Quantum entanglement is a form of correlation between quantum particles that has now become a crucial part in quantum information and communication science. For example, it has been shown to enable or enhance quantum processing tasks such as quantum cryptography, quantum teleportation, and quantum computing. However, quantum entanglement is prone to decoherence as a result of interactions with environmental scatterers, making it an expensive resource. Therefore, it is crucial to understand its creation. We centre our attention to a situation where one would like to distribute quantum entanglement between principal particles that are apart. In this case, it is necessary to use ancillary systems that are communicated between them or interact with them continuously. Cubitt et al. showed that the ancillary systems need not be entangled with the principal particles in order to distribute entanglement. This has been demonstrated experimentally in the case of communicated ancillary particles and it is now known that the bound on the distributed entanglement is given by a communicated quantum discord. On the other hand, little is understood about the setting with continuous interactions, despite its abundant occurrence in nature. The main focus of this thesis is to study the distribution of quantum entanglement via continuous interactions with ancillary particles, which I will call mediators. I will present my work regarding the necessary conditions for entanglement distribution, the factors that are relevant for the distributed amount, and the speed limit to achieving maximum entanglement gain. Finally, I present some notable applications that can benefit from our work. This includes, among others, indirect probing of the quantum nature of optomechanical mirrors, photosynthetic organisms, and gravitational interactions.
Quantum entanglement is a form of correlation between quantum particles that has now become a crucial part in quantum information and communication science. For example, it has been shown to enable or enhance quantum processing tasks such as quantum cryptography, quantum teleportation, and quantum computing. However, quantum entanglement is prone to decoherence as a result of interactions with environmental scatterers, making it an expensive resource. Therefore, it is crucial to understand its creation. We centre our attention to a situation where one would like to distribute quantum entanglement between principal particles that are apart. In this case, it is necessary to use ancillary systems that are communicated between them or interact with them continuously. Cubitt et al. showed that the ancillary systems need not be entangled with the principal particles in order to distribute entanglement. This has been demonstrated experimentally in the case of communicated ancillary particles and it is now known that the bound on the distributed entanglement is given by a communicated quantum discord. On the other hand, little is understood about the setting with continuous interactions, despite its abundant occurrence in nature. The main focus of this thesis is to study the distribution of quantum entanglement via continuous interactions with ancillary particles, which I will call mediators. I will present my work regarding the necessary conditions for entanglement distribution, the factors that are relevant for the distributed amount, and the speed limit to achieving maximum entanglement gain. Finally, I present some notable applications that can benefit from our work. This includes, among others, indirect probing of the quantum nature of optomechanical mirrors, photosynthetic organisms, and gravitational interactions.
Quantum entanglement is a form of correlation between quantum particles that has now become a crucial part in quantum information and communication science. For example, it has been shown to enable or enhance quantum processing tasks such as quantum cryptography, quantum teleportation, and quantum computing. However, quantum entanglement is prone to decoherence as a result of interactions with environmental scatterers, making it an expensive resource. Therefore, it is crucial to understand its creation. We centre our attention to a situation where one would like to distribute quantum entanglement between principal particles that are apart. In this case, it is necessary to use ancillary systems that are communicated between them or interact with them continuously. Cubitt et al. showed that the ancillary systems need not be entangled with the principal particles in order to distribute entanglement. This has been demonstrated experimentally in the case of communicated ancillary particles and it is now known that the bound on the distributed entanglement is given by a communicated quantum discord. On the other hand, little is understood about the setting with continuous interactions, despite its abundant occurrence in nature. The main focus of this thesis is to study the distribution of quantum entanglement via continuous interactions with ancillary particles, which I will call mediators. First, basic concepts and tools that are helpful for this thesis will be introduced. This includes the description of quantum objects within the framework of quantum mechanics, their dynamics, and important properties. Next, I will present my work regarding the necessary conditions for entanglement distribution, the factors that are relevant for the distributed amount, and the speed limit to achieving maximum entanglement gain. Finally, I present some notable applications that can benefit from our work. This includes, among others, indirect probing of the quantum nature of optomechanical mirrors, photosynthetic organisms, and gravitational interactions.
The main focus of this thesis is quantum entanglement and, more specifically, pro- tocols that allow its distribution between distant places. In the last decades, with the rise of attention on quantum technologies, entangle- ment is mostly seen as a resource, that could be exchanged, measured, and ideally stored and used when convenient. However, entanglement is not as stable and cheap as the classical resources. One reason for this is that entanglement between two quantum systems is easily disrupted by the interaction with the environment that surrounds them. The presence of noise is even more significant when we wish to entangle two systems that are very far away, a task that is essential for quantum communication technologies. In this case, what can disrupt the entanglement is not only the noise that the particles experience locally, but also the noise in the quantum channel during the communication process, which can be significant over long distances. For this reason, it is important to design distribution protocols that work well in such conditions. In 2003, Cubitt et al. showed the existence of a very curious class of protocols, often called entanglement distribution with separable states (EDSS). They proved that a non-entangled (separable) carrier can be used to increase, or even gener- ate entanglement between two already distant parties. From this, some questions arise. First, what limits or allows an entanglement distribution protocol, if not the entanglement that is sent. Quantum discord was shown to be a necessary, but not sufficient, resource for entanglement distribution. Another natural question is whether these protocols can beat others, for technological purposes, especially in noisy conditions. I will describe some work I have done in an attempt to partially 9 address both of these questions. After introducing various tools and concepts that are useful to understand this topic (chapter 1) and motivate the necessity of entanglement distribution in quantum communication (chapter 2), I will describe a proof-of-principle experiment demon- strating EDSS schemes (chapter 3). Later (chapter 4), I will introduce a more general classification of distribution protocols (excessive or non-excessive protocols), based on whether the entanglement gain is larger than the one that is communicated, describing specific examples and general properties of such protocols. I will then partially address the problem regarding the robustness to the noise of communica- tion protocols (chapter 5). Finally, in chapter 6, I will describe how entanglement of a bipartite state can grow with a quantum measurement with unknown results. I discuss the properties of the measurement and of the initial state that make this process effective.
Developments in the interdisciplinary field of quantum information open up previously impossible abilities in the realms of information processing and communication. Quantum entanglement has emerged as one property of quantum systems that acts as a resource for quantum information processing and, in particular, enables teleportation and secure cryptography. Therefore, the creation of entangled resources is of key importance for the application of these technologies. Despite a great deal of research the efficient creation of entanglement over long distances is limited by inevitable noise. This problem can be overcome by creating entanglement between nodes in a network and then performing operations to distribute the entanglement over a long distance. This thesis contributes to the field of entanglement distribution within such quantum networks. Entanglement distribution has been extensively studied for one-dimensional networks resulting in "quantum repeater" protocols. However, little work has been done on higher dimensional networks. In these networks a fundamentally different scaling, called "long distance entanglement distribution", can appear between the resources and the distance separating the systems to be entangled. I reveal protocols that enable long distance entanglement distribution for quantum networks composed of mixed state and give a few limitations to the capabilities of entanglement distribution. To aid in the implementation of all entanglement distribution protocols I finish by introducing a new system, composed of an optical nanofibre coupled to a carbon nanotube, that may enable new forms of photo-detectors and quantum memories.
Quantum entanglement, one of the most intriguing phenomena in quantum mechanics, forms the cornerstone of emerging technologies in quantum information science. It describes the non-classical correlations between spatially separated particles, whereby the measurement outcome of one particle instantaneously determines the state of the other, regardless of the distance between them. This paper explores the fundamental principles of quantum entanglement and its critical role in advancing quantum cryptography and quantum computing. In cryptography, entanglement enables protocols such as Ekert’s E91 and BBM92 quantum key distribution schemes, which leverage nonlocal correlations to ensure unconditional security based on the violation of Bell’s inequalities. In quantum computing, entanglement serves as a powerful computational resource, facilitating speedups in algorithms, enabling quantum teleportation, and supporting robust quantum error correction techniques. The discussion integrates both theoretical underpinnings and experimental realizations, highlighting recent advances in photonic entanglement sources, superconducting qubits, and trapped-ion systems. Challenges related to decoherence, scalability, and entanglement distribution in quantum networks are examined, along with potential solutions. By bridging foundational theory with practical applications, this study underscores the pivotal role of entanglement in shaping the future of secure communications and computational paradigms beyond the capabilities of classical systems.
Quantum entanglement is the most popular kind of quantum correlations. Its role in several tasks in quantum information theory like quantum cryptography, quantum dense coding and quantum teleportation is indisputable. The role of entanglement in quantum algorithms has also attracted enormous amount of attention in the literature. The research in this direction is motivated by the finding that a quantum computer can efficiently factorize arbitrary integer numbers. Since no classical algorithm is known today that can solve this problem efficiently, this example clearly demonstrates the superiority of a quantum computer compared to its classical counterpart. In this context, entanglement also plays a crucial role: if the quantum computer operates on a pure state without entanglement, then the computational process can be simulated efficiently on a classical computer. In this case entanglement is the key ingredient that makes the difference between a quantum and a classical computer.
However, the situation is more involved if the computational process is not perfect. In this case the quantum computer operates on a mixed state, and the role of entanglement is less obvious. A seminal result in this direction is a quantum algorithm known under the acronym DQC1. This algorithm operates on an almost maximally mixed state with vanishingly little entanglement. However, DQC1 can still perform tasks efficiently for which no efficient classical algorithm is known today. Thus, it is reasonable to assume that entanglement in general is not related to the efficiency of a quantum algorithm.
Triggered by this observation other kinds of quantum correlations have been studied. In this context a particular measure of quantum correlations known as quantum discord is referred to frequently in the literature. Quantum discord is a measure of quantum correlations beyond entanglement, i.e., a quantum state can have no entanglement, but nonvanishing quantum discord. Quantum discord is also regarded as the key resource in the DQC1 algorithm. Although this statement is still controversial, it was shown under very general assumptions that quantum discord is required if a quantum computer is to show an exponential speedup over any classical algorithm.
In this thesis we give a short introduction into the theory of entanglement and general quantum correlations, and further discuss our results. In particular, we show that two different kinds of entanglement measures can coincide. This result is used to build an algorithm for computing entanglement. We also consider the role of quantum correlations for the distribution of entanglement and for the quantum measurement process. We further discuss the behavior of quantum correlations under local noise, and their monogamy properties.
A fundamental concept in quantum mechanics, quantum entanglement describes a one-of-a-kind occurrence in which two particles, separated by physical distance, become physically connected so that the states of one particle can instantly affect the states of the other. Quantum entanglement has both theoretical and practical consequences for communication and information processing, which are explored in this study. This article explores the latest developments in quantum cryptography, quantum teleportation, and quantum computing, focusing on how these technologies have the ability to transform communication networks, computational speed, and data security. While outlining potential future research directions, the report delves into issues including scalability and decoherence. This manuscript uses entanglement to highlight its revolutionary impact on the development of scientific and technological frontiers.