School of Physics - Theses
Permanent URI for this collection
Search Results
1 - 10 of 17
-
ItemMatrix product states in quantum information processing( 2015)We employ the newly developed Matrix Product State (MPS) formalism to simulate two problems in the context of quantum information processing. One is the Boson sampling problem, the other is the ground state energy density of an n-qubit Hamiltonian. We find that the MPS representation of the Boson sampling problem is inefficient due to large entan- glement as the number of photons increases. In the context of adiabatic quantum computing (AQC), MPS is used to find the first four moments of an n-qubit Hamiltonian to approximate the ground state energy density of the Hamiltonian. We show an advantage of using the first-four-moment method over the conventional adiabatic procedure. Future work around AQC using MPS is discussed.
-
ItemOptical nano-antennas( 2015)Wavelength division multiplexing/demultiplexing in radio frequency communication networks and fibre-optic networks requires sophisticated hardware. One of the challenges in designing subwavelength optical networks is the simplification of the design. When considering the size reduction and feasibility, a design based on fewer constituent elements is highly desirable. Ultra-compact wavelength multiplexers/demultiplexers will play a vital role in all-optical communication networks of the future. The challenge is to use the simplest and fewest antenna elements to achieve a functionality as sophisticated as a wavelength demultiplexer. How a pair of metallic nano-rods with different lengths can achieve such complex functionality is investigate both numerically and analytically. The study is extended to a single asymmetric cross-shaped nanoparticle. Holes perforated in metallic films are considered as slot antennas and are of particular interest in the design of miniaturised colour filters, refractive index sensors, biosensors, wave plates and other miniaturised optical elements. As part of this project, two different techniques of fabricating arrays of holes were employed. The quality and accuracy of fabricated arrays using electron ion beam lithography were compared to those fabricated with focused ion beam lithography. In both cases, periodicities of the arrays were fabricated accurately, whereas the aperture dimensions deviated from those intended. Some authors base their designs on resonant localized surface plasmons associated with the shape resonances of the apertures that are highly susceptible to geometrical defects arising during the fabrication. Detuning such shape resonances to achieve a certain phase requirements, therefore, are also prone to fabrication errors. Arrays of circular holes on the other hand are simpler to fabricate with accurate periodicities. A novel approach in designing miniaturised wave plates based purely on the surface plasmon polaritons that depend solely on the detuned periodicities of the array is proposed with simulation agreeing the experiment. A novel technique in converting a hole array (that is supported on a glass substrate) into a free standing array is also applied to the abovementioned wave plate. Experimental results of such array showed the device acts as a highly efficient refractive index sensor as well as a tuneable quarter wave plate. Prior to this, however, a simple analytical model that explains the origin of the peaks and phase relations in the spectral line associated with hole arrays was developed. The optical responses of arrays based on surface plasmon polaritons are highly sensitive to the angle of incidence. Condition for which such arrays become desensitized to the incident angle was investigate and shown that despite claims regarding depolarization observed in hole arrays, it is possible to steadily control their polarization response. Bullseye antennas with symmetric cross-shaped apertures at their centre are investigated and designed for shaping the radiation pattern of the transmitted light in the far-field. Designs were carried out using finite element methods and experimental data agree with those obtained numerically. A bullseye structure with an asymmetric cross-shaped aperture was also modelled, fabricated and characterised. Experimental data confirms that it is possible to control the radiation pattern as well as the polarization state of the transmitted light when tailoring the surface surrounding an asymmetric cross-shaped aperture with concentric circular corrugations. Another benefit of a resonant slot antenna is its ability to interact with a nearby quantum emitter. Strong and highly localized fields confined to the cavity can interact with those of a quantum emitter positioned inside it. In the strong coupling regime, this leads to an increase in the radiated power by the system as a whole. Although the enhancement to the radiative decay rate is not strictly associated with the antenna theory, one can draw an analogy between the antenna’s gain and the increase in the radiated power observed in a plasmonic antenna when integrated with a quantum emitter. Numerical solutions showed a high yield in the scattered power with a highly directional radiation pattern when a nan-diamond is positioned inside a cross-shaped aperture in a bullseye setting. The integration process of such nanoparticles with subwavelength apertures is cumbersome at the present. A novel approach is therefore proposed in coupling the emission of a NV- colour centre to the plasmonic surface modes based on the utilization of diamond substrates. The influence of the film thickness and the substrate’s refractive index on the surface modes at the superstrate is an important study as the interaction between the holes in a slot antenna array is influenced by these modes. The investigation of such effects, however, is not possible in a period array of holes due to the convolution of the surface plasmons with the Bloch waves. Studies of the surface plasmon polaritons launched by an isolated sub-wavelength slit perforating a metallic thin film showed the existence of a non-travelling interference envelope when the thickness of the film becomes comparable to the skin depth.
-
ItemSearch for the Higgs Boson produced in association with a Vector Boson at ATLAS( 2015)On the 4th of July, 2012, the ATLAS and CMS experiments at CERN announced the discovery of a particle consistent with the Standard Model Higgs boson through its decay to bosons. Subsequent measurements have established that the particle’s spin, parity and coupling strengths are in line with Standard Model expectation, making this the strongest evidence of the Standard Model electroweak symmetry breaking mechanism. This thesis concerns the search for Higgs boson decays to two tau leptons, one of the leading Standard Model Higgs decay modes, with particular focus on events in which the Higgs is produced in association with a weak vector boson. Evidence for the existence of such events constitutes a crucial test of the Higgs’ coupling to leptons. The search is conducted using the 20.3 fb−1 of data collected by the ATLAS detector in 2012 from proton-proton collisions in the Large Hadron Collider at a centre-of-mass √ energy of s = 8 TeV. The search is optimised for events in which the associated vector boson decays to an electron or muon and one or both tau leptons from the Higgs decay subsequently decay to hadrons. The dominant background comes from events in which a jet is misidentified as the hadronic decay products of a tau decay, which is estimated from data samples in a signal-free control region using the rate of misidentification. The trigger and identification efficiencies are measured using dedicated calibration data samples with a tag-and-probe technique. No significant excess above the expected background yield is observed in any signal region. The signal strength of best fit, for a Higgs boson mass of 125 GeV, is 2.3 ± 1.6. Instead, limits are placed on the rate at which the event rate might exceed the Standard Model expectation. The event rate is found to be no greater than 5.6 times that expected by the Standard Model at a 95% confidence interval. This observed limit is less than the expected limit of 3.5 times the SM event rate, under the no signal hypothesis, or 3.7 times the SM event rate with signal injected, but is consistent within uncertainties.
-
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.
-
ItemGravitational waves from low-mass X-ray binaries: a search for Scorpius X-1( 2015)Gravitational wave astronomy is an exciting prospect within the grasp of advanced interferometric detectors such as the Laser Interferometer Gravitational-Wave Observatory (LIGO). Ground-based detectors are sensitive to a range of sources and wave forms, including persistent (i.e. continuous-wave) emission from non-axisymmetric rotating neutron stars in accreting binary systems. The notoriously weak signal amplitudes inherent to continuous-wave sources combined with the extensive parameter space volume introduced by a binary orbit when integrating over long observation times can be a formidable data analysis challenge. In this thesis we design, develop, test and implement a semi-coherent data analysis technique, known as the sideband method, to search for persistent gravitational waves from neutron stars in electromagnetically observed low-mass X-ray binaries (LMXBs). X-ray emission resulting from the accretion in these systems can be used as an indicator of the gravitational wave strain. The brightest LMXB Scorpius X-1 (Sco X-1) is one of the most promising potential sources of continuous gravitational wave emission. We describe the practical implementation of the sideband search for periodic gravitational waves from neutron stars in binary systems. The orbital motion of the neutron star causes frequency modulation in the matched filtering F-statistic. The sideband search is based on the incoherent summation of these F-statistic sidebands. For a known sky position, it uses electromagnetic measurements of the orbital period and semi-major axis to construct a sideband template, which is convolved with the F-statistic to produce a new detection statistic called the C-statistic. The analysis was tested on ten simulated low-mass X-ray binary (LMXB) signals which were hardware injected into LIGO data shortly after the end of the S5 run. Hardware injections are incorporated into the data by physical actuation of the test masses, to mimic an actual signal. The results validate the performance of the sideband algorithm on sources with a range of signal strengths. The search returns a clear detection for five of the ten signals with detected strain amplitudes as low as 3.48 × 10^{−24}. The injected amplitudes of the other five signals were below the expected sensitivity of the search. We present results of the sideband search for persistent gravitational wave emission from Sco X-1 in 10 days of LIGO S5 data ranging from 50-550 Hz. Candidates remaining after the removal of known noise lines and spurious noise artefacts were found to be consistent with noise at a 99% confidence level. We present Bayesian 95% confidence upper limits on the gravitational-wave strain amplitude of Sco X-1 using two different prior distributions: a standard one, with no a priori assumptions about the orientation of the spin axis of Sco X-1 relative to the observer; and an angle-restricted one, which uses a prior on the orientation derived from electromagnetic observations. Median strain upper limits of 1.3 × 10^{−24} and 8 × 10^{−25} are reported at 150 Hz for the standard and angle-restricted searches respectively. This analysis improves upon previous upper limits by factors of 1.4 and 2.3 for the standard and angle-restricted searches. We investigate the application of the search to other known LMXBs. After Sco X-1, the next most energetic X-ray source candidates include 4U 1820-30 and Cyg X-2 which, like Sco X-1, exhibit quasi-periodic oscillations. Although dimmer, burst sources such as 4U 1636-536 and XB 1658-298 could also be interesting targets, as the burst oscillations imply the existence of a hard neutron star surface. The computational efficiency of the search means that it can produce prompt results for LMXB searches, when the Advanced Detector Era dawns.
-
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.
-
ItemMeasurement of the branching fraction of B0 → π0 π0 decays using the final Belle dataset( 2015)This thesis presents a measurement of the branching fraction of B0 → π0 π0 using data collected by the Belle experiment based at the KEKB accelerator in Tsukuba, Japan. This study is performed using the final and complete Belle dataset of 711 fb^−1, comprising about 771 × 10^6 BB pairs collected at the Υ(4S) resonance. The measurement of B0 → π0 π0 is a vital component of the study into the B → ππ system, through which a measurement of the CP violation parameter φ2 (also called α) can be made. This analysis makes use of an artificial neural network to improve continuum suppression as well as making use of timing information pertaining to photons detected in the Belle electromagnetic calorimeter to help distinguish between signal and background originating from QED backgrounds. Photon reconstruction was enhanced through the recovery of photons that converted within the inner detector. The timing information for 20% of the Belle dataset was recovered specifically for the use of this analysis. This is the first time a branching fraction for B → π0 π0 has been presented using an artificial neural network and such QED suppression techniques. The branching fraction of B → π0 π0 is measured to be Br(B → π0 π0 ) = (1.22 +0.28 −0.27,+0.10 −0.12 ) × 10^−6.
-
ItemCharacterization of silicon and diamond semiconductor devices in the low temperature regime( 2015)At ultra low temperatures, materials reveal interesting behaviours that become evident due to the freezing out of thermal vibrations. We report studies of two important group IV material systems using a newly-commissioned dilution refrigerator at temperatures of less than 50 mK, and under axial magnetic fields of up to 7 Tesla and 1 Tesla in the lateral plane. We realized these magnetic fields with a 3D vector magnet and calibrated the system using well-known test samples. The first material studied in detail was nitrogen enhanced grown ultranano- crystalline diamond (N-UNCD). Diamond displays a combination of many extreme physical properties such as a high thermal conductivity, the highest number density of atoms, and a wide optical band gap, and is in addition to these properties also biocompatible [1]. The ultra-nanocrystalline form of diamond is composed of small, 3-5 nm diameter grains of diamond and shares many of the desirable properties of the single crystal form, but is much easier to produce in thin form and can be used in wide variety of applications [2], especially for nanomechanics due to its strength and high Young's modulus [3]. When doped with boron, nano-crystalline diamond (B-NCD) displays superconducting properties below a critical temperature of less than a few Kelvin [4, 5]. The nitrogen doped form has found application in biomedical devices [6], but its superconducting behaviour at very low temperatures has not yet been demonstrated [7]. We fabricated N-UNCD thin films using microwave-enhanced CVD growth and used optical lithography to create Hall bar designs. We found that the conductivity of N-UNCD decreased with decreasing temperature, and between 36 mK and 4.9 K, a negative magneto-resistance was observed. Fitting the temperature and magnetic field dependent data with the 3D weak localization model developed by Kawabata [8], we found that 3D weak localization indeed plays a main role in the conduction mechanism of N-UNCD films even at ultra low temperatures. The second project was on the characterization of erbium doped silicon (Si:Er) semiconductor devices. Erbium has long been known to be important for use in optical fibre amplification in silica [9], and it also shows strong luminescent properties when it is added to Si as a dopant [10]. As such, Er is also of interest as optoelectronic semiconductor material [11]. Since the recent demonstration that it is possible to optically address single erbium ions in the silicon lattice [12] interest in Si:Er has increased. We studied silicon doped with erbium using ion implantation and report on our attempts to create CMOS devices with Er doped channels. The implanted material was characterized by optical spectroscopy, deep level transient spectroscopy, and measured both electrically and magnetically at low temperatures in the dilution refrigerator. Deep level transient spectroscopy performed on devices with varying anneal temperatures showed the emergence of electronically active traps with the minimum trap density occurring at annealing temperatures above 700 °C. Our results reveal that a rapid thermal anneal at 900 °C activates the luminescence from the implanted erbium ions. This effect remains, even if the sample is subjected to subsequent high temperature treatments. MOS devices co-doped with Er and P were fabricated and characterised to the extent they could be, based on the processing issues that arose.
-
ItemDirect shear mapping: the first technique to measure weak gravitational shear directly( 2015)This thesis develops and tests a new technique, called Direct Shear Mapping (DSM), to measure weak gravitational lensing shear, $\gamma$, directly from observations of a single background source. The technique assumes the velocity map of an un-lensed, stably-rotating galaxy will be rotationally symmetric. Lensing distorts the velocity map, making it asymmetric. The degree of lensing can be inferred by determining the transformation required to restore axisymmetry. This technique is in contrast to traditional weak lensing methods, which require averaging an ensemble of background galaxy ellipticity measurements, to obtain a single shear measurement. The accuracy of the fitting algorithm is tested in simulated data with a suite of systematic tests. It is demonstrated that in principle shears as small as $0.01$ can be measured. The shear is then fitted in very low redshift (and hence un-lensed) velocity maps, and a null result is obtained with an error of $\pm 0.01$. The high sensitivity achievable with DSM results from analysing spatially resolved spectroscopic images, including not just shape information (as in traditional weak lensing measurements) but velocity information as well. To investigate the prospects for making nonzero shear measurements with DSM in current and future surveys, a theoretical estimate is made of the frequency of galaxy-galaxy weak lensing at low redshift. The probability of weak lensing at low redshifts is found to be good (1 in 1,000 galaxies at $z\sim 0.2$). An algorithm is then presented to make an empirically driven estimate of the frequency of occurrence of weak lensing in existing low redshift galaxy survey data. This algorithm is applied to the Galaxy and Mass Assembly Phase 1 Survey Data Release 2 catalogue. It is estimated that to a redshift of $z\sim 0.6$, the probability of a galaxy being weakly lensed by at least $\gamma = 0.02$ is $\sim$0.01. A technique is then demonstrated to measure the scatter in the stellar mass-halo mass relation using a simulated sample of low redshift DSM shear measurements. It is estimated that for a shear measurement error of $\Delta\gamma = 0.02$, this measurement could be made with a sample of $\sim$50,000 spatially and spectrally resolved galaxies. Finally, the first step towards extending DSM to incorporate weak lensing flexion is made. Including flexion in a lensing analysis increases the sensitivity of weak lensing measurements, and facilitates measurement of the gradient of the gravitational potential. Weak lensing convergence, shear, and flexion field variables are derived for a generalised lensing mass, without assuming circular symmetry. It is shown that the equations reduce back to the correct expressions for simple lens mass distributions, and when solved numerically for circularly symmetric lenses reproduce the results obtained for analytical solutions. Finally, the equations are solved numerically for a simple non-circularly symmetric lens.
-
ItemGravitational lensing and clustering of galaxies in the epoch of reionization( 2015)The epoch of reionization marks the period where neutral hydrogen in the intergalactic medium was ionized by high energy photons emitted by the first stars and galaxies. Observations of galaxies during this period aim to uncover the types of stars and galaxies that were responsible for producing the ionizing flux to complete reionization within one billion years after the Big Bang (by z~6), and study the formation of the first galaxies in the Universe. These galaxies are observed in the near-infrared (NIR) today, and so require space-based observatories with sensitive NIR cameras such as Wide Field Camera 3 on the Hubble Space Telescope (HST). Considerable effort has been dedicated to ultradeep observations with HST in order to identify galaxies in the epoch of reionization. Surveys such as the eXtreme Deep Field (XDF), Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), Early Release Science (ERS) and the Brightest of Reionizing Galaxies (BoRG) have now discovered hundreds of these galaxies. This thesis makes use of these observations to address two important topics: how gravitational lensing affects the observations of such galaxies, and what their spatial distribution can tell us about the underlying dark matter distribution at this early time. Photons emitted by the first galaxies traverse most of the Universe’s history before reaching our telescopes. As such, they are subject to gravitational lensing by foreground galaxies along the line of sight. Gravitational lensing can result in the magnification of high redshift galaxies, skewing their observed luminosities. Because bright galaxies are significantly rarer than their fainter counterparts, the chance of gravitational magnification for observationally bright galaxies is significantly enhanced. This effect is known as magnification bias. We use the largest samples of Lyman-break galaxy (LBG) candidates observed in the first 1:5 billion years after the Big Bang (46.5. The clustering signal at z~7 is detected at >4 sigma, and corresponds to a real-space correlation length of r_0 = 6.7 +0.9/-1.0 cMpc, a galaxy bias of b = 8.6 +/- 0:9, and dark matter haloes of mass M = 10^(11:3+0:2/-0.3) M_sun . We reassess the clustering of LBGs at z=4–6 and find a trend of increasing bias from z=3.8 (b~3.0) to z=7:2 (b~8.6). We use these measurements to infer the fraction of dark matter haloes hosting UV-bright galaxies, and find that values near unity are preferred at z=7.2, which may be explained by the shorter halo assembly time at high redshift.