School of Physics - Theses
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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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ItemStar formation and galaxy evolution of the Local Universe based on HIPASS( 2007-12)This thesis investigates the star formation and galaxy evolution of the nearby Local Volume based on Neutral Hydrogen (HI) studies. A large portion of this thesis consists of work with the Northern extension of the HI Parkes All Sky Survey (HIPASS). HIPASS is an HI survey of the entire Southern sky up to a declination of +25.5 degrees (including the Northern extension) using the Parkes 64-metre radio telescope. I have also produced a catalogue of the optical counterparts corresponding to the galaxies found in Northern HIPASS. From this optical catalogue, we also conclude that we did not find any isolated dark galaxies. The other half of my thesis consists of work with the SINGG and SUNGG projects. SINGG is the Survey for Ionization in Neutral Gas Galaxies and SUNGG is the Survey of Ultraviolet emission in Neutral Gas Galaxies. Both SINGG and SUNGG are selected from HIPASS and are star formation studies in the H-alpha and ultraviolet (UV), respectively. My work in the SINGG-SUNGG collaboration is mostly based on SUNGG. Using the results of SUNGG, I measured the local luminosity density and the cosmic star formation rate density (SFRD) of the Local Universe. Using far-infrared (FIR) observations from IRAS, the FIR luminosity density was also calculated. Combining the FUV luminosity density and the FIR luminosity density, the bolometric SFRD of the Local Universe was estimated. This thesis also includes the discovery of one of the nearest drop-through ring galaxies, NGC 922, which is a factor of three closer than the infamous Cartwheel galaxy.
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ItemQuantum information engineering: concepts to quantum technologies( 2007-11)This thesis investigates several broad areas related to the effective implementation of quantum information processing, from large scale quantum algorithms and error correction, through to system identification and characterization techniques, efficient designs for quantum computing architectures and the design of small devices which utilize quantum effects. The discussion begins with the introduction of a quantum circuit appropriate for implementing Shor’s factoring algorithm on Linear Nearest Neighbor qubit arrays such as the Kane phosphorus in silicon system. Detailed numerical sim- ulations are then presented, demonstrating the sensitivity of the circuit under coherent quantum errors. The concepts of Quantum Error Correction and Fault-tolerant computation are reviewed with original work carried out to show the relative robustness and practicality of Fault-tolerant computation for logical state preparation. Methods of intrinsic system identification and characterization are proposed. Protocols for characterizing both the confinement of a multi-level system to the qubit subspace and the Hamiltonian dynamics governing two-qubit interactions are presented as well as a brief review of characterization techniques already developed for single qubit dynamics. (For complete abstract open document)
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ItemOn the one-dimensional bose gas( 2007-02)The main work of this thesis involves the calculation, using the Bethe ansatz, of two of the signature quantities of the one-dimensional delta-function Bose gas. These are the density matrix and concomitantly its Fourier transform the occupation numbers, and the correlation function and concomitantly its Fourier transform the structure factor. The coefficient of the delta-function is called the coupling constant; these quantities are calculated in the finite-coupling regime, both expansions around zero coupling and infinite coupling are considered. Further to this, the density matrix in the infinite coupling limit, and its first order correction, is recast into Toeplitz determinant form. From this the occupation numbers are calculated up to 36 particles for the ground state and up to 26 particles for the first and second excited states. This data is used to fit the coefficients of an ansatz for the occupation numbers. The correlation function in the infinite coupling limit, and its first order correction, is recast into a form which is easy to calculate for any N, and is determined explicitly in the thermodynamic limit.
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ItemSuperfluid spherical Couette flow and rotational irregularities in pulsars( 2006-10)Small amplitude rotational irregularities are observed in a number of rotation-powered pulsars. They fall into two classes: (i) glitches, defined as abrupt increases in the angular velocity of a pulsar (accompanied sometimes by changes in the angular acceleration Ω), of which 286 have been observed in 101 objects; and (ii) timing noise, a continuous stochastic fluctuation in phase, or, which is observed mostly in young and adolescent pulsars (with ages ≥ 10 4 yr). Both classes of irregularity seem to arise from some mechanism that couples the angular momentum of the solid crust and superfluid core of the star, which is activated suddenly when differential rotation exceeds a threshold. Coupling mechanisms proposed to date include catastrophic vortex unpinning in the inner crust, triggered by starquakes; vortex creep, due to thermally activated quantum tunnelling; superfluid-superconductor interactions in the core; and superfluid instabilities. The associated theories are phenomenological, not predictive.
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ItemControllable few state quantum systems for information processing( 2006-10)This thesis investigates several different aspects of the physics of few state quantum systems and their use in information processing applications. The main focus is performing high precision computations or experiments using imperfect quantum systems. Specifically looking at methods to calibrate a quantum system once it has been manufactured or performing useful tasks, using a quantum system with only limited spatial or temporal coherence. A novel method for characterising an unknown two-state Hamiltonian is presented which is based on the measurement of coherent oscillations. The method is subsequently extended to include the effects of decoherence and enable the estimation of uncertainties. Using the uncertainty estimates, the achievable precision for a given number of measurements is computed. This method is tested experimentally using the nitrogen-vacancy defect in diamond as an example of a two-state quantum system of interest for quantum information processing. The method of characterisation is extended to higher dimensional systems and this is illustrated using the Heisenberg interaction between spins as an example. The use of buried donors in silicon is investigated as an architecture for realising quantum-dot cellular automata as an example of quantum systems used for classical information processing. The interaction strengths and time scales are calculated and both coherent and incoherent evolution are assessed as possible switching mechanisms. The effects of decoherence on the operation of a single cell and the scaling behaviour of a line of cells is investigated. The use of type-II quantum computers for simulating classical systems is studied as an application of small scale quantum computing. An algorithm is developed for simulating the classical Ising model using Metropolis Monte-Carlo where random number generation is incorporated using quantum superposition. This suggests that several new algorithms could be developed for a type-II quantum computer based on probabilistic cellular automata.
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ItemDefect studies of ion implanted silicon and silicon dioxide for semiconductor devices( 2006-04)We have studied the introduction of defects in silicon wafers with low dose channelling ion implantation. (For complete abstract open document)
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ItemHigh-accuracy measurements of the x-ray mass attenuation coefficients of molybdenum and tin: testing theories of photoabsorption( 2005-08)The x-ray atomic form-factor determines the x-ray optical properties of materials and is a fundamental parameter for critical x-ray investigations. However, despite uncertainty estimates of order 1%, differences of 2-10% between x-ray mass attenuation measurements render comparison with the various theoretical tabulations meaningless. Moreover, such uncertainties impose limits on the accuracy of various quantitative investigations. We determine the imaginary component of the atomic form-factor from measurements of the x-ray mass attenuation coefficient. With the exception of the measurements of Tran et.al. [Phys. Rev. A 64, (062506); 67, (042716); J. Phys. B 38, (89)] with a 0.3% accuracy, previous work has been unable to achieve accuracies below 1%, and differences between results claiming this accuracy often exceed 6%.We have developed a full-foil mapping technique which has improved the measurement accuracy by an order of magnitude. This technique overcomes limitations arising from absorber thickness variations, using the average integrated column density and attenuation measurements across the entire surface of the absorber. We have examined measurements obtained over a wide range of parameter space for systematic deviations indicative of experimental error. Among others, this has led to the identification and correction of a 1% discrepancy arising from the x-ray bandwidth. Resulting measurement accuracies for molybdenum are 0.02-0.15%. Preliminary results for tin suggest a final accuracy of 0.1-1%. We compare these measurements with several commonly-used tabulations and identify a number of systematic discrepancies whose causes are discussed.
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ItemHigh resolution phase contrast x-ray radiography( 2006)The conventional approach for x-ray radiography is absorption contrast. In recent years a new approach that eliminates the usual requirement for absorption and allows the visualization of phase based on refractive index features in a material has been demonstrated. This so-called “phase contrast imaging” has now been applied using a wide range of radiation and samples. In this work we are motivated by the need to find optimal conditions for achieving high quality phase contrast images. We consider image formation using the free space propagation of x-rays from a point source passing through a sample. This imaging model is a lens-less configuration and, as such, is very useful for x-ray wavelengths where lenses are difficult to fabricate. Although no lenses are used, image magnification is still achieved due to the expansion of the wavefront as it propagates from the point source illumination. The short wavelength and penetrating power of x-rays make them ideal for non-destructive studies of microscopic samples. However, these techniques are also important for investigating larger, non-microscopic samples.