School of Physics - Theses
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ItemTowards non-invasive quantum imaging of neuronal activity using optically-active spins in diamond( 2015)Resolving the biological neural network dynamics of the brain with subcellular spatial resolution remains a significant ongoing challenge. It has often been the case, however, that difficult problems in neuroscience have been illuminated by techniques developed at the intersection of various disciplines. This thesis focuses on progress towards such a technique, a nanoscale magnetic sensor for imaging neural networks that functions under physiological conditions. The nitrogen-vacancy (NV) centre defect in biocompatible diamond shows promise due to its nanoscale resolution, sensitivity, stable fluorescence, and room temperature operation. Although there is interest in using the NV centre as a non-invasive magnetic sensor of neural network activity, NV-based sensing techniques are ultimately limited by sources of magnetic noise that destroy the quantum phase coherence these techniques require for their operation. While there are several ways to improve the sensitivity of NV sensors, this thesis investigates the control settings needed to complement the ongoing improvements in diamond quality. We seek to reduce the control errors inherently present in NV sensing protocols: such errors are found to be reduced in the presence of sufficiently large magnetic and microwave fields, although it remains difficult to quantify these errors without accounting for hyperfine NV interactions. The long-term feasibility of NV sensors will be determined by their sensitivity to magnetic fields as well as the magnitude of magnetic fields that individual neurons generate. The external electric potentials generated by individual primary cortical neurons in mice are measured using multi-electrode arrays (MEAs), the prevailing non-invasive technology used to detect electric signals in neural networks. Signals detected by the MEAs are then converted into an equivalent magnetic field, which is found to be in the picotesla range. Although this estimate is lower than the best reported NV sensitivities to date, the actual fields are likely to be larger since MEAs can only detect the extracellular contribution to the magnetic field. This thesis complements existing progress towards realising the long-term goal of a wide-field NV neuron sensor with electrical co-recordings, and suggests that advances in control protocols and material quality may yet be needed to improve the overall sensitivity required to detect activity in neural networks.
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ItemDevelopment of computational relativistic atomic theory with application to the oxygen spectra in astrophysical systems( 2015)Over many decades, atomic spectroscopy has repeatedly proven to be one of the most widely used applications across various different fields as it allows the detection, identification, and quantification of chemical species present within a particular sample. This is particularly true in the studies of astrophysics and cosmology. This thesis includes the development of new codes for the calculation of the electron self-energy – a quantum electrodynamic effect – as part of the relativistic atomic structure package grasp2k . The codes are shown to bring results towards better agreement with experimental as well as some of the most advanced atomic theories to date. Moreover, a review into the Breit interaction was conducted to address claims that the method of implementation can affect the overall results as well as the quality of gauge convergence. The theoretical development was used to study two oxygen forbidden lines that feature prominently in the aurora as observed on Earth, namely the green and ultraviolet lines with nominal wavelengths of 557.7nm and 297.2nm, respectively. Many have researched into these forbidden lines either through observation of the aurora, or experimentally measured in the laboratory, or theoretical calculations, dating back to the 1930s. Despite such tremendous effort spanning many decades, major discrepancies between observation, measurement, and theory remain. We also explore other optically allowed transitions of ionised oxygen, which showed great consistencies in our methodology. Fully relativistic atomic theories are employed for the first time in the calculation of these particular lines, with excellent overall results and convergence.
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ItemMatrix product operator simulations of quantum algorithms( 2015)We develop simulation methods for matrix product operators, and perform simulations of the Quantum Fourier Transform, Shor’s algorithm and Grover’s algorithm using matrix product states and matrix product operators. By doing so, we provide numerical evidence that a constant number of QFTs can be efficiently classically simulated on any state whose Schmidt rank grows only polynomially with the number of qubits, and quantify the amount of entanglement present in Shor’s algorithm. The efficiency of the matrix product state and operator representation allows us to perform moderately large simulations of both Shor’s algorithm with Z errors and Grover’s algorithm with up to 15 X, Y and Z errors. While larger simulations have been performed, our results have been computed with little computational power and provide new methods to perform large-scale quantum algorithm simulations.
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ItemSimulation of noise properties of the Murchison Wide-field Array( 2015)This thesis is focused on better understanding the origins of the noise in an image taken with the Murchison Wide-field Array (MWA). To investigate this, we process images of fields being used for Epoch of Reionization experiments, and use simulations to characterize the expected noise. For our simulation, we use the MIT Array Performance Simulator (MAPS), which was developed at the MIT Haystack Observatory in 2001. MAPS provides a flexible tool for a new generation of simulated data from low frequency radio telescopes. Here we use MAPS to model the performance of the prototype 32 Tiles instrument of the MWA (MWA−32T) and 128 Tiles (MWA−128T). To characterize the noise in the simulation, we need a good astronomical catalogue to provide input sources for the MAPS simulation. By matching source counterparts between the sources observed by the MWA, and the sources in different catalogues, we found that the best catalogue for simulating the model of the sky with respect to the frequency and the flux ranges of the observed sources is the PKSCAT90. In our initial simulation we assume that the sky is dominated by thermal noise. However, we also find noise in the image, which indicates that additional sources of noise are present. We use a counts model to estimate the numbers of unresolved sources in our simulation to better simulate the level of noise in the image. Using number counts of extragalactic radio sources, we find that these are the most dominant noise component. These extragalactic radio sources are usually unresolved at the angular resolution of the observations, and will cause a contamination for detecting the 21-cm line of the neutral hydrogen. In our simulations for the MWA−32T and MWA−128T, we use the EoR1 field, without the presence of resolved sources (FornaxA). Resolved sources can cause errors in the flux densities of the image, and decrease the accuracy of estimated noise in the image.
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ItemThe effects of LO and NLO QCD corrections on the calculations of the CP violation observables S and C for the non-leptonic decays Bod → J/ΨKs and Bod→ ΦKs in the Standard Model and in the General Left-Right Symmetric Model( 2015)For full abstract please refer to abstract file.