Physics
http://hdl.handle.net/10523/98
Tue, 27 Sep 2016 07:21:53 GMT2016-09-27T07:21:53ZControlling Photon Echoes with the Light Shift
http://hdl.handle.net/10523/6787
Controlling Photon Echoes with the Light Shift
2016
McDonald, Hamish Callum
Recent progress with rare-earth-ion ensemble quantum memories has resulted in a family of echo-memory protocols which use sequences of pulsed light fields to recall a pulse stored in the ensemble earlier. The simplest of these protocols, the two-pulse photon echo, has been deemed too noisy to operate in the single-photon regime.
In this work we introduce and investigate the LiSPER protocol, which involves using off-resonant pulses to control the recall of the stored pulse from the ensemble. In the LiSPER protocol, two off-resonant pulses dynamically tune the resonant frequencies of the ions in the ensemble such that an unwanted noisy echo is suppressed and a less noisy echo of the input pulse is emitted at a later time. A theoretical basis for LiSPER is formed with analytical calculations and numerical simulations, and the dependence of the protocol efficiency on parameters of the off-resonant pulses is characterised. Also presented are the results of experiments performed using a praseodymium ion ensemble. These results agree qualitatively with the theoretical work, although the entire LiSPER protocol is not demonstrated.
Fri, 23 Sep 2016 03:12:38 GMThttp://hdl.handle.net/10523/67872016-09-23T03:12:38ZDrivers and Dynamics of the Ross Sea
http://hdl.handle.net/10523/6786
Drivers and Dynamics of the Ross Sea
2016
Jendersie, Stefan
The seasonal control mechanisms of the circulation in the Ross Sea are studied here by utilising a climatological three-dimensional numerical model based on the Regional Ocean Model System (ROMS). Of interest are the physical processes that facilitate exchange across the continental shelf break and the ice shelf front, as these act as effective barriers between different oceanographic regimes in the Ross Sea. In this thesis details of the system of cyclonic and anticyclonic flow structures and their transport variability are described. The analysis identifies external and internal forcing mechanisms, their region of influence and corresponding seasonal evolution. This study determines the temporal scales on which the seasonal signal propagates throughout the ice shelf cavity.
Simulations show significant annual variation in volume fluxes of 2-10 Sv in the Antarctic Slope Current (ASC). Away from the coast the ASC’s transport is dominated by zonal differences in sea surface height (SSH) with wind induced transport playing only a minor role. SSH is generally higher over the continental shelf and its variability is not correlated with the annual sea ice cycle. While the isopycnal structure of the spatially-coincident Antarctic Slope Front is found to modulate current speeds within the ASC it has no significant effect on the transport magnitude. Over the continental shelf a system of three anticyclonic and one cyclonic circulation cells have been identified that facilitate the dvection in the interior, including the ice shelf cavity. Individual currents carry up to 2 Sv seasonally. Constrained by the banks and depressions, the cells are spatially persistent but experience individually different temporal changes. Horizontal differences in density and subsequent baroclinic pressure gradients are found to be the main control of their dynamics. Two competing mechanisms are found in the model that reinforce the salinity dominated density gradients. Circumpolar Deep Water (CDW), sourced from the ASC, sits next to northward-flowing High Salinity Shelf Water (HSSW) and other dense Shelf Water. CDW resupply events seem triggered by a zonal shift of the ASC on the order of ∼10 km that occurs at different times along the shelf break. The second seasonal process to strengthen density gradients is HSSW production through intense winter sea ice formation in the polynyas of the south-western Ross Sea. The HSSW formation is synchronised with the atmosphere.
The polynya initiates two additional advection drivers. One is the gravity-driven bottom flows that establish the western band of the large anticyclonic circulation cell that ventilates the western cavity up to 175◦ W. Main inflows are located east and west of Ross Island. Approximately 74 % of the 0.87 Sv HSSW exported annually from the Ross Sea Polynya (RSP) and the McMurdo Sound Polynya (MMSP) are advected into remote parts of the cavity, and 10 % exits the shelf along the western coast. No significant HSSW quantities from the Terra Nova Bay Polynya (TNBP) intrude southward underneath the RIS, instead HSSW formed here drains north.
The second advection driver, which has not been described previously, is caused by the significant depression of SSH, and is localized to the deep convection cell, where the entire water column is saturated with high density HSSW. Regional SSH gradients are large enough that the associated barotropic pressure gradient overwrites the counteracting baroclinic pressure gradient. This process drives an additional cyclonic circulation cell in the region of the RSP during winter. SSH gradients are also responsible for driving the Victoria Land Coastal Current, where the meridional density gradient is strengthened by abundant Ice Shelf Water (ISW) supplied from beneath the McMurdo Ice Shelf.
Oceanographic conditions were also derived for heat intrusion under the Ross Ice Shelf at intermediate depth over the eastern continental shelf. Conditions that allow a change in the relevant potential vorticity of flows are a function of the local isopycnal structure and SSH with warm Circumpolar Deep Water (CDW) over Hayes Bank adjacent to Low Salinity Shelf Water (LSSW).
Mon, 19 Sep 2016 21:49:30 GMThttp://hdl.handle.net/10523/67862016-09-19T21:49:30ZInternal gravity waves in McMurdo Sound, Antarctica
http://hdl.handle.net/10523/6668
Internal gravity waves in McMurdo Sound, Antarctica
2006
Albrecht, Nicole
Internal waves have an impact on many oceanic processes such as heat transport and mixing and have been studied intensively in mid and low latitudes. Their presence in polar regions has received less attention due to difficulties involved in data collection. High latitudes, beyond the critical latitude for the M2 tide (74.5°S) are particularly interesting and almost unstudied. The behaviour of internal waves is significantly changed in these regions as they cannot propagate freely.
This thesis describes internal waves in McMurdo Sound, Antarctica. It is unique in three respects: first, the study site is beyond the critical latitude for the M2 tide and is the highest southern latitude (77.5°S) studied for internal waves, second, some of the data were collected during the Antarctic winter, and third, it is the first study of internal waves in McMurdo Sound.
The spatial and temporal variation of internal waves, their frequency distribution and the vertical structure of the water column have been investigated during winter 2003 (April- September) and spring 2004 (November). Two different approaches have been taken. Profiles give a good vertical resolution for short time periods, while moored instruments offer the opportunity to study longer-term changes at particular depths.
Vertical stratification changed significantly throughout the year. Temperature profiles are almost constant from May to November (mean temperature around -1.9°C). Simultaneously, density, which is mainly a function of salinity in this low temperature regime, increases and its variability with depth decreases. This is directly related to the annual cycle of sea-ice formation.
Solar radiation in summer injects heat and fresh meltwater into the surface water layers, enhancing stratification. On the other hand, during winter, salt rejection from the local ice cover forces mixing.
Internal tides in McMurdo Sound are sub-inertial. They are dominated by the diurnal frequency band with constituents having amplitudes of around 4 m followed by the semi-diurnal frequencies. Their energy levels decrease during the year, as expected, coinciding with the decrease of stratification of the water column that is necessary to support internal waves. Internal tides are most likely forced and decay as they propagate away from their generation site. Three possible generation sites emerge, local generation within the Sound, coastal internal Kelvin waves or semi-diurnal waves could have propagated from lower latitudes.
Spectral slopes of higher frequency oscillations are consistent with the Garrett-Munk spectrum and Kolgomoroff's turbulence law. They indicate that both turbulent flows and internal waves could be responsible for the observations.
Mon, 04 Jul 2016 02:31:50 GMThttp://hdl.handle.net/10523/66682016-07-04T02:31:50ZInvestigation of first and second order correlation functions of ultra-cold atomic gases
http://hdl.handle.net/10523/6287
Investigation of first and second order correlation functions of ultra-cold atomic gases
2016
Toth, Emese
In the first part of the thesis we develop a theoretical description of the correlations between ultra-cold bosons after free expansion from confinement in an optical lattice. We describe the single particle evolution of the system during expansion and establish criteria for a far-field regime. We develop expressions for first and second order two-point correlations based on a variety of commonly used approximations to the many-body state of the system including: Bogoliubov theory, mean-field decoupling, and particle-hole perturbative expantion about the perfect Mott-insulator state. Using these approaches we examine the effects of quantum depletion and pairing on system correlations. We justify a Gaussian approximation from which we develop a general three-dimensional theoretical formalism for an inhomogeneous lattice system by comparison with the directly calculated correlation functions.
In the second part of the thesis we show how the conventional wisdom that increasing tem- perature means decreasing quantum coherence in system can be faulty. We explore a practical example that can be explored in optical lattice experiments. Using finite temperature perturbation theory and exact calculations we show that the short-range coherence of the Mott-insulating phase of bosons in an optical lattice can increase substantially with increasing temperature. We demonstrate that this phenomenon originates from thermally produced defects that can tunnel with ease. Since the near zero temperature coherence properties have been measured with high precision we expect these results to be verifiable in current experiments.
In the last part of the thesis we turn our attention to one dimensional Bose gases. We fo- cus on the moderate temperature weakly interacting regime namely the high temperature quasicondensate and quantum decoherent regimes, which we investigate using the stochas- tic projected Gross-Pitaevskii equation (SPGPE) formalism. Here we compare the SPGPE results against the Bethe ansatz and a mean-field formalism adapted to the quasicondensate regime. This theory covers a regime not addressed by other formalism.
Fri, 18 Mar 2016 03:16:47 GMThttp://hdl.handle.net/10523/62872016-03-18T03:16:47ZMarkov Chain Monte Carlo Based Analysis of Bearing Vibrations
http://hdl.handle.net/10523/6214
Markov Chain Monte Carlo Based Analysis of Bearing Vibrations
2016
Lawrence, Hilary Jane
The aim of the work in this thesis is to use bearing vibration data to infer the condition of bearings, in particular to detect the presence or absence of damage, and identify the location and cause of damage to parts of the bearing structure. Traditional methods of vibration analysis are evaluated and compared with Markov chain Monte Carlo (MCMC) methods.
The vibration datasets come from a selection of bearings that have been run from new. After running-in bearings, some are allowed to continue running for several hours. Others have seeded defects applied to balls or bearing races, and are then run for a short time. These data are used to evaluate the effectiveness of time-domain, frequency-domain and MCMC methods in the field of bearing vibration analysis.
Time-domain measures can detect the presence or absence of damage in most cases. Different defect types affect vibrations differently, and this can be seen in plots of vibration data. The conventional time-domain methods investigated take a discrete measure or measures over each data. They are simple to implement, but do not have the ability to detect these differences in vibrations, and therefore cannot detect the nature of damage.
Analysis in the frequency-domain is based on the fast Fourier transform (FFT) and filtering. It is successful in detecting the nature of some defects. Bearings with some defect types, or with low levels of damage, are difficult to distinguish from undamaged bearings, and each defect type is better detected using different filtering. These differences suggest that prominent vibrations occur at modes of flexural vibration of the outer race, and that the inner race can be approximated as a rigid body. A classification scheme is implemented using a mixture of time-domain and frequency-domain measures. This scheme can correctly classify data from many - but not all - bearings, but it gives no information on the certainty of classifications.
The findings from the time-domain and frequency-domain analysis have been well explored previously. In this thesis they are combined with Bayesian inference, using a correlation based measure of likelihood to infer bearing condition. MCMC methods are implemented using an adaptive Metropolis (AM) algorithm, existing models of bearing behaviour, and vibration data in the time-domain. MCMC outputs give estimations of the empirical marginal distribution of parameters, so that not only can parameter values - and therefore the underlying physical mechanism - be estimated, but uncertainties on these estimates can also be quantified.
An aims of this thesis is to see whether MCMC methods can be used to infer properties of bearing condition. This thesis shows this is possible, but there are difficulties to overcome. In particular, the use of the correlation based measure of likelihood results in the chain failing to converge in some cases. These states are identifiable, as the likelihood is lower than when convergence occurs. Possible solutions to this problem are discussed.
Analysis of MCMC outputs leads to model refinements to better measure bearing properties. Marginal distributions relating to vibration frequency have properties that allow the physical cause of vibration sources to be inferred. Using data in the time-domain has some advantages when separating the individual sources of these periodic vibrations. In the frequency domain some of these sources have harmonics or sidebands that coincide with predicted frequencies of other sources. In addition, some of these frequencies are not at their predicted values. This is not unexpected, as slippage and the effect of loads are known to alter these frequencies. The analysis of MCMC outputs allows the physical cause of discrepancies to be investigated. Vibrations during the normal operation of undamaged bearings have certain properties that differ from vibrations caused by bearing defects, and relevant parameters show these differences. Distributions of parameters relating to the amplitude and duration of vibrations caused by defects are shown to have a relationship to defect dimensions. Further development is required before defect dimensions can be directly inferred.
This thesis shows that traditional methods of vibration analysis can distinguish between damaged and undamaged bearings in most cases, and detect the nature of damage in some cases. MCMC methods have potential in the field of bearing vibration analysis, and can provide meaningful inference of the physical properties of bearings.
Mon, 15 Feb 2016 01:44:10 GMThttp://hdl.handle.net/10523/62142016-02-15T01:44:10ZSequential Inference for Dynamical Systems
http://hdl.handle.net/10523/6183
Sequential Inference for Dynamical Systems
2016
Morrison, Malcolm Erik King
Sequential inference methods have played a crucial role in many of the technological marvels that we use today, from GPS and navigation systems to machine learning. Most current methods, such as the unscented Kalman filter (UKF) make several, occasionally crippling assumptions which allow them to work efficiently and accurately for approximately linear dynamics. The problem with this is that the majority of systems are not linear. Inference methods fully representing the dynamics and probability distributions were considered infeasible in the early days of sequential inference. However, with the capabilities of modern computers this is no longer the case. In this thesis we propose a method to evolve a probability distribution on a dynamical system explicitly. This is done by using a finite volume partial differential equation solver to solve the continuity equation, combined with Bayesian observations. We present an example case of the simple pendulum and compare this with the UKF to examine several advantages.
Mon, 25 Jan 2016 22:50:57 GMThttp://hdl.handle.net/10523/61832016-01-25T22:50:57ZPower Efficient, Telemetry-Enabled Position Sensor for Animal Tracking
http://hdl.handle.net/10523/6137
Power Efficient, Telemetry-Enabled Position Sensor for Animal Tracking
2016
Panckhurst, Bradley
This thesis describes the design, development and characterisation of an animal position sensor system. It focuses on the implementation of a telemetry system capable of retrieving position data wirelessly.
The position sensor electronics weigh only 10 g and utilises the 868 MHz frequency band for telemetry. Implementing a solar energy harvesting system, the sensors demonstrated the potential for an indefinite deployable lifetime. With over 300,000 location acquisitions, the static position sensors were found to have a Circular Error Probable accuracy of < 4 m. The telemetry system showed reliable data transmission out to 1 km. However, it is capable of a range of 2.4 km with a clear line of sight.
All aspects of the position sensors were developed to be power efficient. The development included details of the hardware circuit design and the software for both the position sensor devices and the base station. Also included is a customised telemetry protocol centred around power efficient strategies. Characterisation and testing of the position sensor included the evaluation of three key factors. The factors investigated were location accuracy, power consumption and the telemetry transmission performance.
Finally, future revisions to the position sensor system are detailed, with exciting possibilities from the hardware layer up to the telemetry protocol.
Tue, 05 Jan 2016 01:50:08 GMThttp://hdl.handle.net/10523/61372016-01-05T01:50:08ZC-Field Theory of Dynamics in One Dimensional Bose Gases: The Kibble-Zurek Mechanism and Bright Soliton Arrest
http://hdl.handle.net/10523/5871
C-Field Theory of Dynamics in One Dimensional Bose Gases: The Kibble-Zurek Mechanism and Bright Soliton Arrest
2015
McDonald, Rob
A grand canonical C-field theory has been previously developed for modelling finite temperature Bose gases, the stochastic projected Gross-Pitaesvkii equation (SPGPE) [1]. Previous investigations have shown quantitative agreement between experiments and the SPGPE, with a fitted growth rate [2,3] or no fitted parameters at all [4]. These and other works [5,6] have used the number-damping SPGPE, a sub-theory of the SPGPE neglecting a scattering process between the coherent and incoherent regions that conserves the particle number of each region, known as energy-damping. The systems in these works were in quasi-equilibirium and as such a growth process, also known as number-damping, is thought to be dominant. Evidence suggests that energy-damping is significant when the system is far from equilibrium [7]; we may also postulate systems where energy-damping is the only allowed process. In this thesis we use the full SPGPE including the energy-damping reservoir interaction in systems where this process plays an important role in the dissipative evolution.
We model quenches of chemical potential across the Bose-Einstein condensation transition in a one dimensional Bose gas confined to a toroid. We use two different models; the full SPGPE and the number-damping SPGPE. The purpose of this is to test the results of our simulations against the predictions of the Kibble-Zurek mechanism (KZM), a theory of defect formation in second order phase transitions. We find that both models give results consistent with KZM, in that various measurable quantities obey a power law with respect to the quench time. The power law exponents are determined by critical exponents, which depend on the universality class of the phase transition. We find the number-damping SPGPE results are consistent with the critical exponents predicted by mean field theory. We are unable to find a universality class with critical exponents consistent with the results of the full theory, and in particular the dynamical critical exponent differs from that predicted by mean field theory.
We also use the SPGPE to simulate the motion of a bright soliton in a one dimensional attractive Bose gas confined to a toroid and in contact with a thermal cloud of a second component. The bright soliton is an analytical solution of the one-dimensional Gross-Pitaevskii equation for an attractive Bose-Einstein condensate, which can propagate in space without changing its functional form. We derive a stochastic differential equation for the soliton velocity, finding that the energy-damping reservoir interaction manifests as an Ornstein-Uhlenbeck process for velocity decay, affording a complete analytic solution for the damping and diffusion rates of the bright soliton. The results of simulating the bright soliton using the SPGPE are compared against the analytic solutions of the velocity stochastic differential equation, including the mean, variance, two-time correlations, and power spectra of the velocity. We find that the numerical and analytical solutions show excellent agreement for all these quantities, validating our procedure for obtaining the velocity equation of motion.
Tue, 08 Sep 2015 03:56:11 GMThttp://hdl.handle.net/10523/58712015-09-08T03:56:11Z