1,218 research outputs found

    Precision Surface Processing and Software Modelling Using Shear-Thickening Polishing Slurries

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    Mid-spatial frequency surface error is a known manufacturing defect for aspherical and freeform precision surfaces. These surface ripples decrease imaging contrast and system signal-to-noise ratio. Existing sub-aperture polishing techniques are limited in their abilities to smooth mid-spatial frequency errors. Shear-thickening slurries have been hypothesised to reduce mid-spatial frequency errors on precision optical surfaces by increasing the viscosity at the tool-part interface. Currently, controlling the generation and mitigating existing mid-spatial frequency surface errors for aspherical and freeform surfaces requires extensive setup and the experience of seasoned workers. This thesis reports on the experimental trials of shear-thickening polishing slurries on glass surfaces. By incorporating shear-thickening slurries with the precessed bonnet technology, the aim is to enhance the ability of the precessions technology in mitigating mid-spatial frequency errors. The findings could facilitate a more streamlined manufacturing chain for precision optics for the versatile precessions technology from form correction and texture improvement, to MSF mitigation, without needing to rely on other polishing technologies. Such improvement on the existing bonnet polishing would provide a vital steppingstone towards building a fully autonomous manufacturing cell in a market of continual economic growth. The experiments in this thesis analysed the capabilities of two shear-thickening slurry systems: (1) polyethylene glycol with silica nanoparticle suspension, and (2) water and cornstarch suspension. Both slurry systems demonstrated the ability at mitigating existing surface ripples. Looking at power spectral density graphs, polyethylene glycol slurries reduced the power of the mid-spatial frequencies by ~50% and cornstarch suspension slurries by 60-90%. Experiments of a novel polishing approach are also reported in this thesis to rotate a precessed bonnet at a predetermined working distance above the workpiece surface. The rapidly rotating tool draws in the shear-thickening slurry through the gap to stiffen the fluid for polishing. This technique demonstrated material removal capabilities using cornstarch suspension slurries at a working distance of 1.0-1.5mm. The volumetric removal rate from this process is ~5% of that of contact bonnet polishing, so this aligns more as a finishing process. This polishing technique was given the term rheological bonnet finishing. The rheological properties of cornstarch suspension slurries were tested using a rheometer and modelled through CFD simulation. Using the empirical rheological data, polishing simulations of the rheological bonnet finishing process were modelled in Ansys to analyse the effects of various input parameters such as working distance, tool headspeed, precess angle, and slurry viscosity

    Advanced techniques for continuous-variable quantum communications over the atmosphere

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    This thesis analyses the application of various techniques to enhance the free-space transmission of Continuous-Variable (CV) quantum communications via the atmosphere. The techniques studied encompass a wide range of methods, from classical techniques to entanglement distillation and quantum error correction. A new realistic model of the atmospheric quantum channel is constructed. This model simulates the detrimental effects incurred on quantum information as it traverses the atmosphere. The model allows us to determine the feasibility of satellite-based quantum communications and develop new techniques to enhance free-space CV quantum communication. Entanglement distillation via non-Gaussian operations is analyzed to enhance Quantum Key Distribution and quantum teleportation in satellite-based quantum communications. While many non-Gaussian states exist, their use to obtain an advantage in any quantum communications protocol depends on the specifics of the quantum state and the channel involved in the quantum communications. Determination of which non-Gaussian states and the conditions in which such an advantage can be obtained in the context of free-space transmission is one of the contributions of this thesis. In satellite-based communications, the uplink channel is considerably more destructive than the downlink channel. A new technique for uplink state transfer that improves transmission by employing quantum teleportation via the downlink channel is introduced in this thesis. In line with the theme of this thesis, the enhancement of this technique using non-Gaussian entangled states during quantum teleportation is also analyzed. Finally, a protocol to perform error correction applied to the free-space transmission of quantum information is presented. In this protocol, quantum information transfer can be augmented by carefully monitoring the free space channel and following an optimization process. This thesis provides novel and significant developments that can be applied to advance CV quantum communications through the atmosphere for satellite-based and ground-level horizontal communications. Such developments should prove beneficial for realizing the future global quantum internet

    Deciphering Radio Emission from Solar Coronal Mass Ejections using High-fidelity Spectropolarimetric Radio Imaging

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    Coronal mass ejections (CMEs) are large-scale expulsions of plasma and magnetic fields from the Sun into the heliosphere and are the most important driver of space weather. The geo-effectiveness of a CME is primarily determined by its magnetic field strength and topology. Measurement of CME magnetic fields, both in the corona and heliosphere, is essential for improving space weather forecasting. Observations at radio wavelengths can provide several remote measurement tools for estimating both strength and topology of the CME magnetic fields. Among them, gyrosynchrotron (GS) emission produced by mildly-relativistic electrons trapped in CME magnetic fields is one of the promising methods to estimate magnetic field strength of CMEs at lower and middle coronal heights. However, GS emissions from some parts of the CME are much fainter than the quiet Sun emission and require high dynamic range (DR) imaging for their detection. This thesis presents a state-of-the-art calibration and imaging algorithm capable of routinely producing high DR spectropolarimetric snapshot solar radio images using data from a new technology radio telescope, the Murchison Widefield Array. This allows us to detect much fainter GS emissions from CME plasma at much higher coronal heights. For the first time, robust circular polarization measurements have been jointly used with total intensity measurements to constrain the GS model parameters, which has significantly improved the robustness of the estimated GS model parameters. A piece of observational evidence is also found that routinely used homogeneous and isotropic GS models may not always be sufficient to model the observations. In the future, with upcoming sensitive telescopes and physics-based forward models, it should be possible to relax some of these assumptions and make this method more robust for estimating CME plasma parameters at coronal heights.Comment: 297 pages, 100 figures, 9 tables. Submitted at Tata Institute of Fundamental Research, Mumbai, India, Ph.D Thesi

    Gyroscopic polynomials

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    Gyroscopic alignment of a fluid occurs when flow structures align with the rotation axis. This often gives rise to highly spatially anisotropic columnar structures that in combination with complex domain boundaries pose challenges for efficient numerical discretizations and computations. We define gyroscopic polynomials to be three-dimensional polynomials expressed in a coordinate system that conforms to rotational alignment. We remap the original domain with radius-dependent boundaries onto a right cylindrical or annular domain to create the computational domain in this coordinate system. We find the volume element expressed in gyroscopic coordinates leads naturally to a hierarchy of orthonormal bases. We build the bases out of Jacobi polynomials in the vertical and generalized Jacobi polynomials in the radial. Because these coordinates explicitly conform to flow structures found in rapidly rotating systems the bases represent fields with a relatively small number of modes. We develop the operator structure for one-dimensional semi-classical orthogonal polynomials as a building block for differential operators in the full three-dimensional cylindrical and annular domains. The differential operators of generalized Jacobi polynomials generate a sparse linear system for discretization of differential operators acting on the gyroscopic bases. This enables efficient simulation of systems with strong gyroscopic alignment

    Assembling Single RbCs Molecules with Optical Tweezers

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    Optical tweezer arrays are useful tools for manipulating single atoms and molecules. An exciting avenue for research with optical tweezers is using the interactions between polar molecules for quantum computation or quantum simulation. Molecules can be assembled in an optical tweezer array starting from pairs of atoms. The atoms must be initialised in the relative motional ground state of a common trap. This work outlines the design of a Raman sideband cooling protocol which is implemented to prepare an 87-Rubidium atom in the motional ground state of an 817 nm tweezer, and a 133-Caesium atom in the motional ground state of a 938 nm tweezer. The protocol circumvents strong heating and dephasing associated with the trap by operating at lower trap depths and cooling from outside the Lamb-Dicke regime. By analysing several sources of heating, we design and implement a merging sequence that transfers the Rb atom and the Cs atom to a common trap with minimal motional excitation. Subsequently, we perform a detailed characterisation of AC Stark shifts caused by the tweezer light, and identify several situations in which the confinement of the atom pair influences their interactions. Then, we demonstrate the preparation of a molecular bound state after an adiabatic ramp across a magnetic Feshbach resonance. Measurements of molecular loss rates provide evidence that the atoms are in fact associated during the merging sequence, before the magnetic field ramp. By preparing a weakly-bound molecule in an optical tweezer, we carry out important steps towards assembling an array of ultracold RbCs molecules in their rovibrational ground states

    Analog Photonics Computing for Information Processing, Inference and Optimisation

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    This review presents an overview of the current state-of-the-art in photonics computing, which leverages photons, photons coupled with matter, and optics-related technologies for effective and efficient computational purposes. It covers the history and development of photonics computing and modern analogue computing platforms and architectures, focusing on optimization tasks and neural network implementations. The authors examine special-purpose optimizers, mathematical descriptions of photonics optimizers, and their various interconnections. Disparate applications are discussed, including direct encoding, logistics, finance, phase retrieval, machine learning, neural networks, probabilistic graphical models, and image processing, among many others. The main directions of technological advancement and associated challenges in photonics computing are explored, along with an assessment of its efficiency. Finally, the paper discusses prospects and the field of optical quantum computing, providing insights into the potential applications of this technology.Comment: Invited submission by Journal of Advanced Quantum Technologies; accepted version 5/06/202

    ICEBEAR-3D: An Advanced Low Elevation Angle Auroral E region Imaging Radar

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    The Ionospheric Continuous-wave E region Bistatic Experimental Auroral Radar (ICEBEAR) is an auroral E~region radar which has operated from 7 December 2017 until the September 2019. During the first two years of operation, ICEBEAR was only capable of spatially locating E~region scatter and meteor trail targets in range and azimuth. Elevation angles were not determinable due to its East-West uniform linear receiving antenna array. Measuring elevation angles of targets when viewing from low elevation angles with radar interferometers has been a long standing problem. Past high latitude radars have attempted to obtain elevation angles of E~region targets using North-South baselines, but have always resulted in erroneous elevation angles being measured in the low elevation regime (0° to ≈30° above the horizon), leaving interesting scientific questions about scatter altitudes in the auroral E~region unanswered. The work entailed in this thesis encompasses the design of the ICEBEAR-3D system for the acquisition of these important elevation angles. The receiver antenna array was redesigned using a custom phase error minimization and stochastic antenna location perturbation technique, which produces phase tolerant receiver antenna arrays. The resulting 45-baseline sparse non-uniform coplanar T-shaped array was designed for aperture synthesis radar imaging. Conventional aperture synthesis radar imaging techniques assume point-like incoherent targets and image using a Cartesian basis over a narrow field of view. These methods are incompatible with horizon pointing E~region radars such as ICEBEAR. Instead, radar targets were imaged using the Suppressed Spherical Wave Harmonic Transform (Suppressed-SWHT) technique. This imaging method uses precalculated spherical harmonic coefficient matrices to transform the visibilities to brightness maps by direct matrix multiplication. The under sampled image domain artefacts (dirty beam) were suppressed by the products of differing harmonic order brightness maps. From the images, elevation and azimuth angles of arrival were obtained. Due to the excellent phase tolerance of ICEBEAR new light was shed on the long standing low elevation angle problem. This led to the development of the proper phase reference vertical interferometry geometry, which allowed horizon pointing radar interferometers to unambiguously measure elevation angles near the horizon. Ultimately resulting in accurate elevation angles from zenith to horizon

    The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry

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    The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations

    Caractérisation de l'amortissement des structures complexes par la méthode de corrélation

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    Abstract : The thesis presents inverse correlation techniques able to measure accurately the damping loss factor of complex plane structures. In the first chapter, the state of the art gathering numerous local and global characterization methods is presented. In the second part of the chapter, various topics of direct interest to the thesis such as classical damping loss factor measurement techniques and the analytical solution based on the discrete general laminate model (GLM) are briefly discussed. In the second chapter, the inhomogeneous wave correlation (IWC) method based on the maximization of the correlation between an inhomogeneous wave and the measured displacement field as a function of the wave heading angle is revisited. A new variant that considers the exponential decay with distance from the excitation point in the inhomogeneous wave formulation is introduced. The purpose of introducing this variant is to improve the estimation of the damping loss factor. The validity of the proposed method is investigated numerically on flat thin structures and sandwich damped structures. The performance of the method related to the excitation point location and the size of the observation window are also investigated. A new Green's function-based model correlation (GFC) method able to estimate the equivalent elastic parameters of complex structures at different propagation angles is detailed in the third chapter. Contrary to the IWC method, the measured displacement field is correlated with a Green's function-based model. This approach is more adapted to describe the field near the excitation point and offers more stability in estimating the damping loss factor compared to previous methods. Several results, with simulated and measured data, are compared with an analytical discrete laminate model and show the accuracy of this technique to recover the damping loss factor of complex structures as function of the frequency and the heading angle. In the second part of the chapter, the impact of different excitation location on the estimation of the wavenumber and the damping loss factor is investigated. A spatial angular filter to rectify the estimation of the damping loss factor is introduced. In the fourth chapter, the image source method with an objective of improving the previous GFC method in the low frequency range and for lightly damped structures is introduced. The proposed method takes into account the reflection at boundaries which is ignored in the free field Green's function used in the previous chapter. The performance of the method is investigated for two types of boundary conditions: simply supported and free edges. The identified parameters of the numerical simulations are compared to the previous GFC method and to the analytical discrete laminate model.Le travail de thèse porte sur la caractérisation de l'amortissement des structures complexes par la méthode de corrélation. Dans le premier chapitre, un état de l'art rassemblant de nombreuses méthodes de caractérisations locales et globales est présenté. Dans la deuxième partie du chapitre, les méthodes de mesures expérimentales de l'amortissement et un modèle analytique de référence sont abordés. Dans le deuxième chapitre, la methode Inhomogeneous Wave Correlation (IWC) qui calcule la corrélation entre le champ de déplacement mesuré et une onde plane inhomogène est revisitée. Une nouvelle variante qui considère la décroissance exponentielle avec la distance du point d'excitation dans la formulation d'onde inhomogène est introduite. L'introduction de cette variante a pour but d'améliorer l'estimation de l'amortissement. La validité de la méthode proposée est étudiée numériquement sur des structures planes avec différents degrés de complexité. Les performances de la méthode en fonction la position du point d'excitation et de la taille de la fenêtre d'observation sont également étudiées. Un nouveau modèle de corrélation basé sur la fonction de Green permettant d'estimer les paramètres élastiques équivalents des structures complexes en fonction de l'angle de propagation est détaillé dans le troisième chapitre. Contrairement à la méthode IWC mentionnée ci-dessus, le champ de déplacement mesuré est désormais corrélé avec un modèle basé sur la fonction de Green. Cette dernière est plus adaptée pour décrire le champ proche du point d'excitation et offre plus de stabilité sur l'estimation de l'amortissement comparée aux méthodes précédentes. Plusieurs résultats, avec des données simulées et mesurées, sont comparés au modèle analytique et montrent la précision de cette technique pour estimer précisément l'amortissement des structures complexes en fonction de la fréquence et de l'angle de propagation des ondes. Dans la deuxième partie du chapitre, la performance de la méthode sur l'estimation de l'amortissement en fonction des différents points d'excitation est également étudiée et un filtre angulaire spatial est introduit pour améliorer le résultat. Dans le quatrième chapitre, la méthode des sources images qui a pour objectif d'améliorer l'estimation de l'amortissement en basses fréquences des structures faiblement amorties est introduite. Cette approche prend en compte les réflexions des ondes de flexion aux frontières. La performance de la méthode est étudiée sur deux types de conditions limites : bords simplement appuyés et bords libres. Le résultat est comparé à la méthode introduite dans le troisième chapitre ainsi qu'au modèle analytique GLM
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