The positron is the antiparticle of the electron. In general, for any particle there exists a corresponding antiparticle. The two are identical except for the charges, i.e. electric charge, leptonic number, muonic number, ..., which are equal in module but opposite sign. Examples are the electron and the positron, the first has negative electric charge, while the second has positive charge. Similarly for proton, which is positive charged, and the antiproton, which is negative charged. In the case of photons, they are their own antiparticle. When a particle and its antiparticle interact, they are destroyed in a process called annihilation which converts all their mass into energy following Einstein’s equation E=mc2. The inverse is also possible, a high-energy event creates a particle-antiparticle couple, this is called pair production. Needless to say, the products have total mass less than the one corresponding to initial energy from Einstein’s equation. For this reason, from an annihilation event, a cascade of particle-antiparticle pairs is generated, the mass of the created particle and antiparticle is less than the sum of the mass of the original particles. Still, in the annihilation process, the momentum and angular momentum of the initial particle-antiparticle system is conserved. The annihilation of a stationary positron-electron pair generates two photons. Due to the conservation of momentum, the two photons are emitted in opposite direction both with 511keV energy. The direction of emission is random. Due to their light mass, few MeV gamma-rays are capable of producing positron with pair production. In fact, the positrons are the most available antiparticle in the universe, the characteristic 511keV annihilation photons have been observed in active galactic nuclei [1], in the sun [2], and even in thunderstorm clouds on Earth [3]. The antiparticles are not easily available, the observable universe is mainly composed of matter, so any interaction would result in the annihilation. From here one of the main unanswered questions of modern physics: given the big bang was a high energy event, it should have generated matter and antimatter in equal quantity, however this symmetry is not observed in the universe around us. High-energy photons capable to produce positron-electron pairs can be generated in a controlled environment here on Earth with the use of LINACs [4] or nuclear reactor [5]. Moreover, positrons can be generated by radioisotope decay. The β^+ decay transforms a proton in a neutron in the atom nucleus, the process frees a positron, other than an electronic neutrino. This makes positrons the easiest available antiparticle and the first to be discovered and studied.In the 1920s, special relativity and quantum mechanics were two of the pillars of modern physics. One of the first attempts to combine the two was Dirac’s equation [6]. Dirac tried to explain the behavior of spin one-half particles like the electron when moving at relativistic speed, however the resulting equation admits free-particle solutions with positive and negative energies. Obviously, negative energies are not physically possible. An explanation proposed by Dirac involved a sea of particle [7]. The positive energy solution of the equation represents a particle excited from the sea, the hole left by this process corresponds to the negative energy solution. Then for any particle there is a correspond hole, called antiparticle. In the 1930s, Anderson was studying the behavior of cosmic rays interacting with a cloud chamber in the presence of a magnetic field [8]. Between the photographed particles, he demonstrated for the first time the existence if a particle with mass and charge equal in absolute value to the electron but positively charged. He called this particle positron, following studies confirmed the positron is the antiparticle of the electron. Nowadays, the positrons have found two main applications: in the medical field and in material studies. In medicine, β^+ radioisotopes are used as tracer for the individuation of cancer in patient with Positron Emission Tomography (PET). By detecting the two counterpropagating annihilation gamma-rays, it is possible to reconstruct the annihilation spot, and so the distribution of the absorption of the molecules with the radioisotope in the body. Cancerous cells have a different metabolism with respect normal cell, so their absorption of particular molecules is amplified. By selecting the correct molecular vector and radioisotope, the area in the patient body affected by the cancer is highlighted by the PET. In the case of material science, positrons are implanted in the material with energies up to tens of kiloelectronvolts. Interacting with the material, the positrons lose energy and diffuse in the material surrounding few tens of nanometers until they annihilate with an electron in the material, or they escape from the material surface. This makes the positrons a good probe because the information on the electrons transmitted outside the material by the annihilation gammas. How much time the positrons live in the material depends on the electron density. The presence of defects in the atomic structures creates spaces with lower electron density where the positron can live longer. This is studied with the Positron Annihilation Lifetime Spectroscopy (PALS). Because the positrons are generally consider thermalized at the annihilation, they have much less energy than the electron in the material. Then we can obtain information on the electron from any deviation in the direction and energy of the two annihilation photons form the case of stationary particles. The deviation in direction of the two photons is studied with Angular Correlation Annihilation Radiation (ACAR), the annihilation gammas energy with Doppler Broadening Spectroscopy (DBS). From the study of positron interaction with the matter, the bound state of the positron and the electron was discovered for the first time in the 1950s [9]. This bound state is called positronium (Ps) and it is the lightest bound matter-antimatter system. It is a hydrogen-like atom, with the positron substituting for the proton, this gives it particular properties [10]. The Ps is not stable, and, in its ground level, it is divided based on the total spin S in para- (S=0) and ortho- (S=1) positronium (p- and o- Ps, respectively) with different behaviors. Para-positronium is in a singlet spin state S=0 and m=0, where m is the projection of the spin on the z-axis. It tends to annihilate in two counterpropagating photons with 511keV energy and it has a vacuum lifetime of 125ps. Ortho-positronium corresponds to the triplet of spin states S=1 and m=-1, 0, +1. It annihilates mainly into three gamma-rays with a lifetime of 142ns in vacuum. This longer lifetime makes it possible to manipulate the o-Ps level with laser excitation [10], bringing it in longer lived levels for the study of its properties. In both the case of p-Ps and o-Ps, the conservation of energy and momentum in the annihilation fixes the gamma-rays direction and energy. For p-Ps like for free positrons, the two photons have a fixed energy and direction of one respect to the other, however the emission direction is random. The three photons resulting from the annihilation of o-Ps are emitted on a plane, called annihilation plane, with a wide range of energies and direction, the inclination of the annihilation plane is randomly distributed. In this discussion, we did not consider the conservation of the angular momentum in the annihilation process. This brings a constrain in the direction of polarization of the annihilation gamma-rays. For positronium in the ground level, the total angular momentum is given by the spin. For para-positronium and free positrons, the spin conservation means the two photons are entangled in the polarization state: the polarization of a gamma-ray is orthogonal to the other [11,12]. For ortho-positronium, the three gamma polarizations are genuinely multiparticle entangled, however the exact entanglement state depends on the emission direction of the three [13]. The correlation in the annihilation radiation of the two annihilation gammas was first experimentally studied in the 1940s [14–16]. Only a decade later, the experimental results demonstrated for the first time the existence of entanglement [17]. The entanglement in the case of three gammas has not yet been experimentally demonstrated. This is due to the complexity in the realization of a detector capable of measuring the polarization of three high-energy photon at the same time and of a source of positronium in a spin selected state in a free-field environment. This work thesis is centered on the design and study of an apparatus with the objective of study the entanglement of gamma-rays polarization generated by the annihilation of ortho-positronium. This apparatus is called PSICO (Positronium Inertial and Correlation Observations) apparatus, and it is under construction at the Antimatter laboratory (AML) of the University of Trento.At the center of the PSICO apparatus is the realization of a dense bunched positron beam capable of implanting the positrons in a positron/positronium converter in a field-free region with energy up to tens of kiloelectronvolt. No other positron beamline in the literature satisfies all these requirements. The creation of the PSICO positron beamline is based on four steps: the creation of a monoenergetic continuous positron beam, the trapping of the positrons in a buffer-gas trap (BGT) [18,19] and the generation of a dense bunched beam, the extraction of the bunched beam from the magnetic field of the trap, the acceleration, time-compression, and focalization of the positron bunches into a target in a free field region. To each step corresponds a part of the PSICO positron beamline. During this thesis work all fours parts have been designed, the last three parts are now under construction, the first part has been completed and commissioned. The creation of the monoenergetic continuous positron beam in the first step of the PSICO apparatus requires a radioactive source, a moderator, and a magnetic transport system. The design of this part of the apparatus is based on a commercial solution from First Point Scientific [20]. The source generates continuously positrons, which are emitted with a wide energy distribution. So, a moderator is required for the creation of a monoenergetic beam. The transport system guides the beam to the next section while eliminating the non-moderated positrons that are anyway emitted from the moderator. In the implementation, a sodium-22 source is used for its long half-life of 2.6 years. A solid noble gas moderator is used for the moderation process due to its high efficiency [21]. The design of the magnetic transport is based on raytracing simulation in order to optimize the speed-selection and transport of the positrons from the moderator with the minimal number of magnets. Once constructed, the first part of the PSICO apparatus can generate a continuous positron beam with up to 50 000 positrons per second with a total efficiency from the source to the end of the magnetic transport higher than 0.15%. Three solid noble gas moderators were tested: Neon, Argon, and Krypton. The advantages and disadvantages of the moderator realized with the three gases have been studied. The commissioning of the continuous positron beam has been completed by measuring the dimension of the beam spot, energy distribution, and polarization at the end of the magnetic transport system for the three gases. The measured beam dimensions are compatible with the transport simulation at the same position. The second part of the PSICO apparatus consists of the buffer-gas trap. The BGT is a modified Penning trap, where a 700G magnetic field confines radially the positrons which are accumulated in an electrostatic potential well along the magnet axis. When the positrons enter the trap, they are cooled by inelastic scattering with gas introduced into the chamber until they fall to the bottom of the potential well. The design of the PSICO BGT is based on a commercial design from First Point Scientific with changes in the magnet and terminal electron for the optimal release of the positron bunches from the BGT and their extraction from the trap magnetic field in the third stage of the PSICO beamline. From the simulation with the new trap design, the positron bunches are formed in a region with a field homogeneity ΔB\/B better than 0.1%, and the bunches are released from the trap with a temporal width less than 5ns [22].The extraction of magnetic field is done in the third step of the PSICO apparatus immediately after the trap. In the literature, a few configurations for the magnetic field extraction of positrons from the BGT have been proposed and implemented [23–25]. However, the present design is the first one where the positron extraction from the magnetic field is performed directly at the exit of the BGT. According to the simulations of our design, 60% of the positron can be extracted from the magnetic field of the trap [22].In order to perform the fourth step, a buncher-elevator followed by four lenses has been designed. After the extraction from the BGT, the positrons enter the buncher-elevator whose potential is shaped in 5.5 ns [26]. The final potential is given by a constant value superimposed by a parabolic potential. The parabolic potential has a height of 1kV at the start of the buncher-elevator and the vertex at its end, it is required for the time compression of the bunch. The constant potential value gives, instead, the majority of the implantation energy to the positrons and it can reach up to 21kV [22]. The positron bunch is then focused by the last four lenses onto the target in a spot smaller than 5mm in diameter and temporal width lower than 2ns. This will be the first positron beam from a BGT capable to operate at high implantation energy with a target in a field-free region. These four parts complete the dense bunched positron beam. For the study of the entanglement in the polarization of the o-Ps annihilation photons, a good positron/positronium converter is needed. In the literature, there are example of good converters [27,28], however they work in reflection geometry, i.e. they emit Ps on the same side where positrons are implanted. This presents some limitation in the manipulation and study of positronium in vacuum. Positron/positron converter capable of emitting Ps on the other side of the implantation can be found in the literature, however their efficiency is low [29]. So, a new kind of converter has been studied for the production of positronium in transmission [30]. It consists of silicon membranes of few microns of thickness where pass through nanochannels have been electrochemically etched. This kind of converters have shown a conversion efficiency of at least 16%. The last element needed for the entanglement measurement is the detector. This needs to be capable of measuring the polarization of the gamma-rays at the same time. The only way of measuring the polarization of hundreds of kiloelectronvolt photons is by applying the Compton scattering. For each annihilation gamma, the detector records the electron scattered from the Compton scattering and the scattered photons. The direction of the scattered photon depends on the polarization of the annihilation gammas. To reconstruct the three polarizations, the detector records six events, this requires a complex detection system. After an extensive study of literature, we are oriented into the use of modules of plastic scintillators originally developed for PET measurements by a group of the Jagiellonian University (Krakow, Poland) [31]. Two modules have been tested with the continuous positron beam from the first part of the apparatus. Just two modules were enough to reconstruct the beam spot with an uncertainty of few centimeters [31]. Using more modules, a higher precision can be obtained. Thanks to all the work done in the design, testing, and implementation of the different components of the PSICO apparatus it makes possible in the near future the realization of the first test of entanglement in the polarization of the three gamma-rays generated by annihilation of ortho-positronium.

Direct detection of dark matter remains an important outstanding problem since abundant astrophysical evidence points towards its existence, but no experiment has succeeded in detecting it. Axions and axion-like-particles are some of the most compelling candidates for dark matter given their appearance in many theories of physics beyond the Standard Model and their relatively unexplored parameter space compared to other candidates. Recently, the Cosmic Axion Spin Precession Experiment-Electric (CASPEr-e) has used nuclear magnetic resonance (NMR) to search for effective magnetic fields created by axionic dark matter. By decreasing technical noise sources, CASPEr-e is projected to reach the standard quantum limit where spin projection noise is the dominant noise source limiting sensitivity. However, some axion models predict axion couplings to normal matter that would be too small for even a quantum limited CASPEr-e experiment to detect. This creates a need for surpassing the spin projection noise limit in NMR dark matter searches. In this thesis, I explore the prospects for surpassing the quantum limit in NMR by using spin squeezed states, entangled states with variance in one projection reduced below the standard quantum limit. First, I propose an experimental scheme for generating squeezed states by coupling the spins to an off-resonant circuit to create a One-Axis-Twist Hamiltonian. Then, using exact results and numerical simulations, I determine the amount of squeezing that can be achieved given decoherence and noise. Next, I perform modeling to show that squeezing can accelerate dark matter searches despite earlier results that argued squeezing cannot improve experimental sensitivity when subject to decoherence. Finally, I apply these results to the CASPEr-e experiment and show that at axion frequencies near 100MHz, squeezing can speed up the experiment by a factor of up to 30, corresponding to a sensitivity improvement by a factor of over 5.

Quantum communication protocols require maximally entangled state of pair of qubits (spin-1/2 states in this context) to be shared between sender and the receiver. The entangled qubits lose entanglement because of random magnetic field disturbances. The dynamics in the form of joint density matrix of random pure entangled state provide the steady (joint) state and the associated timescales (time taken by the pair to reach the steady state) providing a scope in future to quantify the effective utilization of quantum communication protocols. / The dynamics of an identical pair of entangled spin-1/2 particles, both subjected to the identical, independent, correlated random magnetic fields is studied. The dynamics of the pure joint state of the pair is derived using stochastic calculus. In case of identical fields, an ensemble of such pure states are combined using the modified spin joint density matrix and the joint relaxation time is obtained for the pair of spin-1/2 particles. These dynamics can be interpreted as special kind of correlations involving the spatial components of the Bloch polarization vectors of the constituent entangled spin-1/2 particles. In case of independent random magnetic fields, the dynamics are obtained by considering a pure joint state of entangled spin-1/2 particles. The disentanglement time defined as the time taken for the particles to become disentangled, is obtained. In case of correlated random magnetic fields, the dynamics of a maximally entangled pair of spin-1/2 particles are derived in terms of the joint density matrix of the entangled pair from which the steady state density matrix and the associated timescale for it to be reached are obtained. The asymptotic density matrix in this case represents a state of (partial) disentanglement. In other words, there is a persistent entanglement in case of correlated field disturbances. / Thesis / Doctor of Philosophy (PhD) / Maximally entangled pair of quantum bits (in the form of spin-1/2 states) lose entanglement either partially or completely depending upon the nature of random magnetic field disturbances around them (correlated/independent/identical fields). The dynamics of entangled states (in the form of density matrix of a random pure state) in the presence of random magnetic fields are obtained using the ideas of stochastic calculus to understand the steady state of the pair and the associated timescales to be reached.

No description available.

<p>The most important results in this thesis are those concerned with the levelling of a stepped film’s height profile. Films are prepared such that their height profiles are well described by a Heaviside step function and to a good approximation, they are invariant in one dimension. The temporal dependence of the levelling gives rheological information about the molecules making up the stepped films. For the range of heights that is much larger that the typical size of molecules making up the film, we use classical hydrodynamics to model the flows in these stepped films. Having measured the temporal and geometric dependence of the energy dissipation in time, we find that the hydrodynamic models are in excellent agreement.</p> / Doctor of Philosophy (PhD)

In this dissertation, applications of thermodynamics at the macroscopic and quantum levels of description are developed. Within the macroscopic level, an upper-level Sustainability Assessment Framework (SAF) is proposed for evaluating the sustainable and resilient synthesis/design and operation of sets of small renewable and non-renewable energy production technologies coupled to power production transmission and distribution networks via microgrids. The upper-level SAF is developed in accord with the four pillars of sustainability, i.e., economic, environmental, technical and social. A superstructure of energy producers with a fixed transmission network initially available is synthesized based on the day with the highest energy demand of the year, resulting in an optimum synthesis, design, and off-design network configuration. The optimization is developed in a quasi-stationary manner with an hourly basis, including partial-load behavior for the producers. Since sustainability indices are typically not expressed in the same units, multicriteria decision making methods are employed to obtain a composite sustainability index. Within the quantum level of description, steepest-entropy-ascent quantum thermodynamics (SEA-QT) is used to model the phenomenon of decoherence. The two smallest microscopic composite systems encountered in Nature are studied. The first of these is composed of two two-level-type particles, while the second one is composed of a two-level-type particle and an electromagnetic field. Starting from a non-equilibrium state of the composite and for each of the two different composite systems, the time evolution of the state of the composite as well as that of the reduced and locally-perceived states of the constituents are traced along their relaxation towards stable equilibrium at constant system energy. The modeling shows how the initial entanglement and coherence between constituents are reduced during the relaxation towards a state of stable equilibrium. When the constituents are non-interacting, the initial coherence is lost once stable equilibrium is reached. When they are interacting, the coherence in the final stable equilibrium state is only that due to the interaction. For the atom-photon field composite system, decoherence is compared with data obtained experimentally by the CQED group at Paris. The SEA-QT method applied in this dissertation provides an alternative and comprehensive explanation to that obtained with the "open system" approach of Quantum Thermodynamics (QT) and its associated quantum master equations of the Kossakowski-Lindblad-Gorini-Sudarshan type. / Ph. D.

Step-growth polymerization enabled the synthesis of novel polyester regioisomers and pendant functionalized polyetherimides (PEI)s. Novel monomers incorporated at targeted mol % produced series of polyesters and PEIs, suitable for systematic analysis of key polymer properties. Subsequent compositional, thermal, mechanical, and rheological characterization forged structure-property relationships to further understand the influence of composition on performance. Altering regiochemistry is a subtle way to maintain the same polymer composition but tune desired properties. Similarly, introducing functional pendant groups expands the property profile of common industrial polymers and installs a handle for secondary chemistry after synthesizing the main polymer. Both altering regiochemistry and adding pendant groups alters polymer properties without the need for large changes in synthetic requirements or reaction conditions, ideal for industrial adoption. Incorporation of a kinked bibenzoate (BB)-based diester monomers into the commonly utilized linear regioisomer afforded processable amorphous and semi-aromatic (co)polyesters. BB-(co)polyesters with ethylene glycol (EG) possessed improved barrier performance compared to poly(ethylene terephthalate) (PET) while improving on mechanical properties, including tensile and flexural modulus/strength, rivaling bisphenol-A polycarbonate (BPA-PC). Replacement of EG with 1,4-cyclohexanedimethanol (CHDM) improved thermal properties closer to BPA-PC, while enabling melt rheological analysis due to its amorphous morphology. Time-temperature superposition (TTS) analysis produced master curves provided insight into the entanglement molecular weight (Me) and entanglement density. More kinked structures possessed a lower Me and more entanglements. Introducing kinked monomers posed the question of cyclic speices generation during polymerization, common in step-growth reactions. Thus, systematic incorporation of meta-substituted hydroxyethylresorcinol and para-substituted hydroxyethylhydroquinone regioisomers into PET analogues enabled the characterization of cyclic formation due to monomer regioisomers. Increased meta substitution produced increased amounts of cylic species, analyzed by size exclusion chromatography (SEC). Adding functionality to high performance polyetherimides (PEI)s is difficult due to the high temperatures required for processing. The lack of thermal stability for commonly utilized H-bonding/reactive groups limits viable moieties. Utilizing the high temperture processing, PEIs incorporating pendant carboxylic acids reacted in the melt to form branched PEIs. These branched PEIs exhibited steeper shear thinning as well as improved flame resistance, limited in thin film commercial PEIs. / Doctor of Philosophy / My research focused on making new plastics (polymers) for use in consumer and performance markets. Typical applications utilizing these plastics include food packaging, consumer goods, automotive, aerospace, microelectronics, construction, and medical devices. Large changes such as intricate new chemicals used to make the plastics increase the difficulty in incorporating these new materials into existing synthesis and processing techniques and infrastructure. Thus, my research revolved around subtle changes to the chemical structure of the plastic, suitable for easy industrial adoption while also improving targeted properties necessary for the aforementioned applications. Polyesters are a class of polymers commonly used for food packaging and consumer goods. Thus, improving gas barrier performance and mechanical integrity/strength is crucial when designing new polyesters. Changing the bond angles through the linear versus kinked nature of the polymer chain imparts processability and improved gas barrier, compared to commercial poly(ethylene terepthalate) (PET), commonly used in food packaging applications. The density of the polyesters is also increased, which improves mechanical strength. The specific structures used also increased the thermal resistivity compared to PET. This higher thermal resistivity enables use in applications where high temperature cleaning such as steam sterilization and dish-washing could deform products or processing such as filling food packaging containers with hot foods. Similar types of polymers which possess much higher thermal resistivity are classified as high performance polymers. One class of these include polyetherimides (PEIs). The specific chemical structures and their high thermal resistance makes them great candidates for applications in automotive, aerospace, and microelectronic applications; although, these same properties make these polymers very difficult and expensive to process into the desired parts. Thus, adding functionality to the polymer by putting specific chemical groups off of the main chain enabled easier processing and improved other polymer properties. Adding the functionality to these polymers allowed them to react and change structure at high temperatures (during processing) to achieve a different shape, thus improving desired properties, such as how easy they flow like liquids at high temperatures and processing conditions. Another benefit realized from this change during processing was the improvement of flame resistance. Due to the chemical structure of the PEIs, they inherently possess resistance to catching on fire, remaining on fire, and dripping flaming material. Although PEIs typically possess good flame resistance, thin films or small parts made from these polymers do not possess the same flame resistance and can produce flaming drips, undesirable for applications requiring flame resistance. Chemically modifying these polymers with the aforementioned functionality and processing them increased the flame resistance to eliminate flaming drips and lessen the amount of time the polymer was on fire.

No / A system consisting of two Josephson qubits coupled through a quantum monochromatic electromagnetic field mode of a resonant tank circuit is studied. It is shown that for certain values of the parameters, it can be used as an entangling gate, which entangles the two qubits whilst the electromagnetic field remains disentangled. The gate operates with decent fidelity to a gate and could form the basis for initial experimental investigations of coupled superconducting qubits.

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.