School of Physics - Theses

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    Realistic read-out and control for Si:P based quantum computers
    Testolin, Matthew J. ( 2008)
    This thesis identifies problems with the current operation proposals for Si:P based solid-state quantum computing architectures and outlines realistic alternatives as an effective fix. The focus is qubit read-out and robust two-qubit control of the exchange interaction in the presence of both systematic and environmental errors. We develop a theoretical model of the doubly occupied D- read-out state found in Si:P based nuclear spin architectures. We test our theory by using it to determine the binding energy of the D- state, comparing to known results. Our model can be used in detailed calculations of the adiabatic read-out protocol proposed for these devices. Regarding this protocol, preliminary calculations suggest the small binding energy of the doubly occupied read-out state will result in a state dwell-time less than that required for measurement using a single electron transistor (SET). We propose and analyse an alternative approach to single-spin read-out using optically induced spin to charge transduction, showing that the top gate biases required for qubit selection are significantly less than those demanded by the adiabatic scheme, thereby increasing the D+D- lifetime. Implications for singlet-triplet discrimination for electron spin qubits are also discussed.
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    Controllable few state quantum systems for information processing
    Cole, Jared H. ( 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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    Towards large-scale quantum computation
    FOWLER, AUSTIN GREIG ( 2005-05)
    This thesis deals with a series of quantum computer implementation issues from the Kane 31P in 28Si architecture to Shor’s integer factoring algorithm and beyond. The discussion begins with simulations of the adiabatic Kane CNOT and readout gates, followed by linear nearest neighbor implementations of 5-qubit quantum error correction with and without fast measurement. A linear nearest neighbor circuit implementing Shor’s algorithm is presented, then modified to remove the need for exponentially small rotation gates. Finally, a method of constructing optimal approximations of arbitrary single-qubit fault-tolerant gates is described and applied to the specific case of the remaining rotation gates required by Shor’s algorithm.