The role that quantum correlations play in Quantum Information Science and the prospects of new developments and newe applications encourage physicists to search for a deeper understanding of these correlations, and of their non-locality. This thesis follows this line of research. After illustrating their practical applications in the field of Quantum Key Distribution, we present more fundamental studies. We thus analyze two different alternative models for non-local correlations : Leggett's model, and a model that attempt to explain the correlations with some hypothetical superluminal "spooky action at a distance". We also tackle the problem of quantifying the non-locality of quantum correlations, with two different approaches. Finally, we address the intriguing question, wheter the quantum world could somehow be brought closer to our direct perception, and if quantum correlations could be revealed with the naked eye.
The role that quantum correlations play in Quantum Information Science and the prospects of new developments and newe applications encourage physicists to search for a deeper understanding of these correlations, and of their non-locality. This thesis follows this line of research. After illustrating their practical applications in the field of Quantum Key Distribution, we present more fundamental studies. We thus analyze two different alternative models for non-local correlations : Leggett's model, and a model that attempt to explain the correlations with some hypothetical superluminal "spooky action at a distance". We also tackle the problem of quantifying the non-locality of quantum correlations, with two different approaches. Finally, we address the intriguing question, wheter the quantum world could somehow be brought closer to our direct perception, and if quantum correlations could be revealed with the naked eye.
This thesis is devoted to the study of quantum non-locality, in the framework of quantum information science. The research presented here includes both theoretical works on some fundamental concepts, such as the measurement process and quantum correlations, as well as some applications in modern quantum optics. As it is often the case in quantum information science, the joint study of fundamental notions and applications is very productive. While a better understanding of the fundamental concepts may lead to new applications, it happens also that applications shed new light on some fundamental notions...
Despite the success of quantum mechanics in predicting the outcomes of experiments in many branches of physics, the foundations of the theory have remained subject of research and dispute. At the basis of this struggle with the theory lie the phenomena of nonlocality and entanglement. Since it was first predicted by Bell in 1964, nonlocality was not only verified in numerous experiments, but also identified as a useful resource for quantum information processing. Thus, the study of nonlocality is important both from a fundamental point of view and with respect to new applications in quantum information theory, such as secure cryptography and randomness generation. The identification of entanglement as a resource for quantum information led to a strong theoretical effort devoted to its characterisation and detection. Many of the resulting mathematical tools find application in several domains of physics. Although the only known way to create nonlocal correlations is to measure entangled quantum systems, it has been shown that entanglement and nonlocality constitute two inequivalent properties. Therefore, in the light of the success of entanglement theory, it is of interest to also devise a resource theory of nonlocality. The task of this thesis is to develop such a theoretical framework for the characterisation of nonlocality as a resource. To gain a better understanding of nonlocal correlations it will be necessary to investigate correlation scenarios that go beyond the situation originally considered by Bell. In doing so, this thesis provides new description of nonlocality that also have implications for the characterisation of quantum correlations and the detection of new forms of nonlocality. The first question we address is how nonlocality can consistently be defined in a scenario of arbitrarily many parties that may collaborate among each other. To this end we recognise which are the allowed physical operations in this situation and then define nonlocality as the resource that cannot be created by these operations. Our approach shows that the conventional definition of multipartite nonlocality, adopted by the community so far, is inconsistent with this operational definition; we further propose and analyse new models that do not suffer from these inconsistencies. Furthermore, we show that our findings have implications for the characterisation of quantum correlations. A recent approach to describe the set of quantum correlation consists in using principles inspired from information theory. By using a special instance of the models we defined earlier we show a fundamental limitation of this approach: no bipartite information principle is sufficient to single out the set of quantum correlations from the set of nonsignalling correlations. We then developed a description of nonlocality in an even more generalised scenario of several parties. Motivated by a result of Popescu we study scenarios where the parties are allowed to perform not only a single but sequences of measurements on their systems. Characterising nonlocality also in this scenario in operational terms and defining local models compatible with this definition, we show that a new form of nonlocality can be detected Lastly, we examine the problem of detecting the presence of nonlocality in a multipartite scenario when one is given only partial access to the global system. We find that one can verify that the total system must display nonlocality, even though the accessible subsystems only exhibit local correlations.
Quantum correlations arising from measurements on pairs of entangled particles are often referred to as the most non-classical feature of quantum mechanics. As such, they have found a role in many fundamental tests of quantum mechanics and most emerging applications in the field of quantum communication. The most well-known fundamental test is the Bell inequality. Its repeated violation in countless experiments has convinced physicists that local hidden variables cannot describe the correlations arising from measurements on entangled particles. On the application side, quantum correlations have been used in quantum key distribution, which aims to provably secure messages during transmission, and quantum repeaters, which are essential for future long-distance quantum communication. The main goal of this thesis was to use quantum entanglement for new fundamental studies and quantum communication applications. Towards this end, we developed a high-fidelity source of entanglement and used it in a fundamental test where we bounded the predictive power any physical theory could have about the outcomes of measurements on entangled particles. Secondly, we made use of entangling measurements to develop a new quantum cryptography system based on the promising MDI-QKD protocol, which protects users from otherwise undetectable hacking attacks. We developed a detailed model of MDI-QKD systems with which one can optimize any implementation, deployed our MDI-QKD system across the city of Calgary, and demonstrated the feasibility of this protocol and of long-distance entangling measurements. Thirdly, we have built a source of entanglement compatible with quantum memories, which is an essential ingredient of quantum repeaters. We then demonstrated several crucial steps towards a functioning quantum repeater, including the preservation of entanglement during storage and, more generally, the entire photonic wavefunction. While further work is required to bring our demonstrations to real-world applications, we are confident that they will prove useful in guiding future developments.
Quantum entanglement is a fundamental and intriguing phenomenon in quantum mechanics, wherein two or more particles become correlated in such a way that the state of one instantaneously influences the state of the other, regardless of the distance separating them. This research delves into the theoretical foundation of quantum entanglement, exploring its mathematical formulation, key experimental validations, and implications for emerging technologies. A primary focus is placed on its application in secure communication, particularly in Quantum Key Distribution (QKD), where entanglement enables fundamentally secure cryptographic protocols resistant to eavesdropping. We present an in-depth analysis of various QKD protocols, including BB84, E91, and decoy-state methods, assessing their security, efficiency, and practical feasibility. Recent experimental advancements in entanglement-based cryptography are examined, alongside real-world implementations and their technical challenges, such as photon loss, decoherence, and scalability in long-distance quantum communication networks. Additionally, we provide comparative insights into the efficiency of different QKD protocols using a bar chart, while figures and tables illustrate core quantum principles and experimental setups. By addressing both theoretical and practical aspects, this study contributes to a comprehensive understanding of quantum entanglement’s role in secure communication and highlights future directions for overcoming existing technological limitations.
This paper investigates the intricate relationship between distant quantum states through the application of quantum entanglement. The core focus is to develop a novel method for quantifying and analyzing correlations within these states, offering a potential pathway to unlock deeper insights into complex quantum phenomena. The method leverages quantum entanglement to establish a global state representation, enabling the analysis of correlations across vast distances. This research aims to address limitations of existing approaches, particularly concerning long-range correlations, and presents a promising direction for advancing our understanding of quantum systems.
Characterization and categorization of quantum correlations are both fundamentally and practically important in quantum information science. Although quantum correlations such as non-separability, steerability, and non-locality can be characterized by different theoretical models in different scenarios with either known (trusted) or unknown (untrusted) knowledge of the associated systems, such characterization sometimes lacks unambiguous to experimentalist. In this work, we propose the physical interpretation of nonlocal quantum correlation between two systems. In the absence of complete local description of one of the subsystems quantified by the local uncertainty relation, the correlation between subsystems becomes nonlocal. Remarkably, different nonlocal quantum correlations can be discriminated from a single uncertainty relation derived under local hidden state (LHS)-LHS model only. We experimentally characterize the two-qubit Werner state in different scenarios.