School of Physics - Theses
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ItemDistributed Matrix Product State Simulations of Large-Scale Quantum Circuits( 2017)Before large-scale, robust quantum computers are developed, it is valuable to be able to classically simulate quantum algorithms to study their properties. To do so, we developed a numerical library for simulating quantum circuits via the matrix product state formalism on distributed memory architectures. By examining the multipartite entanglement present across Shor’s algorithm, we were able to effectively map a high-level circuit of Shor’s algorithm to the one-dimensional structure of a matrix product state, enabling us to perform a simulation of a specific 60 qubit instance in approximately 14 TB of memory: potentially the largest non-trivial quantum circuit simulation ever performed. We then applied matrix product state and matrix product density operator techniques to simulating one-dimensional circuits from Google’s quantum supremacy problem with errors and found it mostly resistant to our methods.
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ItemErbium for optical modulation and quantum computation( 2017)Erbium (Er) is a lanthanide element, mainly used in its trivalent ionic form (Er3+), as an active dopant in optical devices, for light amplification or generation. The luminescence of Er3+ lies within the conventional wavelength band, 1530-1565 nm, for fiber-optic communication. The low noise, linear response, and stability of the optical gain provided by the Er3+ luminescence are ideal for applications in photonic systems that operate in the fiber-optic frequency. While much research has been done to understand the Er3+ luminescence in various lasing media, few studies have been conducted to tap the potential of Er for applications other than amplifiers or lasers. This thesis delves into two new areas, namely optical modulation and quantum computation, where the Er3+ luminescence may be able to be applied in a novel way. By incorporating Er3+ into a switchable optical material, an optical modulator could potentially be made that is capable of not only switching but also amplifying signal transmission or sustaining the signal intensity from propagation losses. This integrated approach could reduce device footprint and latency for on-chip as well as synchronous applications. Successful integration of Er, however, has never been demonstrated in conventional optical modulators because their reliance on electro-optic effects conflicts with the carrier-sensitive mechanism of the Er3+ luminescence. The compatibility between Er and a recently advocated optical material, namely vanadium dioxide (VO2), is examined in the first part of this thesis. VO2 exhibits a hysteretic, bistable phase transition that is accompanied by a high-contrast optical switching in infrared, including the fiber-optic, wavelength band. The phase transition can be triggered thermally as well as optically. When triggered optically, it can occur in picosecond timescale, making VO2 a promising material for ultrafast optical switching applications. Experimental characterizations of the Er3+ luminescence and the optical switching were performed on selectively prepared thin-film samples of VO2. The Er3+ luminescence could be observed after the samples were implanted with Er and then annealed between 800*C and 1000*C. The optical switching could also be measured in the implanted and annealed samples as they were thermally heated up and then cooled down past the critical temperature of the phase transition. The Er-implanted samples, however, were found to have broader hysteresis and lower switching contrasts than the pure VO2 samples. It is concluded that although Er-implanted VO2 could probably work as a combined optical switch and amplifier, the poorer switching qualities do not guarantee that a device based on the material could provide better utility than a separated system of optical switches and Er amplifiers. The Er3+ luminescence could also be utilized for quantum frequency conversion, for implementation in interconnects that interface superconducting quantum computers to a fiber-optic quantum network. For two superconducting quantum computers to be able to communicate over a fiber-optic quantum network, the frequency of the signals transmitted from either computer needs to be converted into the fiber-optic frequency, and then back into the microwave frequency upon receipt at the other computer. Early proposals suggested that the interconnect be at least comprised of Er3+ ions, a microwave resonator, and an optical resonator. The realization of this system has been attempted recently in basic experiments, but the conversion efficiency was found to be too low. The weak couplings between the Er3+ ions and the two resonators were identified as one of the main reasons for the low conversion efficiency. One way to mitigate the weak coupling in the microwave part is to have a superconducting flux qubit bridge the interaction between the Er3+ ions and the microwave resonator. The second part of this thesis presents a theoretical and simulation study of the dynamics of a coupled system consisting of Er3+ ions, a superconducting microwave resonator, an optical resonator, and a superconducting flux qubit. It is shown that quantum information can be exchanged between the Er3+ ions and the microwave resonator with a high fidelity via the qubit coupling, and the exchange process is controllable by changing the frequency of the qubit. The frequency conversion between the microwave and the optical regime is shown to be infeasible to be realized at the limit where the number of optical excitations (n) is much less than the number of ions (N). A high-efficiency frequency down-conversion is demonstrated to be achievable in the case where there is no decoherence, and both n and N are small. However, the time it takes to complete the down-conversion is very long, leaving the efficiency prone to decoherence. It is argued that for the frequency conversion to be able to be accomplished in a typical decoherent environment, both n and N need to be large. The study of the dynamics, in this case, is left for future research.
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ItemUltrafast spectroscopy of nanostructures( 2017)This thesis presents studies of ultrafast laser spectroscopy of semiconductor and gold nanostructures, aiming to advance our understanding of, and consequently control, photoinduced charge carrier dynamics in nanostructures to further improve their performance in practical applications. Artificial nanostructures have drawn significant attention in applications such as optoelectronic devices, photo-catalysts, and solar cells. Compared to bulk materials, nanostructures provide unique optical properties, which more importantly can be directly and easily tailored through changing size or shapes of the structures, during their synthesis procedures. Photoinduced charge carrier dynamics in the nanostructures play an important role in the photon conversion processes. However, in contrast to the fast development of nanostructure-based devices, the mechanisms of these processes are still being experimentally unravelled. In this study, a range of ultrafast optical spectroscopy methods have been applied to investigate the carrier dynamics, with a focus on the electron transfer (ET) process. Semiconductor nanoparticles, or quantum dots (QDs), of core/shell heterostructures are promising for their good photostability and high photoluminescence quantum yields. The ET dynamics from the 1S$_\mathrm{e}$ electron state to adsorbed methyl viologen electron acceptors, in CdSe/CdS and CdSe/CdS/ZnS QDs, were studied using femtosecond transient absorption and time-resolved photoluminescence spectroscopy. By changing shell thickness or alloying the shell interface, significant modulation of the ET dynamics was observed. In CdSe/CdS QDs, the 1S$_\mathrm{e}$ ET dynamics exhibited a hole-coupled effect, which is ascribed to the Auger-assisted ET process. In CdSe/CdS/ZnS QDs, the formation of alloyed shell interfaces at elevated shelling temperatures reduced the shell potential barrier, leading to an observed greater ET rate. Photoinduced ET processes from gold nanorod and nanowire structures to TiO$_{2}$ were also investigated, using a visible pump-NIR probe transient absorption spectroscopy method. Partially embedded Au nanorods on a TiO$_{2}$ layer exhibited an enhanced but directional ET process. An Au nanowire grating supported on a TiO$_{2}$ layer structure underwent the plasmon-waveguide hybridisation mechanism. The ET dynamics from the split states showed a dependence on the light-matter coupling effect that can be varied with the Au grating period. In summary, this thesis shows the great ability of ultrafast optical spectroscopy to reveal photoinduced processes in nanostructures. Results indicate ways for rational design of nanostructure-based devices. A greater understanding in underlying physics leads to better control of the performance of these nano-systems in potential practical applications.
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ItemMeasurements of the ATLAS tau trigger reconstruction and identification efficiencies using 2016 data from \(pp\) collisions at\(\sqrt s \) = 13 T\(e\)V( 2017)This thesis presents the performance of the tau trigger algorithm used by the ATLAS experiment to select hadronically decaying tau leptons in the LHC Run 2. Using the 33.3 \(f{b^{ - 1}}\) of \(pp\) collisions data recorded in 2016 at\(\sqrt s \) = 13 T\(e\)V, the performance of this algorithm is studied using a `tag-and-probe' based analysis in order to select Z boson decays to tau leptons, where one tau decays hadronically and the other leptonically. The reconstruction and identification efficiencies of the tau trigger algorithm are measured, and good performance is observed. The efficiency of the tau trigger in data is compared with that in simulation, and is parametrised as a function of the tau decay topology, its kinematics, and the average number of interactions per bunch crossing. The selection efficiency at each step of the high level trigger is measured, using dedicated intermediary triggers, and good agreement between data and simulation is observed. Using the comparison between reconstruction and identification efficiencies in data and simulation, correction factors for simulated events are measured, which are utilised by the entire ATLAS collaboration.
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ItemNon-invasive imaging techniques for the investigation of cultural objects( 2017)Laser speckle is used in cultural materials conservation to study changes in the physical condition and movement on the surface and within the structure of objects. This thesis investigates three speckle methods, Laser Speckle Contrast Imaging (LSCI), Dynamic Speckle Imaging (DSI) and Electronic Speckle Pattern Interferometry (ESPI) to: image obscured text, examine paint application methods, record physical transformations during paint drying processes, and monitor canvas strain due to climate variations in- situ. It extends the boundaries of use of speckle-based methods in conservation through a set of four case studies that cover a broad range of cultural conservation problems faced in Australian collections. These studies were contextualised within an examination the properties of viscoelastic materials, a mathematical description of speckle, an explanation of the speckle-based methods and the experimental equipment and design considerations that provide information about the challenges and considerations required when deploying speckle-based methods. In the first case study, LSCI was used to image obscured text on books that had been repaired with layers of paper. The quality of the results, which was dependent on the weight of the paper, colour of the subsurface medium, and temperature of the object, was similar to that of Infrared Photography. In the second case study LSCI was used to monitor the curing process of modified alkyd resins over 120 hours to investigate whether the application method used by the artist was robust to typical ageing conditions. The results correlated well with traditional gravimetric results, which only provide information about the mass changes to the samples with no spatial information. LSCI was able to demonstrate where and when the alkyd resin surface was moving, most likely due to off gassing processes. The third case study used DSI to monitor the drying process of artists’ acrylic paints, allowing for the novel non-invasive detection of when the paint was touch dry, a task that normally involves destructive methods or has only been applied to rapid drying industrial paints. The final case study demonstrated the in-situ application of ESPI to monitor two archetypical colonial easel paintings in assessing physical movement due to variations in the display environment. ESPI was used in-situ to record changes in the strain of the canvas. Furthermore, it was shown that DSI analysis could be applied to ESPI data to monitor for vibrational disturbances that can decorrelate results The speckle-based techniques used in this thesis were shown to satisfy six criteria for preferred conservation methods: all information is gathered non-invasively and can be analysed in a variety of ways; off the shelf optical components are used, thus it is highly suitable for fields in which specialised optical components are not available; no prior analysis about the sample is required; experimental challenges can be diagnosed and overcome through the analysis of data sets and/or computer simulations; the techniques can be applied to the investigation of a range of materials with different optical properties; and multiple speckle-based methods can be combined to provide complementary information. The case-studies explored in this thesis further demonstrated that speckle is capable of non-invasive characterisation of many physical properties and can be applied both in the laboratory and in-situ. Through the assessment of theory and case studies this work has extended the application of speckle analysis within conservation studies, delivering practical outcomes to aid conservation efforts.
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ItemClassical processing for topological quantum error correction( 2017)This thesis deals with a series of issues implementing the classical software stack to control a large-scale quantum computer. Due to the relatively high error rates of quantum hardware expected for the foreseeable future, high threshold error correction schemes such as the topological cluster state and the surface code, will be essential to performing robust quantum computation. The work begins by bringing new functionality to the Autotune software library to support the presence of runtime qubit loss when using the topological cluster state for error correction. Through simulation, it is shown that <1% runtime loss is required for minimal space-time overhead. The thesis continues by constructing new software, LogiQSim, which enables fully-fault-tolerant quantum simulation and error analysis of logical quantum gates. Through the implementation of byproduct operator tracking and the placement of defects and correlation fragments, it was possible to demonstrate a full quantum simulation of a working logical controlled NOT gate (CNOT), as well as estimate its logical error rate directly through simulation. This approach was further improved to allow the simulation of the Clifford portion of A-state distillation, significant as this circuit is the predominant source of overhead in quantum computation when using the topological cluster state. The simulated overhead of the distillation process was compared to estimates calculated from the results of the logical CNOT simulations in order to determine the accuracy of estimating a larger circuit using the simulations of a smaller one. Furthermore, we found that the relationship between the structure of a correlation fragment and its contribution to the logical error rate is complex. This is due to an asymmetry in the frequency of logical errors that are the result of error chains connecting two defects as compared to rings of errors around defects. The work presented in this thesis makes substantial progress towards the understanding and development of a practical classical software stack that can be used to run a topologically error corrected quantum computer, and utilizes this (in the absence of physical hardware) to simulate and provide more accurate overhead estimates for key logical operations.
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ItemNano-electronics at graphene-electrolyte interfaces for biological application( 2017)Research of graphene for life science has attracted significant attention from not only the scientific community but also the general public. Graphene possesses many exciting properties which are beneficial for biological applications, such as biocompatibility, tunable chemical properties, atomic thickness, among others. Detection of biomolecules in liquids, such as the electrolytes encountered in biological environments, is a key application. However, the physical processes at the graphene-electrolyte interface are not fully understood. This thesis presents a theoretical and experimental study of this system, and demonstrates its application in bio-sensing. Electrical double layer (EDL) is a critical component necessary for the investigation of the processes at solid-electrolyte interfaces. A new ionic vibration model for the EDL as an improvement to the conventional Gouy-Chapman-Stern (GCS) theory is proposed in chapter 3, as the GCS fails to describe the EDL capacitance over a wider range of electrode potentials. This new theory elucidates the mechanism behind the ionic dynamics at the interface of solid-electrolyte systems, as ions are found to vibrate near the electrode surface in response to the applied electric field. The calculated results are consistent with measurements, and this provide useful insight not only important for biosensing (as we may need to change the potential on graphene over a wide range), but also for energy storage, neuron stimulation, among others. Since some of these applications (e.g.: neuron stimulation) require large capacitance, and this theory indicates that the EDL capacitance decreases with the electrode potential, one of the approaches is to increase the surface area of the electrode material (graphene). Conformally coating graphene onto a nanoporous three-dimensional (3D) structure gives 3D graphene with very large surface area. A novel method of directly fabricating 3D graphene by plasma assisted graphitization is presented in chapter 4. The obtained material shows very large electrochemical capacitance values, as high as 2.1 mF for a sample of 10 mm$^3$. It also exhibits excellent chemical stability, providing a good platform for electrochemical applications (e.g.: energy storage, neuron stimulation, etc.). This sample is then used to capacitively detect a protein (bovine serum albumin) as a non-destructive label-free method. The results can inspire the work on developing bio-sensors which are able to work as bio-filters simultaneously in future, due to the nano-porous structure. However, the unclear sensing mechanism (whether EDL capacitance or quantum capacitance is dominant), low sensitivity, and long response time of the sensor raise the task for searching for different sensing mechanisms. Based on the discussion of electron transport of graphene in solution in chapter 5, a new method for the detection of biomolecules at very low concentrations using Hall effect measurements on liquid gated graphene based devices is proposed in chapter 6. The results of L-histidine detection suggests high sensitivity in the pM range, exceeding the performance of the conventional amperometric and potentiometric techniques under the same conditions. Through the determination of changes of the mobility and the charge carrier density in graphene we provide new clues for understanding different mechanisms of molecular detection on graphene-based devices in liquids. Theoretical modeling of the experiments indicates that the asymmetric electron-hole mobilities can explain the enhanced detection sensitivity. Quantum capacitance is only dominant near the "Dirac" point, while EDL capacitance dominates at higher potential. This explains the drastic changes of the measured electronic properties close to the "Dirac" point. The detailed theoretical and experimental studies presented in this thesis provide a deep understanding for graphene-electrolyte systems, which are important not only for biological applications (such as biosensing, neuron stimulation and recording, bionic implants, etc), but also for energy storage systems.
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ItemInvestigation of B+ mesons decay to K+K−π+ at the Belle experiment( 2017)Charmless decays of B mesons to three charged hadrons are suppressed in the Standard Model, and thus provide an opportunity to search for physics beyond the Standard Model. An unexpected excess and a large CP asymmetry in the low invariant mass spectrum of the K+K− system for the decay B+ → K+K−π+ were observed by BaBar and LHCb in recent years. We present the measurements of branching fraction and direct CP asymmetry of the charmless decay B+ → K+K−π+. This analysis is performed on a data sample of 772 × 10^6BB pairs produced at the Υ(4S) resonance by the KEKB asymmetric-energy e+e− collider and collected by the Belle detector. We perform a blind analysis, examining signal reconstruc- tion and background suppression with Monte Carlo simulated samples, and extract signal yield and direct CP asymmetry with a 2D extended maximum likelihood fit to the data. The measured branching fraction and direct CP asymmetry are B(B+ → K+K−π+) = (5.38 ± 0.40 ± 0.35) × 10^−6 and ACP = −0.170 ± 0.073 ± 0.017, respectively, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the current world average. We extract the branching fraction and direct CP asymmetry as a function of the K+K− invariant mass. The K+K− invariant mass distribution of the signal candidates shows an excess in the region below 1.5 GeV/c^2, which is consistent with the previous studies from BaBar and LHCb. Strong evidence of a large direct CP asymmetry of −0.90 ± 0.17 ± 0.03 with 4.8σ significance is found in the K+K− low-invariant-mass region.
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ItemElectron diffraction using a cold atom source( 2017)Observing matter on atomic length and time scales simultaneously is now routinely achieved in ultrafast electron and X-ray imaging techniques, but continued advances in both approaches promise to deliver huge leaps in our understanding of all kinds of atomic structures and processes. Both technologies rely on the generation of ultrabright, ultrashort duration electron bunches, with these bunches being used directly to probe the sample in electron diffraction, or to generate ultrabright X-ray pulses in X-ray free electron lasers. The Cold Atom Electron and Ion Source (CAEIS) was conceived as a possible way to generate ultrafast electron bunches that are brighter than can currently be produced, with the aim of enabling next-generation structural determination techniques, particularly those based on electron diffraction. The CAEIS generates electrons by near-threshold photoionisation of an atomic gas, which has been shown to produce electron bunches with temperature as low as 10K. Such cold electron bunches have the potential to be much brighter than those generated from solid photocathode sources, which typically have temperatures in the thousands of Kelvin range. Extremely cold ions are also generated in the CAEIS, which show great potential for use in ion microscopy and milling. This thesis presents work on a number of different aspects of the continued development of the cold atom electron and ion source, with a particular emphasis on progress towards ultrafast single-shot electron diffraction based experiments. The brightness degrading effects of space-charge repulsion are investigated using nanosecond duration ion bunches as analogues of ultrafast, picosecond duration electron bunches. Ion bunch shaping was achieved through tailoring of the spatial profile of lasers used in ionisation of the atomic gas. It was found that atomic fluorescence could substantially reduce the fidelity with which the ion bunch profile could be controlled, but methods were developed to circumvent the fluorescence problem. The improved shaping procedures allowed generation of uniformly filled ellipsoidal bunches, which theoretically will not suffer emittance degradation under space-charge expansion. Emittance measurements following space-charge driven expansion showed that these uniformly filled ellipsoids did indeed have reduced emittance growth compared to other profiles. Photoexcitation and field-ionisation processes involved in generation of cold electrons on ultrafast timescales were investigated, with the aim of determining the mechanisms that affect the ultimate electron bunch duration. Bunch duration was measured for a range of excitation conditions, with the finding that previously assumed ultrafast excitation pathways in fact generated fairly slow nanosecond long bunches. Ionisation time could also be a million times slower than assumed if atoms were excited below the classical ionisation threshold. Identification of the conditions required for ultrafast excitation and ionisation ultimately allowed generation of ultrafast cold electron bunches with duration of tens of picoseconds. Electron diffraction using nanosecond long electron bunches was achieved in both transmission and reflection modes for a variety of large samples of inorganic crystals. Bunches were of sufficiently high charge to allow identification of features of a crystal structure using only a single shot. Bragg peaks could also be identified by averaging together many images formed using ultrafast, but low-charge bunches. Simulations were performed to determine the feasibility of using electrons generated in the CAEIS for electron coherent diffractive imaging of nanoscale apertures. It was found that it should be possible to successfully reconstruct the object plane wavefield, even taking into account realistic experimental parameters for partial coherence and noise.
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ItemB0→K0π0 and direct CP violation at Belle( 2017)Rare B-meson decays such as the B0 → Ksπ0 which proceed without a charm quark provide a probe for physics beyond the standard model. This decay proceeds mainly via the b → s penguin transition, with the b → u transition being colour suppressed, allowing CP-violating effects to be observable. The asymmetric e+e− KEKB collider and the Belle detector provide the large luminosity and data collection required to observe these rare B decays. Methods to reduce the large qq backgrounds are investigated. The use of optimised neural networks using TensorFlow shows a significant improvement compared to the commonly used NeuroBayes software. Techniques for reducing correlations between variables introduced by TensorFlow are also investigated, proving that the use of adversarial neural networks can provide an improved background suppression as compared to NeuroBayes, whilst minimising correlations introduced by the neural network. An improved method of measuring the direct CP violation is introduced. Using Monte Carlo data with sample sizes corresponding to the full Belle datatset of (771.581 ± 10.566) × 106 BB events, the statistical uncertainty in ACP using this method is reduced from the latest Belle result of 0.13 to 0.1035 ± 0.0032. This method would also provide an up to date measurement on B(B0 → K0π0).