School of Physics - Theses
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ItemQuasar Broad Emission Line Regions and Gravitational Microlensing( 2023-08)This thesis has focussed on hydrogen and helium emission line generation in the Broad Emission Line Region (BELR) of quasars and also on the gravitationally lensed quasar LBQS1009-0252. Quasars are a class of galaxy characterised by an active nuclear region in which a high rate of accretion onto a central supermassive black hole results in the release of vast amounts of broadband energy over a prolonged period of time. These incredible luminosities—often up to 100,000 times that of a standard galaxy—make quasars some of the most distant, and therefore earliest, sources ever observed in the universe. The relatively tiny size of the innermost region, however, precludes direct observation, and so the physics and geometry of quasars remains enigmatic. Emitted by the accretion disk, light interacts with surrounding gasses in the BELR—so-named because light which is reprocessed and re-emitted from this region tends to be significantly broadened due to wholescale Doppler motions of the gas. As the first gas to ‘see’ light from the accretion disk, understanding and constraining the nature and dynamics of the BELR remains of interest in many astrophysical fields. Following methods pioneered by Ruff (2012), microphysical simulations (such as those produced by the photoionisation code, Cloudy) can be combined with observed spectral data of real sources to model hydrogen line emission from the BELR. This research has confirmed and built further upon the methods presented in Ruff (2012) through the use of updated code and new, high-quality NIR spectral data of 14 quasar sources acquired from the Flamingos-2 (F2) instrument on Gemini South at the Gemini Observatory. The thesis has also gone a step beyond the original method and includes the modelling and analysis of helium emission lines for the first time. Broadly, the results lend support to the conclusions presented in Ruff (2012): hydrogen lines tend to be produced optimally in regions of low incident ionising flux and high gas number density. Helium lines also appear to follow this trend, clustering in a similar parameter space, although there appears to be a tendency towards a flatter distribution across the value for maximum gas number density. This suggests that despite similarities in their physical production and spectral appearance, there are some slight differences in the behaviour of hydrogen and helium emission lines in the BELR, and it is prudent to analyse them separately where possible. This thesis also examined the double-image gravitationally lensed quasar, LBQS1009-0252, via new and relatively high-resolution data from the Gemini Multi-Object Spectrograph (GMOS) at Gemini Observatory. The project investigated both the emission and absorption lines present in the spectra of components A and B, seeking to better understand the spectral differences apparent between the two images. The analysis confirms that the LBQS1009-0252 system likely consists of two images of the same, gravitationally lensed background quasar source, with a third component, LBQS1009-0252C, most likely a foreground and physically unrelated quasar. A comparison and analysis of the overall spectra and emission lines attempts to separate the effects of different light paths through the lensing galaxy, previously identified at z~0.869. A combination of differential extinction due to dust in the macrolens plus a minor component of microlensing is a reasonable explanation for the origin of chromaticity between the spectra of the two component images. The new dataset was of high resolution, allowing for the identification of many more absorption lines than had previously been catalogued. These have been matched to those previously classified as belonging to the lensing galaxy and another known absorber situated at z~1.627. Newly-observed lines were analysed to identify likely absorption species candidates, showing that the presence of at least one more intervening absorption system at z~1.116–1.117 is highly likely.
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ItemMechanisms of ergodicity breaking - from time crystals to Hilbert space fragmentation( 2023-11)The concept of ergodicity reaches back to Boltzmann’s attempts at formalizing and clarifying the foundations of statistical mechanics. The postulate of thermal equilibrium seems to describe the behavior of sufficiently complex real-world systems, whether they are classical or quantum, exceedingly well. However, our understanding of the fundamental laws of physics that govern the dynamics of these systems on a microscopic scale can at first glance seem incompatible with this postulate. A complete description of the dynamical process of thermalization is subject of ongoing research even more than a century after Boltzmann’s contribution. Significant advances have been made, including a formal description of this process in the context of closed quantum systems that is linked to properties of its energy eigenstates. Besides deepening our understanding of what seems to be quite generic behavior, the possibility of robust exceptions to the fundamental postulates of thermodynamics have been unveiled as a consequence. Among them are potentially novel phases of matter, such as many-body localization, time crystals and Hilbert space fragmentation, that do not readily fit within the usual frameworks of phase transitions and may even find application in quantum technologies. In this thesis, I discuss the current state of research in regards to thermalization in quantum systems and some of the presumable exceptions. I then present my own contribution to the field in the form of published works on time crystals and fragmentation. In particular, I demonstrate the realization of a discrete time crystal on a quantum computer, a rapidly evolving technology at the forefront of quantum information science that promises to revolutionize our ability to model and simulate quantum systems. The demonstration of a novel phase of matter in the form of a time crystal represents one of the first successful applications of a quantum device in the field of quantum many-body physics. Furthermore, I present results on interaction-induced fragmentation that address questions about the relationship between localization and fragmentation. They also demonstrate possible transitions between different dynamical regimes within a fragmented system based on tunable emergent conserved charges. Lastly, I present work on the robustness of Hilbert space fragmentation under more general perturbations than are typically considered in other works. Introducing a new definition of block inverse participation ratio, I show that fragmentation may constitute a finite regime within the space of Hamiltonians with a possible phase transition into ergodic dynamics, with the block inverse participation ratio serving as the order parameter.
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ItemHydrogen in the first billion years: a study of the 21-cm signal from the high-redshift Universe( 2024-01)The history of our Universe is reflected in the state of its hydrogen (HI) atoms. After recombination (redshift z ~ 1000), the intergalactic medium (IGM) is composed primarily of neutral hydrogen (HI). The formation of the first stars and the first galaxies in the early Universe during Cosmic Dawn (z ~ 30) triggered the last major phase change of the HI. During the Epoch of Reionisation (EoR), the intense ultraviolet (UV) and X-ray radiation emitted by the first luminous sources carve out ionised hydrogen (HII) bubbles in the IGM. These HII bubbles expand and fill the whole Universe (z ~ 5). By altering the thermal and ionisation state of the IGM, the EoR directly impacts the subsequent formation and evolution of galaxies in the Universe. The 21-cm hyperfine spin-flip of HI is the primary probe of this period, and dedicated observational campaigns are ongoing/under construction to observe this redshifted 21-cm emission. Theoretical models must be on hand to interpret current upper limits as well as future observations. Semi-analytical models (SAMs) are well-suited for this purpose because of their computationally efficient and physically motivated prescriptions of relevant physics. Galaxy formation SAMs typically work by post-processing the dark-matter halo merger trees from dark-matter-only N-body simulations. This thesis updated the Meraxes SAM of coupled galaxy formation and reionisation in this thesis. Specifically, the explicit calculation of the spin temperature of the HI gas was implemented. This involves tracking the thermal state of the IGM, which is influenced primarily by the X-rays. This updated version of Meraxes was deployed on an N-body simulation of side 210 h^(-1) Mpc. Such large cosmological volumes are necessitated by the long mean free paths, ~ O(100 Mpc), of X-rays in the early Universe. At the same time, for a given number of particles, the mass resolution of an N-body simulation is inversely proportional to its volume. Hence, the simulations will not capture the full source population. To overcome this, the dark matter merger trees are augmented by introducing low-mass haloes into the simulations. The augmented simulation is one of the large-volume simulations in the literature that is simultaneously capable of resolving all atomically cooled haloes down from z = 20 and is sufficiently large enough to track the impact of X-rays on the thermal state of the IGM. Taking advantage of the computational efficiency of the Meraxes SAM, the impact of the galaxy X-ray luminosity on the 21-cm statistics, i.e. the 21-cm global signal and 21-cm power spectra (21-cm PS), are explored. Exploiting the large dynamic range of the model, the thesis also shows that the magnitude of the non-Gaussian term in the sample variance of the 21-cm PS is more than twice the magnitude of the Gaussian term at scales relevant to the upcoming Square Kilometre Array (SKA). The thesis then explores the astrophysical constraints that will be achievable with a future detection of the 21-cm PS. Using the Fisher matrix formalism, the fractional uncertainties in the model parameters enabled by a 21-cm detection spanning z in [5, 24] from a 1000 h mock observation with the SKA are forecasted. This work focused on the X-ray luminosity, ionising UV photon escape fraction, star formation and supernova feedback of the first galaxies. It is shown that it is possible to recover 5 of the 8 parameters describing these properties with better than 50 per cent precision using just the 21-cm PS. By combining UV luminosity functions with the 21-cm PS, we can improve our forecast, with 5 of the 8 parameters constrained to better than 10 per cent (and all below 50 per cent).
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ItemTopological quantum computing with magnet-superconductor hybrid systems( 2023-11)Developing a practical general purpose quantum computer is this eras moonshot project, enabling fundamental advances in simulating quantum many-body systems, as well as promising new classical-computer-beating algorithms with applications in cryptography, meteorology, economics, and logistics. Current quantum processors struggle with short coherence times --- meaning that the fragile quantum bits (qubits) break down --- resulting in high error rates. Thus complicated or long calculations are prohibitive to run on current devices. Quantum error correction could be the solution, however, many physical qubits are required to encode a single logical qubit. Thus a massive scaling up of hardware is required to realise even a modest number of fault-tolerant logical qubits. Over the past twenty years the idea of engineering an inherently fault-tolerant, or topological, quantum computer has been developed. In principle, these fault-tolerant qubits do not decohere due to a topological protection; the information is distributed across a physical system such that local perturbations do not damage the whole information encoding. Majorana zero-modes, characteristic quasiparticles in topological superconductors, have emerged as a leading candidate for the building blocks of a fault-tolerant qubit. Many experimental platforms which might yield Majorana zero-modes have been proposed, but as of writing unambiguous evidence for Majorana zero-modes and topological superconductivity has not been presented in any experiment. Here I study magnet-superconductor hybrid (MSH) systems, which involve networks of magnetic adatoms assembled on a superconducting surface via lateral atom manipulation using a scanning tunneling microscope tip. These systems are clean and crystalline, and thus are an ideal platform for experiments. I present compelling theoretical and experimental evidence for topological superconductivity in Mn and Fe chains on Nb(110). However, the systems investigated so far experimentally have long localisation lengths, resulting in hybridised Majorana modes. Because these modes cannot be used to build a fault-tolerant qubit, I theoretically investigate several extensions to these experiments. I propose constructing quasi one-dimensional chains consisting of several rows of magnetic adatoms, with ferromagnetic order in one crystalline direction and antiferromagnetic in the other. I also suggest engineering the Nb(110) surface with an alloy to dramatically increase the Rashba splitting. Both of these proposals are readily accessible in experiment, and could yield non-hybridised Majorana zero-modes. Having established the viability of the platform, I introduce a numerical apparatus for studying many-body nonequilibrium superconducting physics. While this is generic and can be applied to any superconducting problem, here I use it to study topological quantum computing on a MSH platform. I first show that quantum gates can indeed be implemented via braiding Majorana zero-modes. I then show how single-molecule magnets can be use to initialise and readout MSH qubits. I build on this protocol and introduced a dressed Majorana qubit, which combines an MSH network with single-molecule magnets. These could be easier to initialise and readout than a conventional Majorana qubit.
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ItemDetecting and characterising extrasolar planets in reflected light(University of Melbourne, 2009)
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ItemFault-tolerant quantum computation with local interactions(University of Melbourne, 2009)
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ItemAddressing domain shift in deeply-learned jet tagging at the LHC( 2023-09)Over the last fifteen years, deep learning has emerged as an extremely powerful tool for exploiting large datasets. At the Large Hadron Collider, which has been in operation over the same time span, an important use case is to identify the initiating particles of hadronic jets. Due to the complexity of the radiation patterns within jets, neural network-based classifiers are able to out-perform traditional techniques for jet tagging. While these approaches are powerful, neural networks must be applied carefully to avoid performance losses in the presence of domain shift—where the data on which a model is evaluated follows different statistics to the training dataset. This thesis presents studies of possible strategies to mitigate domain shift in the application of deep learning to jet tagging. Firstly, we develop a deep generative model that can separately learn the distribution of quark and gluon jets from mixed samples. Building on the jet topics framework, this model provides the ability to sample quark and gluon jets in high dimension without taking input from Monte Carlo simulations. We demonstrate the advantage of the model over a conventional approach in terms of estimating the performance of a quark/gluon classifier on experimental data. One can also use likelihoods under the model to perform classification that is robust to outliers. We go on to evaluate fully- and weakly-supervised classifiers using real datasets collected at the CMS experiment. Two measurements of the quark/gluon mixture proportions of the datasets are made under different assumptions. Compared to the predictions based on simulation, we either over- or under-estimate the quark fractions of each sample depending on which assumption is made. When estimating the discrimination power of the classifiers in real data we find that while the absolute performance depends on the choice of fractions, the rankings among the models are stable. In particular, weakly-supervised models trained on real jets outperform both simulation-trained models. Our generative networks yield competitive classification and provide a better model for the quark and gluon jet topic distributions in data than the simulation. Finally, we investigate the performance of a number of methods for training mass-generalised jet taggers, with a focus on algorithms that leverage meta-learning. We study the discrimination of jets from boosted Z' bosons against a QCD background and evaluate the networks' performance at masses distant from those used in training. We find that a simple data augmentation strategy that standardises the angular scale of jets with different masses is sufficient to produce strong generalisation. The meta-learning algorithms provide only a small improvement in generalisation when combined with this augmentation.
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ItemMonte Carlo characterisation of narrow photon fields for intensity modulated radiotherapy(University of Melbourne, 2006)
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ItemFocusing of an atomic beam using a TEM01 mode lens(University of Melbourne, 2006)
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ItemFunctional Renormalization Group Methods for Spin-Orbit Coupled Hubbard Systems( 2023-08)This thesis establishes the extension of the functional renormalization group to systems of arbitrary lattice complexity with additional spin or orbital degrees of freedom. Using these capabilities, we investigate the effects of spin-orbit coupling on square and triangular lattice structures, which describe for example cuprates, iron-pnictides, strontium ruthenate, tin layers on silicon and lead layers on silicon-carbide. For the methodological advances, we build on previous studies of the truncated-unity functional renormalization group, but remedy existing symmetry breaking issues. These were incurred when combining a sublattice degree of freedom with the expansion of non- transfer momentum dependencies in a plane-wave basis, and can be alleviated by careful selection of considered bonds. We furthermore demonstrate a wide range of intricacies, paramount for correct functional renormalization calculations, all of which we resolved. The obtained algorithms we validate at certainty not hitherto achieved, heralding a novel approach of quantitative comparison. All of this is contained and published in a high- performance C++ implementation, already in use by junior researchers. Motivated by experimental results, we study the effect of Rashba spin-orbit coupling in the square-lattice Hubbard model. We find the superconducting instabilities to be robust under weak-to-moderate Rashba-coupling strengths. When the coupling is increased further the transition scale decreases significantly. We furthermore measure the contribution of triplet superconductivity, to indicate regions of interest for topological effects. Taking advantage of the functional renormalization group’s capability to produce phase diagrams, we also investigate particle-hole instabilities in the system. Here we find a complex interplay of commensurate and incommensurate spin-density waves and unexpected regions of accidental nesting. The weak-to-intermediate coupling phase diagram in filling and spin-orbit coupling strength is presented. We lastly turn our attention to triangular lattice materials. Here, recent ab-initio calculations predict high Rashba spin-orbit coupling strengths in, for example, Pb on SiC. We introduce Rashba spin-orbit coupling to the Hubbard model, finding a wide range of spin-density waves with differing ordering vectors, some of which appear favorable for multi-q instabilities. We further find superconducting phases around half-filling and at low filling. The region around half-filling is singlet-dominated, gaining triplet weight with increased spin-orbit coupling. Contrarily the pure triplet region at low filling is an extended phase, persisting under spin-orbit coupling. We present a phase diagram for the triangular lattice Rashba-Hubbard model in filling and spin-orbit coupling strength.