The future quantum Internet (QI) will transform today's communication networks and user experiences by providing unparalleled security levels, superior quantum computational powers, along with enhanced sensing accuracy and data processing capabilities. These features will be enabled through applications like quantum key distribution (QKD) and quantum machine learning (QML). Towards enabling these applications, the QI requires the development of global quantum communication networks (QCNs) that enable the distribution of entangled resources between distant nodes. This dissertation addresses two major challenges facing QCNs, which are the scalability and coverage of their architectures, and the efficiency of their operations. Additionally, the dissertation studies the near-term deployment of QML applications over today's noisy quantum devices, essential for realizing the future QI. In doing so, the scalability and efficiency challenges facing the different QCN elements are explored, and practical noise-aware and physics-informed approaches are developed to optimize the QCN performance given heterogeneous quantum application-specific quality of service (QoS) user requirements on entanglement rate and fidelity. Towards achieving this goal, this dissertation makes a number of key contributions. First, the scaling limits of quantum repeaters is investigated, and a holistic optimization framework is proposed to optimize the geographical coverage of quantum repeater networks (QRNs), including the number of quantum repeaters, their placement and separating distances, quantum memory management, and quantum operations scheduling. Then, a novel framework is proposed to address the scalability challenge of free-space optical (FSO) quantum channels in the presence of blockages and environmental effects. Particularly, the utilization of a reconfigurable intelligent surface (RIS) in QCNs is proposed to maintain a line-of-sight (LoS) connection between quantum nodes separated by blockages, and a novel analytical model of quantum noise and end-to-end (e2e) fidelity in such QCNs is developed. The results show enhanced entangled state fidelity and entanglement distribution rates, improving user fairness by around 40% compared to benchmark approaches. The dissertation then investigates the efficiency challenges in a practical use-case of QCNs with a single quantum switch (QS). Particularly, the average quantum memory noise effects are analytically analyzed and their impacts on the allocation of entanglement generation sources and minimization of entanglement distribution delay while optimizing QS entanglement distillation operations are investigated. The results show an enhanced e2e fidelity and a minimized e2e entanglement distribution delay compared to existing approaches, and a unique capability of satisfying all users QoS requirements. This QCN architecture is scaled up with multiple QSs serving heterogeneous user requests, necessary for scalable quantum applications over the QI. Here, a novel efficient matching theory-based framework for optimizing the request-QS association in such QCNs while managing quantum memories and optimizing QS operations is proposed. Finally, after scaling QCNs and ensuring their efficient operations, the dissertation proposes novel distributed QML frameworks that can leverage both classical networks and QCNs to enable collaborative learning between today's noisy quantum devices. In particular, the first quantum federated learning (QFL) frameworks incorporating different quantum neural networks and leveraging quantum and classical data are developed, and the first publicly available federated quantum dataset is introduced. The results show enhanced performance and reductions in the communication overhead and number of training epochs needed until convergence, compared to classical counterpart frameworks. Overall, this dissertation develops robust frameworks and algorithms that advance the theoretical understanding of QCNs and offers practical insights for the future development of the QI and its applications. The dissertation concludes by analyzing some open challenges facing QCNs and proposing a vision for physics-informed QCNs, along with important future directions. / Doctor of Philosophy / In today's digital age, we are generating vast amounts of data through videos, live streams, and various online activities. This explosion of data brings not only incredible opportunities for innovation but also heightened security concerns. The current Internet infrastructure struggles to keep up with the demand for speed and security. In this regard, the quantum Internet (QI) emerges as a revolutionary technology poised to make the communication and data sharing processes faster and more secure than ever before. The QI requires the development of quantum communication networks (QCNs) that will be seamlessly integrated with today's existing communication systems that form today's Internet. This way, the QI enables ultra-secure communication and advanced computing applications that can transform various sectors, from finance to healthcare. However, building such global QCNs, requires overcoming significant challenges, including the sensitive nature and limitations of quantum devices. In this regard, the goal of this dissertation is to develop scalable and efficient QCNs that overcome the different challenges facing different QCN elements and enable a wide coverage and robust performance towards realizing the QI at a global scale. Simultaneously, machine learning (ML), which is driving significant advancements and transforming industries in today's world. Here, quantum technologies are anticipated to make a breakthrough in ML through quantum machine learning (QML) models that can handle today's large and complex data. However, quantum computers are still limited in scale and efficiency, often being noisy and unreliable. Throughout this dissertation, these limitations of QML are addressed by developing frameworks that allow multiple quantum computers to work together collaboratively in a distributed manner over classical networks and QCNs. By leveraging distributed QML, it is possible to achieve remarkable advancements in privacy and data utilization. For instance, distributed QML can enhance navigation systems by providing more accurate and secure route planning or revolutionize healthcare by enabling secure and efficient analysis of medical data. In summary, this dissertation addresses the critical challenges of building scalable and efficient QCNs to support the QI and develops distributed QML frameworks to enable near-term utilization of QML in transformative applications. By doing so, it paves the way for a future where quantum technology is integral to our daily lives, enhancing security, efficiency, and innovation across various domains.

Quantum networks play an indispensable role in quantum information tasks such as secure communications, enhanced quantum sensing, and distributed computing. In recent years several platforms are being developed for such tasks, witnessing breakthrough technological advancement in terms of fabrication techniques, precise control methods, and information transfer. Among the most mature and promising platforms are color centers in solids. These systems provide an optically active electronic spin and long-lived nuclear spins for information storage. The first part of this dissertation is concerned with error mechanisms in the control of electronic and nuclear spins. First, I will focus on control protocols for improved electron-spin rotations tailored to specific color centers in diamond. I will then discuss how to manipulate the entanglement between the electron and the always-coupled nuclear spin register. I will describe a general formalism to quantify and control the generation of en- tanglement in an arbitrarily large nuclear spin register. This formalism incorporates exactly the dynamics with unwanted nuclei, and quantifies the performance of entangling gates in the presence of unwanted residual entanglement links. Using experimental parameters from a well-characterized multinuclear spin register, I will show that preparation of multipartite entanglement in a single-shot is possible, which drastically reduces the total gate time of conventional protocols. Then, I will present a new formalism for describing all-way entanglement and show how to design gates that prepare GHZM states. I will show how to incorporate errors such as unwanted correlations, electronic dephasing errors or pulse control errors. The second part of this thesis focuses on the preparation of all-photonic graph states from a few quantum emitters. I will introduce heuristic algorithms that exploit graph theory concepts in order to reduce the entangling gate counts, and also discuss the role of locally equivalent graphs in the optimization of the generation circuits. / Doctor of Philosophy / Quantum information science emerged by combining ideas and principles of information theory, nanoscale engineering, photonics, atomic and solid-state physics in a unified effort to realize and fabricate efficient quantum-based architectures. Spin-based solid-state quantum computers are one of the leading candidates for quantum architectures. For these types of devices, the quantum bit of information can be encoded in the spin states of electron/nuclear memories, while the logical operations are performed by driving transitions between a multi- level spin structure. In this thesis, I will describe the role of color centers for quantum computations and networking. I will explain the error sources and dynamics of SiV− and SnV− color centers in diamond and show how to drive with high fidelity optical rotations of their spin states. Additionally, I will explain how periodic driving of the electronic spin can serve as a method to control the nuclear spin memories and show how to precisely prepare multipartite entangled states within an arbitrarily large electron-nuclear spin register. Lastly, I will focus on the preparation of all-photonic graph states and show how to prepare them with optimal resources.

“Doctor Moreau and The Hunt for The Ethical Figure. From unsuccessful man-making to respectful relations with Jacques Derrida, Donna Haraway and H. G. Wells” In H. G. Wells’ science fiction novel, The Island of Doctor Moreau (1896), a scientist on a remote island in the Pacific Ocean attempts to create humans from animals. Wells draws on Charles Darwin’s theories on the origins of species and the descent of man to create a horror story in which the uniqueness of the human is called into question. This study uses the novel to investigate the re-emergent interest in human-animal relations, within the natural sciences as well as the arts and humanities, in the past twenty years. In what is often termed “the Animal Turn,” theorists such as Jacques Derrida and Donna Haraway, who are at the centre of this study, have dedicated a significant amount of their work to the animal question and in particular to the ethics of inter-species relationships. Furthermore this essay stages the interaction of fiction and theoretical discourse in an analysis that probes challenges inherent to the relations of humans to other species, such as the practice of eating meat, the killing of animals, and animal rights. Moreover, it considers how the figure of the animal has been used to define the human, as well as to dehumanize people in the justification of abuse and persecution. However, human-animal ethics also has positive connotations, discussed through the figures of positivity and possibility in play, sharing, contact and responsive responsibility. Taking its inspiration from Karen Barad’s method of diffraction, this study foregrounds new pattern-making while exploring how Derrida’s and Haraway’s strategies for formulating a new ethics are present in their use of tropes and figures. / Science fiction-romanen The Island of Doctor Moreau skrevs 1896 och handlar om hur en vetenskapsman på en avlägsen ö i Stilla havet försöker skapa människor av djur. Författaren H. G. Wells inspirerades av Darwins teorier om arternas uppkomst till att skriva en skräckberättelse där mänsklighetens unikhet sätts ur spel. Den här studien använder Wells text i syfte att undersöka hur relationen mellan människor och andra arter har kommit att få ett nytt intresse inom en mängd olika vetenskapliga, praktiska och konstnärliga ämnen i slutet av 1900-talet och början av 2000-talet, något som har kommit att kallas ”The Animal Turn”.  I centrum för studien återfinns teorier formulerade av filosofen Jacques Derrida och den feministiska vetenskapsteoretikern och biologen Donna Haraway. Båda två har ägnat stor del av sina respektive arbeten åt djurfrågan, framför allt med enträget intresse för hur ett etiskt förhållande mellan arter kan se ut. Studien låter romanen och de teoretiska texterna samverka i en analys som diskuterar utmaningar i människa-djur-relationer – som köttätande, dödande och rättigheter. Människan har till exempel använt sig av djuret för att definiera sig som människa, men också för att dehumanisera andra människor, och därmed kunnat utnyttja eller våldföra sig på dessa människor. Men människa-djur-etiken har också positiva aspekter: kontakt över språkgränser, att tillsammans utgöra världsmedborgare som alla består av och är beroende av varandra, att dela lek och arbete, att se och upptäcka den andre.  Med inspiration från Karen Barads användning av metoden diffraktion vill den här uppsatsen utforska Derridas och Haraways strategier för att formulera en ny etik genom bruket av figurer och troper.

Nous analysons les caractéristiques d'intrication de quelques matériaux à l'état solide ainsi que des systèmes particuliers d'atomes et de champs en interaction. Une étude détaillée de la riche structure de phase des modèles de spins de basse dimension, décrivant le minéral naturel d'azurite et les composés de coordination à base de cuivre, a révélé des régimes à comportement d'intrication des plus robustes. En utilisant l'approche des systèmes dynamiques, la structure de phase de certains modèles classiques en réseaux hiérarchiques (récursifs) a aussi été étudiée et, pour la première fois, la transition entre régime chaotique et régime périodique au moyen de la bifurcation tangente a été détectée.Nous présentons une description détaillée des propriétés d'intrication de trois atomes piégés dans la limite dispersive. Une relativement simple accordabilité de la force atomique d'interaction de ce système et son étroite relation aux problèmes de frustration magnétique est démontrée. Les effets de propagation de pulses laser intenses dans un système atomique de type [lambda] avec des forces d'oscillateurs différentes sont analysés. Les résultats obtenus sont d'extrême importance dans des problèmes d'information quantique, comme par exemple, dans l'analyse du mécanisme de transfert de population dans des milieux ayant les propriétés définies ci-avant. Enfin, nous avons analysé les effets dissipatifs dans un protocole de distillation de l'intrication à variable continue récemment proposé. Malgré des contraintes additionnelles sur les paramètres du protocole, il est encore possible d'implémenter ce schéma de distillation évoqué ci-avant dans les technologies émergentes. / The entanglement features of some solid state materials, as well as of particular systems of interacting atoms and fields are analyzed. A detailed investigation of the rich phase structure of low dimensional spin models, describing the natural mineral azurite and copper based coordination compounds, has revealed regimes with the most robust entanglement behavior. Using the dynamical system approach, the phase structure of some classical models on hierarchical (recursive) lattices has been also studied and, for the first time, the transition between chaotic and periodic regimes by means of tangent bifurcation has been detected.A detailed description of entanglement properties of three atoms trapped in a cavity within the dispersive limit is presented. A relatively simple tunability of the atomic interaction strength of the above system and its close relation to the problems of frustrated magnetism is shown. Furthermore, the propagation effects of two intense laser pulses in a medium of [lambda] atoms with unequal oscillator strengths are investigated. Obtained results are crucial in some problems of quantum information theory, as, e.g., in the analysis of population transfer mechanism in media possessing the above properties. Finally, the dissipation effects in a recently proposed compact continuous-variable entanglement distillation protocol have been analyzed. Despite additional constraints on the parameters of the protocol, the discussed entanglement distillation scheme in quantum memories is still possible to implement within emerging technologies.

The deterministic generation and manipulation of quantum states has attracted much interest ever since the rise of quantum mechanics. Large-scale, distributed quantum states are the basis for novel applications such as quantum communication, quantum remote sensing, distributed quantum computing or quantum voting protocols. The necessary infrastructure will be provided by distributed quantum networks, allowing for quantum bit processing and storage at single nodes. Quantum states of light then allow for inter-node transmission of quantum information. Transmission losses in optical fibers may be overcome by quantum repeaters, the quantum equivalent of classical signal amplifiers. The fragility of quantum superposition states makes building such networks very challenging. Hybrid solutions combine the strengths of different physical systems: Efficient quantum memories can be realized using alkali atoms such as rubidium. Leading in the deterministic generation of single photons and polarization entangled photon pairs are semiconductor InAs/GaAs quantum dots grown by the Stranski-Krastanov method. Despite remarkable progress in the last twenty years, complex quantum optical protocols could not be realized due to low degree of entanglement, low brightness and broad wavelength distribution. In this work, an emerging family of epitaxially grown GaAs/AlGaAs quantum dots obtained by droplet etching and nanohole infilling is studied. Under pulsed resonant two-photon excitation, they emit single pairs of entangled photons with high purity and unprecedented degree of entanglement. Entanglement fidelities up to f = 0.94 are observed, which are only limited by the optical setup or a residual exciton fine structure. The samples exhibit a very narrow wavelength distribution at rubidium memory transitions. Strain tuning is applied via piezoelectric actuators to allow for reversible fine-tuning of the emission frequency. In a next step, active feedback is employed to stabilize the frequency of single photons emitted by two separate quantum dots to an atomic rubidium standard. The transmission of a rubidium-based Faraday filter serves as the error signal for frequency stabilization. A residual frequency deviation of < 30MHz is achieved, which is less than 1.5% of the quantum dot linewidth. Long-term stability is demonstrated by Hong-Ou-Mandel interference between photons from the two quantum dots. Their internal dephasing limits the expected visibility to V = 40%. For frequency-stabilized dots, V = (41 ± 5)% is observed as opposed to V = (31 ± 7)% for free-running emission. This technique reaches the maximally expected visibility for the given system and therefore facilitates quantum networks with indistinguishable photons from distributed sources. Based on the presented techniques and improved emission quality, pivotal quantum communication protocols can now be implemented with quantum dots, such as transferring entanglement between photon pairs. Embedding quantum dots in a dielectric antenna ensures a bright emission. For the first time, entanglement swapping between two pairs of photons emitted by a single quantum dot is realized. A joint Bell measurement heralds the successful generation of the Bell state Ψ+ with a fidelity of up to (0.81 ± 0.04). The state's nonlocal nature is confirmed by violating the CHSH-Bell inequality with S = (2.28 ± 0.13). The photon source is tuned into resonance with rubidium transitions, facilitating implementation of hybrid quantum repeaters. This work thus represents a major step forward for the application of semiconductor based entangled photon sources in real-world scenarios.

The final voyages of H.M.S Erebus and H.M.S Terror marked a pivotal chapter in the tapestry of international relations with the Arctic frontier. The aim of this study is to analyze how these wrecks impact territorial disputes and the geopolitical dynamics between coastal states and international actors. By adopting an interdisciplinary perspective, where archaeology is supplemented by law and geopolitical theory, it has been possible to gain increased knowledge of how actors such as Canada, the United Kingdom, the United States, and the Inuit of Nunavut perceive and utilize the wrecks for their respective territorial interests. With the intention of further deepening the understanding of the complex conditions that shape the wreckages legal status and management in the modern world, an analysis of the legal frameworks regulating the Northwest Passage is also conducted. The study exposes contemporary jurisdictional gaps and emphasizes the necessity of including Inuit perspectives into Arctic Management. / Den sista expeditionen med H.M.S Erebus och H.M.S Terror har kommit att få central betydelse för de komplexa internationella relationerna med Arktis. Syftet med den här studien är att analysera hur dessa vrak påverkar de territoriella tvisterna och det geopolitiska samspelet mellan kuststater och internationella aktörer. Genom att anlägga ett tvärvetenskapligt perspektiv, där arkeologi kompletteras med juridik och geopolitisk teori har det varit möjligt att nå ökad kunskap om hur aktörer som Kanada, Storbritannien, USA och Inuiterna i Nunavut uppfattar och använder vraken för sina respektive territoriella intressen. Med avsikt att ytterligare fördjupa förståelsen av de komplexa förhållanden som formar vrakens juridiska status och hantering i den moderna världen, genomförs också en analys av de rättsliga ramverk som reglerar Nordvästpassagen. Studien avslöjar bristerna i nuvarande lagstiftning och framhäver vikten av att inkludera Inuiternas perspektiv i diskussioner om förvaltning.

This dissertation analyzes narratives of queer partner migration, that is, a family-tie migration in which one of the partners of a relationship has migrated in order for the partners to be together, and where the partners queer the migration in the sense that they have a non-normative sexuality and/or gender identity. The purpose of the study is to examine how queer partner migrants and their Swedish partners experience the migration process – which continues also once the administrative process has been completed – by analyzing the emotions and feelings that emerge in the process. The study is a contribution to research on privileged migration as well as intimate migration. The focus is the queer partner migration relationship, and what emotions and feelings ‘do’ to this relationship, but also how emotions and feelings structure the migration process. The study analyzes the work three different emotions – love, loss, and belonging – do in these migration processes, and how this work is described in the participant narratives. Migrant participants have migrated from different parts of the world (Africa, Europe, Latin America, and North America), making it possible to analyze what emotions and feelings do in this particular migration process from the point of view of nationality and, in particular, proximity to ‘Western-ness,’ race, and language as well as how privileges connected to these positions come to matter in the process. The dissertation is an ethnographic interview study in which both migrants and Swedish partners have been interviewed. The interview material consists of a combination of couple interviews and individual interviews. By using affect theories and the concept of queer phenomenology, the dissertation shows how the work that emotions and feelings do in migration processes is connected to gender identity, sexual identity, race and whiteness, nationality, perceived proximity to Western-ness, class, language, and the migration narrative the migrating partner is (or is not) written into by way of the country they have migrated from. This is analyzed in relation to the theoretical frameworks of entanglement, homonationalism, and intimate citizenship. The analysis shows that emotions and feelings structure the migration process for both more privileged and less privileged migrants, but in different ways. The understanding of who ‘is’ a migrant, and the preparedness for the feelings that arise in a migration process, are tied to the positions mentioned above and the privileges these positions give, or do not give, the migrant access to. By focusing on emotions and feelings and what these do, the study also illustrates how the migration process affects the non-migrating partner as this partner engages in emotional labour to ‘make’ the migrating partner ‘Swedish.’ Through their the migrating partner, the non-migrating partner is also aligned in a way that makes them a little bit less ‘Swedish,’ contributing to the non-migrating partner being ‘stopped’ in ways they have usually not experienced before. The study further shows how migration processes produces inequality, and the difficulties that arise when the couples try to live up to the Swedish ideal of the equal relationship. The interviews are analyzed as narratives, and both narratives and storytelling are important throughout the dissertation, not only as the method used in the analysis but as the form of the dissertation, making it a kind of super structure organizing the writing. Writing (how to write accessibly and interesting) and reading (how to write in order to invite an open and active reading) are important aspects of the dissertation. / Avhandlingen analyserar narrativ om queer partnermigration, dvs en familjebandsmigration i vilken en av de två personerna i ett parförhållande migrerar för att de två ska kunna leva i samma land och i vilken de två individerna queerar migrationen på så sätt att de har en icke-normativ sexualitet och/eller könsidentitet. Syftet med studien är att undersöka hur queera partnermigranter och deras svenska partners upplever migrationsprocessen – vilken pågår även efter att den administrativa processen är avslutad – genom att analysera de känslor som uppstår i processen. Studien är ett bidrag till forskning om så väl privilegierad migration som intim migration. I fokus står det queera partnermigrationsförhållandet och vad känslor ”gör” med detta förhållande, men också hur känslor strukturerar migrationsprocessen på olika sätt. Studien analyserar det arbete tre olika känslor – kärlek, förlust och tillhörighet – gör i migrationsprocessen och hur detta arbete beskrivs i deltagarnas narrativ. Migrantdeltagarna i studien kommer från olika delar av världen (Afrika, Europa, Latinamerika och Nordamerika), vilket gör det möjligt att analysera vad känslor gör i den här specifika migrationsprocessen utifrån nationalitet, och specifikt närhet till västerländskhet, ras och språkbakgrund samt hur privilegier kopplade till dessa positioner spelar in i processen. Avhandlingen är en etnografisk intervjustudie där både migranter och svenska partners har intervjuats. Intervjumaterialet består av en blandning av parintervjuer och enskilda intervjuer. Genom att använda affektteorier och queer fenomenologi visar avhandlingen hur det arbete känslor utför i migrationsprocesser är kopplat till könsidentitet, sexuell identitet, ras och vithet, nationalitet, upplevd närhet till västerländskhet, klass, språk och det migrationsnarrativ den migrerande partnern är inskriven i (eller inte) genom det land den migrerat från. Detta analyseras i relation till de teoretiska ramverken trassel (entanglement), homonationalism och intimt medborgarskap. I analysen framkommer att känslor strukturerar migrationsprocessen för både mer privilegierade och mindre privilegierade migranter men på olika sätt. Förståelsen av vem som ”är” en migrant och beredskapen för de känslor som uppstår i migrationsprocessen är till stor del kopplade till de positioner som nämns ovan samt de privilegier migranten har tillgång till genom dessa. Genom att fokusera på känslor och vad dessa gör visar studien också att migrationsprocessen påverkar den icke-migrerande partnern genom att denna förutsätts utföra känsloarbete för att ”göra” den migrerande partnern ”svensk.” Samtidigt blir den icke-migrerande partnern själv, genom sin migrerande partner, riktad på ett sätt som gör den lite mindre ”svensk”, vilket bidrar till att också den icke-migrerande partnerns liv ”stoppas” på sätt den vanligtvis inte tidigare upplevt. Studien visar vidare på hur migrationsprocesser producerar ojämlikhet och de svårigheter som då uppstår när paren försöker leva upp till det jämlika svenska idealförhållandet. Intervjuerna är analyserade som narrativ och både narrativ och berättande är genomgående viktiga i avhandlingen, inte bara som metod utan också som avhandlingens form och en slags struktur som organiserar texten. Skrivande (att skriva tillgängligt och intressant) och läsande (att skriva på ett sätt som inbjuder till öppet och aktivt läsande) är viktiga aspekter i avhandlingen.

Resource identification and quantification is an essential element of both classical and quantum information theory. Entanglement is one of these resources, arising when quantum communication and nonlocal operations are expensive to perform. In the first part of this thesis we quantify the effective entanglement when operations are additionally restricted to account for both fundamental restrictions on operations, such as those arising from superselection rules, as well as experimental errors arising from the imperfections in the apparatus. For an important class of errors we find a linear relationship between the usual and effective higher dimensional generalization of concurrence, a measure of entanglement. Following the treatment of effective entanglement, we focus on a related concept of nonlocality in the presence of superselection rules (SSR). Here we propose a scheme that may be used to activate nongenuinely multipartite nonlocality, in that a single copy of a state is not multipartite nonlocal, while two or more copies exhibit nongenuinely multipartite nonlocality. The states used exhibit the more powerful genuinely multipartite nonlocality when SSR are not enforced, but not when they are, raising the question of what is needed for genuinely multipartite nonlocality. We show that whenever the number of particles is insufficient, the degrading of genuinely multipartite to nongenuinely multipartite nonlocality is necessary. While in the first few chapters we focus our attention on understanding the resources present in quantum states, in the final part we turn the picture around and instead treat operations themselves as a resource. We provide our observers with free access to classical operations - ie. those that cannot detect or generate quantum coherence. We show that the operation of interest can then be used to either generate or detect quantum coherence if and only if it violates a particular commutation relation. Using the relative entropy, the commutation relation provides us with a measure of nonclassicality of operations. We show that the measure is a sum of two contributions, the generating power and the distinguishing power, each of which is separately an essential ingredient in quantum communication and information processing. The measure also sheds light on the operational meaning of quantum discord - we show it can be interpreted as the difference in superdense coding capacity between a quantum state and a classical state.

The highly parallel nature of the fundamental principles of quantum mechanics means that certain key resource-intensive tasks --- including searching, code decryption and medical, chemical and material simulations --- can be computed polynomially or even exponentially faster with a quantum computer. In spite of its remarkably fast development, the field of quantum computing is still young, and a large-scale prototype using any one of the candidate quantum bits (or 'qubits') under investigation has yet to be developed. Spin-based qubits in condensed matter systems are excellent candidates. Spins controlled using magnetic resonance have provided the first, most advanced, and highest fidelity experimental demonstrations of quantum algorithms to date. Despite having highly promising control characteristics, most physical ensembles investigated using magnetic resonance are unable to produce entanglement, a critical missing ingredient for a pure-state quantum computer. Quantum objects are said to be entangled if they cannot be described individually: they remain fundamentally linked regardless of their physical separation. Such highly non-classical states can be exploited for a host of quantum technologies including teleportation, metrology, and quantum computation. Here I describe how to experimentally create, control and characterise entangled quantum ensembles using magnetic resonance. I first explore the relationship between entanglement and quantum metrology and demonstrate a sensitivity enhancement over classical resources using molecular sensors controlled with liquid-state nuclear magnetic resonance. I then examine the computational potential of irreversible relaxation processes in combination with traditional reversible magnetic resonance control techniques. I show how irreversible processes can polarise both nuclear and electronic spins, which improves the quality of qubit initialisation. I discuss the process of quantum state tomography, where an arbitrary quantum state can be accurately measured and characterised, including components which go undetected using traditional magnetic resonance techniques. Lastly, I combine the above findings to initialise, create and characterise entanglement between an ensemble of electron and nuclear spin defects in silicon. I further this by generating pseudo-entanglement between an ensemble of nuclear spins mediated by a transient electron spin in a molecular system. These findings help pave the way towards a particular architecture for a scalable, spin-based quantum computer.

This thesis is concerned with the collective dynamics in different spin chains and spin ensembles in solid-state materials. The focus is on the manipulation of electron spins, through spin-spin and spin-photon couplings controlled by voltage potentials or electromagnetic fields. A brief review of various systems is provided to describe the possible physical implementation of the ideas, and also outlines the basis of the adopted effective interaction models. The first two ideas presented explore the collective behaviour of non-interacting spin chains with external couplings. One focuses on mapping the identical state of spin-singlet pairs in two currents onto two distant, static spins downstream, creating distributed entanglement that may be accessed. The other studies a quantum memory consisting of an array of non-interacting, static spins, which may encode and decode multiple flying spins. Both chains could effectively `enhance' weak couplings in a cumulative fashion, and neither scheme requires active quantum control. Moreover, the distributed entanglement generated can offer larger separation between the qubits than more conventional protocols that only exploit the tunnelling effects between quantum dots. The quantum memory can also `smooth' the statistical fluctuations in the effects of local errors when the stored information is spread. Next, an interacting chain of static spins with nearest-neighbour interactions is introduced to connect distant end spins. Previously, it has been shown that this approach provides a cubic speed-up when compared with the direct coupling between the target spins. The practicality of this scheme is investigated by analysing realistic error effects via numerical simulations, and from that perspective relaxation of the nearest-neighbour assumption is proposed. Finally, a non-interacting electron spin ensemble is reviewed as a quantum memory to store single photons from an on-chip stripline cavity. It is then promoted to a full quantum processor, with major error effects analysed.