathway toward achieving global security in upcoming sixth-generation (6G) wireless communications. Our study focuses on a superior type of these systems, the satellite EB/CV-QKD/FSO, which utilizes the continuous-variable (CV) method for quantum state representation and the entanglement-based (EB) scheme for QKD implementation. We propose the use of optical phase-shift keying (QPSK) signaling and dual-threshold/heterodyne detection (DT/HD) receivers to bolster the reliability and feasibility of satellite EB/CV-QKD/FSO systems. Closed-form expressions for key system performance metrics are derived using improved channel modeling. Numerical results are presented to showcase the effects of channel impairments on the system performance. We also provide recommendations for optimal system setup parameters, aiming to enhance performance. / Ministry of Information and Communications (Vietnam) (Grant Number: DT.26/23). Asia Pacific Network Information Centre (APNIC) Foundation under the Switch! Project