Physics and Astronomy

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This is the collection for the University of Waterloo's Department of Physics and Astronomy.

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1H MAS NMR Study of Water on Pd-MCM-41
(University of Waterloo, 2015-06-18) Crone, Joshua
Hydrogen spillover represents one of the most promising avenues to achieve hydrogen storage at the density required for transportation applications. The spillover phenomenon, however, is a topic of much debate, with many conflicting results existing in the literature. The goal of this work is to thoroughly study and understand a system where spillover has been reported, palladium loaded MCM-41 (Pd-MCM-41), thus laying a groundwork favorable to accurate, conclusive work on the spillover phenomenon in the future. Specifically, the nature of the material and how water (an important factor in certain spillover systems) interacts with it are investigated. This information would not only benefit the study of spillover, but also any other application of Pd-MCM-41 where water is involved. 1H MAS NMR measurements were carried out on three mesoporous silica samples at a range of water hydration levels. The materials were unmodified (pristine) MCM-41, Pd-MCM-41 and reduced (ie. treated with H2 gas) Pd-MCM-41. The spectra from these samples were fit using a previously published model for water on pristine MCM-41, which was extended to account for differences in the current samples. The model was applied first to the pristine MCM-41 spectra. In the hydration range from 0.0 to 0.2 monolayers (ML) of water adsorbed on the pore surface, the results were similar enough to those from Walia's publication that they were used to aid and assess the fitting results of the Pd loaded samples. Additional features observed in the results for pristine MCM-41 were the appearance of a new peak at 0.5 ML, attributed to water condensing and filling sections of the pore volume, and the development of water-water interactions, which are typically absent at lower hydration levels. Two additional peaks, designated Pd Water Groups 1 and 2, are observed in the spectra of the palladized samples. Once these were added to the fitting model, the spectra were determined to be adequately fit by the model; features of the remaining peaks matched those in the pristine sample. Two models, labeled A and B, are presented to explain the differences between the results from pristine MCM-41 and Pd-MCM-41. In Model A, water dissociatively and preferentially adsorbs onto the Pd, causing water to condense around the metal clusters. This multilayer water phase exchanges with water on the pore surface, resulting in Pd Water Group 1. Pd Water Group 2 is attributed to the first layers of water molecules strongly bound to Pd. Model B, which relies on Pd causing nearby water to have a large chemical shift, is shown to be unlikely. The main reason is that the chemical shift of this Pd shifted water is required to increase with increasing water hydration level in order to reproduce the observed chemical shift of Pd Water Group 1. For this and other reasons, Model A is concluded to be the most probable description for the behavior of water on Pd-MCM-41, based on the results presented.
400 kHz Spectral Domain Optical Coherence Tomography for Corneal Imaging
(University of Waterloo, 2021-12-23) Chen, Lin Kun
The cornea is the transparent, outermost layer of the human eye that contributes approximately 70% of the refractive power of the eye in air. It is composed of five major tissue layers: the epithelium, the Bowman’s membrane, the stroma, the Descemet’s membrane, and the endothelium. Corneal diseases such as Keratoconus and Fuchs’ dystrophy can change the morphology of some or all of the corneal layers, which can lead to vision impairment and eventually blindness. For example, Keratoconus causes localizes thinning and thickening of the corneal epithelium, damage to the collagen structure of the corneal stroma (scarring) and alteration of the corneal curvature. All of these changes result in blurred and double vision, and in severe cases can lead to corneal blindness that would require corneal replacement surgery. Fuchs’ dystrophy is a genetic disease that damages the endothelium cell. Since the endothelial cells are responsible for maintaining the fluid level in the stroma, impairment or death of the endothelial cells leads to dehydration or edema of the cornea that results in partial or full corneal blindness. Systemic diseases such as diabetes also affect the physiology and morphology of the cornea. Diabetes affects all the corneal cells and leads to abnormalities such as neuropathy, keratopathy, stromal edema, decrease in endothelial cell density, low tear secretion etc. Although there have been many clinical studies of these diseases, knowledge of the early-stage changes in the corneal morphology at the cellular level remains unclear. Understanding the early stage of disease development with the help of high speed and ultra-high resolution optical coherence tomography (UHR-OCT) corneal imaging can improve the early diagnostics of corneal diseases and well as monitoring the effectiveness of different therapies such as surgical intervention or administration of pharmaceutical drugs. The main objectives of my research project were: a) to upgrade the 34 kHz OCT system with a new camera that offered a 400 kHz data acquisition rate and 8192-pixel linear array sensor, b) test the performance of the 400 kHz OCT system for ex-vivo and in-vivo corneal imaging, and c) develop pre-processing for the interferogram and post-processing algorithms for the images. Implementing a camera with a faster acquisition rate will help to reduce the motion artifact caused by involuntary eye motions. Also, compared to 4500 pixels used in the 34 kHz camera, the new system utilizes approximately 7500 pixels, resulting in a larger scanning range. Although new camera has smaller sensor size (30% smaller), vertical binning is applied to ensure the light signal is all captured. However, due to the faster acquisition rate (~11 times faster), about 10 dB of SNR will suffers from the reduced integration time. Doubling the sample arm power while keep all other conditions the same can boost the SNR by about 3 dB. Therefore, incident power at the sample arm will be raised carefully according to the maximum permissible exposure calculated using the American National Standard for Ophthalmics – Light Hazard Protection for Ophthalmics instruments provided by ANSI. The result from the technical tests shows that the 400 kHz SD-OCT system offers 1 µm axial resolution in biological tissue with an extended scanning range of 2.8 mm (compared to 1.2 mm of the 34 kHz system). It has a lateral resolution of 1.04 μm/pix and can resolve group 7 element 6 of the USAF target with a 20x objective. It can provide 83 dB SNR with 0.95 mW of incident power at a 400 kHz image acquisition rate which should be sufficient to image semi-transparent biological tissues such as the human retina and cornea. However, given the much higher image acquisition rate (> 10x higher), the imaging power can be increased safely to ~ 4 mW, which will increase the system’s SNR to ~ 90 dB. So far, the performance of the 400 kHz OCT system has been tested by imaging plant tissues (cucumber) and ex-vivo pig corneas, due to the cancellation of all in-vivo human and animal studies imposed by COVID-19.
A 4d Lorentzian Spin Foam Model With Timelike Surfaces
(University of Waterloo, 2010-10-07T18:19:40Z) Hnybida, Jeffrey
We construct a 4d Lorentzian spin foam model capable of describing both spacelike and timelike surfaces. To do so we use a coherent state approach inspired by the Riemannian FK model. Using the coherent state method we reproduce the results of the EPRL model for Euclidean tetrahedra and extend the model to include Lorentzian tetrahedra. The coherent states of spacelike/timelike triangles are found to correspond to elements of the discrete/continuous series of SU(1,1). It is found that the area spectrum of both spacelike and timelike surfaces is quantized. A path integral for the quantum theory is defined as a product of vertex amplitudes. The states corresponding to timelike triangles are constructed in a basis diagonalised with respect to a noncompact generator. A derivation of the matrix elements of the generators of SL(2,C) in this basis is provided.
A Multi-Phase Analysis of Gas Dynamics and Perturbations in the Galaxy Cluster Cores
(University of Waterloo, 2025-02-14) Li, Muzi
This thesis provides a detailed analysis of gas kinematics and their interactions across various phases within galaxy cluster cores. It examines the processes that generate gas perturbations and the factors that contribute to the thermal stability of the intracluster medium (ICM). A focus is placed on exploring the origins of multi-phase gas and the mechanisms—particularly AGN feedback—that either couple or decouple their motions. Radio-mechanical AGN feedback is identified as one of the most promising heating mechanisms that prevent the cooling of gas. However, the debate on the details of the heating transport processes has remained open. The atmospheres of 5 cool-core clusters, Abell 2029, Abell 2107, Abell 2151, RBS0533 and RBS0540, have short central cooling times but little evidence of cold gas, and jet-inflated bubbles. The amplitudes of gas density fluctuations were measured using a new statistical analysis of X-ray surface brightness fluctuations within the cool cores of these ‘spoil’ clusters in Chapter 2. The derived velocities of gas motions, typically around 100 - 200 km/s, are comparable to those in atmospheres around central galaxies experiencing energetic feedback, such as in the Perseus Cluster, and align well with the turbulent velocities expected in the ICM. Regardless of the mechanisms driving these perturbations, turbulent heating appears sufficient to counteract radiative losses in four of the five spoiler cluster cores. We thus suggest that other mechanisms, such as gas sloshing, may be responsible for generating turbulence, offering a plausible solution to suppress cooling in these structureless atmospheres. Multiphase filaments, key byproducts of AGN feedback, are frequently observed near central galaxies, with their morphologies and kinematics closely linked to bubbles. In Chapter 3, we analyzed the velocity structure functions (VSFs) of warm ionized gas and cold molecular gas, identified through [OII] emission and CO emissions observed by the Keck Cosmic Web Imager (KCWI) and the Atacama Large Millimeter/submillimeter Array (ALMA), respectively, in four clusters: Abell 1835, PKS 0745-191, Abell 262, and RXJ0820.9+0752. Excluding Abell 262, where gas forms a circumnuclear disk, the remaining clusters exhibit VSFs steeper than the Kolmogorov slope. The VSFs of CO and [OII] in RXJ0820 and Abell 262 show close alignment, whereas in PKS 0745 and Abell 1835, were differentiated across most scales, likely due to the churning caused by the radio-AGN. The large-scale consistency in Abell 1835 and RXJ0820, together with scale-dependent velocity amplitudes of the hot atmospheres obtained from Chandra X-ray data, may support the idea of cold gas condensation from the hot atmospheres. X-ray observations have previously been constrained by low energy resolution, which has impeded direct measurements of velocity fields in galaxy clusters. However, the recent release of initial data from the X-ray Imaging and Spectroscopy Mission (XRISM) provides a non-dispersive energy resolution of about 5 eV, facilitating the measurement of line broadening and shifts. In Chapter 4 of this thesis, I detail my contributions to calibrating the optical blocking filters for XRISM using synchrotron beamlines at the Canadian Light Source (CLS) and Advanced Light Source (ALS) prior to its launch, and I discuss the model-based estimation of the parameters of the calibrated filters. This capability for direct measurement of plasma velocities is expected to greatly improve our understanding of the ICM dynamics with high accuracy.
Ablation loading and qudit measurements with barium ions
(University of Waterloo, 2023-08-31) Bramman, Brendan
Barium is one of the best ions for performing quantum information in a trapped-ion system. Its long-lived metastable D5/2 state allows for some interesting quantum operations, including the current best state preparation and measurement fidelity in qubits. This metastable state also opens up the possibility of implementing higher-dimensional qudits instead of qubits. However, installing a barium metal source in a vacuum chamber has shown to be somewhat of a challenge. Here, we present a loading technique which uses a barium chloride source instead, making it much easier to install. Laser ablation with a high-energy pulsed laser is used to generate neutral atoms, and a two-step photoionization technique is used to selectively load different isotopes of barium in our ion trap. The process of laser ablation and the plume of atoms it generates are characterized, informing us on how to best load ions. Loading is achieved, and selectivity of our method is demonstrated, giving us a reliable way to load 138Ba+ and 137Ba+ ions. The quadrupole transition into the metastable D5/2 state is investigated, with all of the individual transitions successfully found and characterized for 138Ba+ and 137Ba+. Coherent operations are performed on these transitions, allowing us to use them to define a 13-level qudit, on which we perform a state preparation and measurement experiment. The main error source in operations using this transition is identified to be magnetic field noise, and so we present attempts at mitigating this noise. An ac-line noise compensation method is used, which marginally improved the coherence time of the quadrupole transitions, and an additional method of using permanent magnets is proposed for future work. These efforts will help to make trapping barium more reliable, making it an even more attractive option for trapped ion systems. The state preparation and measurement results using the quadrupole transition to the long-lived metastable D5/2 state establish barium as an interesting platform for performing high-dimensional qudit quantum computing.
Above Bandgap Hyperpolarization Mechanism in Isotopically Purified Silicon and Optimal Bayesian Experiment Design for $T_1$ Estimation
(University of Waterloo, 2018-05-24) Alexander, Thomas
This thesis is concerned with the mechanism underlying the above bandgap illumination Dynamic Nuclear Polarization (DNP) of phosphorus donors in isotopically purified silicon-28. Two proposed DNP models are introduced and compared. A series of NMR saturation experiments are performed in which modified buildup dynamics are observed when the saturation tone is applied at the bare phosphorus resonance. This effect is attributed to the phosphorus donor being ionized via the Auger process resulting in dynamics which are modelled as a set of coupled Bloch equations. The donor bound exciton capture and neutralization rates are extracted, and a paramagnetic shift of the bare phosphorus resonance is observed. These observed dynamics strongly imply the DNP mechanism is due to phononic modulation of the donor electron spatial wavefunction inducing cross-relaxation between the hyperfine coupled electron and nuclear spins. The framework of Bayesian parameter estimation and its Sequential Monte Carlo(SMC) numerical implementation for continuous outcome probability distributions are introduced. Next, an introduction to Bayesian experiment design and its incorporation within the SMC framework is provided. A discussion of the computational challenges for continuous outcome distributions is given. To resolve these difficulties Monte Carlo Maximum Importance Sampling(MIS) numerical methods are developed which allow the evaluation of Bayesian experimental design heuristics such as the Bayes risk. These design strategies are applied to the problem of $T_1$ relaxation rate estimation with inversion recovery experiments. Experiments are optimized both respect to per-experiment performance and total experiment time. These techniques are shown to have substantial improvements over baseline methods. Furthermore, they compare favourably with previous frequentist experimental designs for IR experiments and demonstrate significant improvements.
Abundance Matching with the Galaxies of the Virgo Cluster and the Stellar-to-Halo Mass Relation
(University of Waterloo, 2012-09-27T21:17:19Z) Grossauer, Jonathan
Using data from the Next Generation Virgo Cluster Survey and high-resolution simulations of Virgo cluster-like halos, we determine the stellar-to-halo mass relation (SHMR) for subhalos, using the technique of abundance matching. The subhalo SHMR differs markedly from its field galaxy counterpart, regardless of how the subhalo mass is defined (mass at z = 0, mass at infall, or maximum mass while in the field). The slope of the relation at low mass (M⋆<10^10 Msun) is in all cases steeper than the same for the field. We find conflicting indicators of whether this difference in slope indicates an increasing or decreasing dark-to-stellar ratio; further modelling is required to reach a definitive conclusion. We also find evidence for the existence of a measurable age gradient in velocity, such that older subhalos have lower velocities than their younger peers. This opens the possibility that good quality redshifts of the lower mass galaxies of the Virgo cluster might provide additional constraints on the SHMR at high redshift and its evolution. Finally, we investigate the degree to which mergers, particularly major mergers, cause mixing of old and new material in halos, which has implications for the robustness of any implied radial age gradient. We find only a slight increase in mixing for major mergers over minor mergers, and little evidence for any large amount of mixing being induced by mergers of any ratio.
Achievable Polarization for Heat-Bath Algorithmic Cooling
(University of Waterloo, 2015-08-17) Rodríguez Briones, Nayeli Azucena
Highly pure quantum states play a central role in applications of quantum information science, both as initial states for quantum algorithms and as resources for quantum error correction. Controlled preparation of pure enough quantum states that satisfy the threshold for quantum error correction remains a challenge, not only for ensemble implementations like nuclear magnetic resonance (NMR) or electron spin resonance (ESR) but also for other technologies. Heat-bath algorithmic cooling (HBAC) is a promising method to increase the purity of a set of qubits coupled to a bath. In this thesis, we investigated the achievable polarization of this technique by analyzing the limit when no more entropy can be extracted from the system. In particular, we give an analytic form of the maximum polarization achievable for the case when the initial state is totally mixed, and the corresponding steady state of the whole system. Furthermore, we give the number of steps needed to get a specific required polarization (the exact number for the two qubit case and an upper bound for more general cases).
Adhesion of Two Cylindrical Particles to a Soft Membrane Tube
(University of Waterloo, 2012-09-26T19:49:05Z) Mkrtchyan, Sergey
The interaction of nanoparticles with biological systems, especially interactions with cell membranes, has been a subject of active research due to its numerous applications in many areas of soft-matter and biological systems. Within only a few relevant physical parameters profound structural properties have been discovered in the context of simple coarse-grained theoretical models. In this Thesis we study the structure of a tubular membrane adhering to two rigid cylindrical particles on a basis of a free-energy model that uses Helfrich energy for the description of the membrane. A numerical procedure is developed to solve the shape equations that determine the state of lowest energy. Several phase transitions exist in the system, arising from the competition between the bending energy of the membrane and the adhesion energy between the membrane and the particles. A continuous adhesion transition between the free and bound states, as well as several discontinuous shape transitions are identified, depending on the physical parameters of the system. The results are then generalized into a single phase diagram separating free, symmetric- and asymmetric-wrapping states in the phase space of the size of the particles and the adhesion energy. We show that for a relatively small size of the membrane tube the interaction between the cylinders becomes attractive in the strong curvature regime, leading to aggregation of the particles in the highly curved area of the tube that is characteristically different from the aggregation in a related three-dimensional system. For a relatively large membrane tube size the cylinders prefer to have a non-zero separation, even in the completely engulfed state. This indicates that, i) the spontaneous curvature of the membrane may play a role in the sign of the interaction of two colloidal particles adhered to a membrane and ii) cylindrical particles can aggregate on membrane tubes and vesicles if the curvature of the membrane around the aggregation region is sufficiently large.
Adsorption of surface-modified silica nanoparticles to the interface of melt poly (lactic acid) and supercritical carbon dioxide
(ACS Publications, 2015-04-28) Thompson, R.B.; Park, Chul B.; Chen, P.; Jeddi, K.; Sarikhani, K.
With the purpose of fabricating polymer nanocomposite foams and preventing coalescence in foaming processes, the interfacial tension of poly (lactic acid) (PLA) -silica composites is investigated in this work. Synthesized silica nanoparticles(SNs) with a CO2 - philic surface modification are used as the dispersednanoparticles. Interfacial tension is a key parameter in processing of polymer foamssince it directly affects the final foam properties, such as cell size and cell density.Interfacial tension of silica-containing PLA and supercritical carbon dioxide (CO2)is measured using Axisymmetric Drop Shape Analysis Profile (ADSA-P) pendantdrop method at high pressures and high temperatures. The interfacial tensionbetween PLA and supercritical CO2 is observed to decrease as a result ofnanoparticles’ adsorption to the interface. These results indicate that the reductionin interfacial tension with increasing silica content significantly deviates from alinear trend; there is a minimum at 2 wt. % loading of the SNs and then the interfacialtension curve reaches a plateau. Contact angle measurements show an affinity of theSNs for the polymer-supercritical CO2 interface, and these obtained results are usedto calculate the binding energy of the nanoparticles to the PLA / CO2 interface. Inaddition to interfacial properties, the adsorption of silica nanoparticles at theinterface is also studied in detail with Scanning Electron Microscopy.
Advances in Superconducting Circuit Quantum Electrodynamics
(University of Waterloo, 2022-01-31) Béjanin, Jérémy H.
The topics of this thesis are based on circuit quantum electrodynamics (cQED), a theoretical and experimental platform allowing the study of light--matter interaction. This platform is rich both in observable physical phenomena and future practical applications. A "circuit" in cQED may comprise various elements, with the two main types being electromagnetic quantum harmonic oscillators, or resonators, and superconducting Josephson quantum bits, qubits. Because of the relative ease to fabricate and control quantum circuits—especially when compared to the more traditional cavity quantum electrodynamics—cQED has quickly grown in popularity in research labs across the world and is regarded as one of the major contenders for quantum computing. The advances referred to in the title of this thesis address three significant challenges to practical applications of cQED; they are relevant not only to quantum computing, but also to other applications, such as simulations of physical systems. The first advance is related to control scalability. Practical applications require large circuits, and the current approaches used to send control signals to those circuits will not scale indefinitely. A solution to this challenge, the quantum socket, is presented and evaluated in depth. The second advance concerns calibration. Any application of cQED requires knowing the precise parameters defining the interactions between the various components of a circuit. Two cutting edge methods for the calibration of interaction parameters are explained and benchmarked; they show a remarkable improvement over existing, inefficient, methods. The third advance involves the physics of dielectric defects in the samples on which circuits are fabricated. These unwanted defects are modeled as two-level systems (TLS) that interact with circuit elements such as qubits. Experimental measurements and novel simulations conclusively demonstrate that interactions between TLS are responsible for the stochastic relaxation-time fluctuations observed in superconducting qubits.
Advancing Techniques for Detecting Dwarf Satellite Populations Beyond the Local Group
(University of Waterloo, 2022-05-20) Xi, Chengyu
This thesis includes material from three papers that develop techniques for quantifying faint dwarf satellite populations outside the Local Group. Dwarf satellites are important for our understanding of small-scale structure formation, the history of galaxy formation, and the nature of dark matter. Identifying faint satellites is technically challenging, however, because accurate distance information for these objects is usually unavailable and very expensive to obtain. In Chapter 2 we test a previously proposed method of estimating the average satellite population around nearby bright galaxies. The method uses structural cuts on size and magnitude to preferentially select low-redshift dwarf galaxies, and clustering to estimate the faction of true satellites within the selected sample. Using the high-precision photometric redshifts of the COSMOS survey, we were able to test the effectiveness of different structural cuts and optimize them for several different redshift ranges. We also describe a set of very nearby dwarf galaxies (at distances D < 200 Mpc) identified morphologically in the COSMOS field. In chapters 3 and 4, we introduce a new method for quantifying satellite abundance specifically designed for samples with high-quality photometric redshifts. The method allows us to measure satellite abundance around primaries in crowded fields, improving on previous methods that considered only isolated systems. The method also avoids the use of spectroscopic redshifts, which makes it much more expensive in observation time to reach a given depth in the satellite population. Chapter 3 focuses on establishing and testing the method, and presents some initial science results. In Chapter 4, we measure various satellite properties, including the satellite stellar mass function, the relative stellar mass function, and the quiescent fraction, as well as their dependence on primary properties such as mass, colour and specific star formation rate.
Advancing the robustness of polarization and time bin quantum key distribution for free-space channels
(University of Waterloo, 2023-05-15) Tannous, Ramy
Quantum networks are an emerging technology that aims to harness the power of quantum mechanics to revolutionize communication and computation. Many countries are establishing national quantum networks to modernizing their communication and computational infrastructure. Satellites are necessary to extend the distances between communication nodes to a global scale. Creating a global quantum network requires many such nodes to be built, increasing the overhead of a network. Thus, to increase adoption, reducing the overhead and increasing the robustness of the systems employed by these nodes are necessary. In this thesis, we begin by developing an upgraded polarization modulation system for the weak coherent pulse source that will be used to connect with the Quantum Science and Encryption Satellite (QEYSSat). This new system is an inline optical fiber solution that completely avoids the stability and alignment issues that were present in previous versions. The inline scheme reduces the need for realignment and maintenance. The performance of the prototype system is analyzed and investigated. Another aspect of the QEYSSat mission is investigated. Particularly the feasibility of the 6-state 4-state reference frame independent (RFI) protocol for a moving free-space channel. By using RFI protocols, the random polarization rotations that occur in optical fibers can be compensated for, particularly in the optical fiber that connects the source to the QEYSSat ground station telescope. Thus eliminating the need for active polarization compensation systems. The robustness of the protocol to overcome polarization misalignment is investigated in the context of a QEYSSat pass. Second, a fully passive time bin quantum key distribution scheme is developed and investigated. This scheme removes the need for active phase alignment of the interferometers between the two communication parties. Proof-of-concept experiments are conducted over several challenging channels, particularly highly multi mode optical fibers. This scheme is then used to investigate the feasibility of using near-infrared time bin encoded photons in a standard telecommunication optical fiber. Near-infrared is particularly interesting as many single quantum sources produce photons within this regime. The passive scheme is also tested in a moving free-space time bin demonstration. The results of these demonstrations are discussed, including the challenges that were encountered. Third, a novel optical design for a field widened interferometer is investigated. The new optical design employs a fully reflective imaging system that is similar to an Offner relay. The new optical design allows for long relative path delays while maintaining a relatively compact physical footprint. The performance of the interferometer is tested for both single mode and multi mode signals. In addition, the achromatic performance of the design is tested. The device is also tested in a quantum sensing scenario, demonstrating its practicality beyond quantum communications. Finally, a prototype of a monolithic chassis for the Offner relay interferometer is built using additive manufacturing with the objective of increasing the robustness of the interferometer. As part of the monolithic chassis, flexure devices are studied to be used instead of standard optomechanical components to provide the necessary degrees of freedom for optical alignment purposes. In addition, the thermal stability of the chassis is studied using finite element analysis with standard materials and an analytical analysis with functionally graded materials. Through various studies, experiments, and component design, this thesis has advanced the practicality of both polarization and time bin encoding for free-space channels. Particularly increasing the potential for satellite deployable time bin interferometers. This work contributes to the long line of progress leading towards realizing a global quantum network.
Adventures in Holography
(University of Waterloo, 2014-07-11) Razieh, Pourhasan
In this thesis, inspired by the holographic theories, we study a variety of interesting problems in gravity, condensed matter and cosmology. First, we explore the entanglement entropy of a general region in a theory of quantum gravity using holographic calculations. In particular, we use holographic entanglement entropy prescription of Ryu-Takayanagi in the context of the Randall-Sundrum 2 model with considering three kind of gravity theory in the bulk: the Einstein gravity, the general f(R) gravity and the Gauss-Bonnet gravity. Showing the leading term is given by the usual Bekenstein-Hawking formula, we confirm the conjecture by Bianchi and Myers for this theory. Further, we calculate the first subleading term to entanglement entropy and show that they agree with the Wald entropy up to the extrinsic curvature terms. Then, we study the holographic dual of what is known as quantum Hall ferromagnetism in condensed matter theory. This phenomenon, which has been observed in graphene sam-ples by applying strong magnetic field, is the emergence of energy gaps and Hall plateaus at integer filling fractions due to occurrence of spontaneous symmetry breaking. This effect is partially understood with certain perturbative calculations at weak coupling. The question is then whether this feature survives in a strongly coupled system as well. To address this question, we apply a well-established string theory dual, namely the D3-D5 system. In this framework, coincident D5 and D7-branes are embedded in the AdS5 × S5 background of the D3-branes. Within this holographic set-up and through the numerical calculations, we investigate the possibility of spontaneous symmetry breaking and find interesting phase transitions at finite temperature. Finally, we introduce a holographic description of our four-dimensional universe through a “brane world” scenario known as the Dvali-Gabadadze-Porrati (DGP) construction, where the brane refers to our universe embedded in a bulk space-time with five or more dimensions. In fact, we examine the DGP model as a theory of five-dimensional Einstein gravity coupled to four-dimensional branes while we assume five-dimensional spherical black hole metric in the bulk. Then, we study the phenomenological viability of the brane around this five-dimensional black hole. Further, we relate bulk, brane, and black hole parameters and the observational constraints on them. We find that viable solutions are indeed possible, hence we propose a holographic origin for the big bang. In particular, we suggest that our four-dimensional brane emerges from the gravitational collapse of matter in five dimensions which avoids the big bang singularity.
An ALMA View of Molecular Gas in Brightest Cluster Galaxies
(University of Waterloo, 2018-09-25) Vantyghem, Adrian
In this thesis I use ALMA observations to map the distribution and kinematics of molecular gas in the brightest cluster galaxies of three galaxy clusters: 2A0335+096, RXJ0821+0752, and RXCJ1504-0248. The goal is to understand how the coldest gas in clusters is formed, identify any long-lived structures that could fuel sustained black hole accretion, and explore star formation in cluster environments. I use the J=1-0 and J=3-2 rotational transitions of carbon monoxide (CO) as tracers of the total molecular gas distribution. The two transitions provide different resolutions and fields of view. The molecular gas in all three central galaxies are complex and disturbed. None show evidence for rotationally-supported nuclear structures, such as a disk or ring, that would be expected from either a merger origin or long-lived cooling flow. Instead, the molecular gas is either clumpy with no clear velocity structure or extends away from the galactic center in filaments that are several kiloparsecs long. The molecular filaments are coincident with nebular and bright X-ray emission, suggesting that they have condensed out of the hot intracluster medium. They are also generally associated with cavities in the X-ray emission inflated by the active galactic nucleus (AGN), suggesting that AGN feedback has stimulated the formation of molecular gas. The narrow velocity gradients along the filaments are only consistent with freefall if the filament is situated close to the plane of the sky. This is a common feature in brightest cluster galaxies. Since ram pressure is ineffective at slowing dense molecular clouds, the filaments must either be pinned to the hot atmosphere by magnetic fields or have condensed in-situ relatively recently. In RXCJ1504-0248 I combine the ALMA analysis with spatially-resolved ultraviolet emission tracing young stars. The central gas falls on the Kennicutt-Schmidt relation, while the filament has elevated star formation surface densities. The ongoing consumption of a finite fuel supply by star formation, or spatial variations in the CO-to-H2 conversion factor, may be diminishing the molecular gas surface density to produce this effect. Despite their drastic differences in morphology and environment, the molecular gas in clusters is still converted into stars following the same relation as in spirals and starbursts. I have also detected the J=3-2 transition from 13CO, an optically thin isotopologue of 12CO, in RXJ0821.0+0752. This enables a measurement of the conversion between CO intensity and molecular column density for the first time in a galaxy cluster. The CO-to-H2 conversion factor in RXJ0821+0752 is half of the Galactic value. If this value applies to other clusters, then it would alleviate the high coupling efficiencies required for molecular filaments to be uplifted by X-ray cavities. This analysis also provides reassurance that the molecular gas masses measured in BCGs are unlikely to be overwhelmingly biased by adopting the Galactic conversion factor.
An alternative derivation of orbital-free density functional theory
(AIP, 2019-05-28) Thompson, Russell B.
Polymer self-consistent field theory techniques are used to derive quantum density functional theory without the use of the theorems of density functional theory. Instead, a free energy is obtained from a partition function that is constructed directly from a Hamiltonian so that the results are, in principle, valid at finite temperatures. The main governing equations are found to be a set of modified diffusion equations, and the set of self-consistent equations are essentially identical to those of a ring polymer system. The equations are shown to be equivalent to Kohn-Sham density functional theory and to reduce to classical density functional theory, each under appropriate conditions. The obtained noninteracting kinetic energy functional is, in principle, exact but suffers from the usual orbital-free approximation of the Pauli exclusion principle in addition to the exchange-correlation approximation. The equations are solved using the spectral method of polymer self-consistent field theory, which allows the set of modified diffusion equations to be evaluated for the same computational cost as solving a single diffusion equation. A simple exchange-correlation functional is chosen, together with a shell-structure-based Pauli potential, in order to compare the ensemble average electron densities of several isolated atom systems to known literature results. The agreement is excellent, justifying the alternative formalism and numerical method. Some speculation is provided on considering the timelike parameter in the diffusion equations, which is related to temperature, as having dimensional significance, and thus picturing pointlike quantum particles instead as nonlocal, polymerlike, threads in a higher dimensional thermal-space. A consideration of the double-slit experiment from this point of view is speculated to provide results equivalent to the Copenhagen interpretation. Thus, the present formalism may be considered as a type of “pilot-wave,” realist, perspective on density functional theory.
Aluminium-Palladium Transition Edge Sensors
(University of Waterloo, 2008-09-15T19:47:06Z) Persaud, Lauren Margaret
A superconducting Transition Edge Sensor (TES) can be used to make the most sensitive thermometer which operates in a very narrow temperature range. The thin film bi-layer fabrication details are discussed as well as application in condensed matter physics. These include: measurement of quasi-adiabatic latent heat of superconducting transition, cobalt thermometry and photon detection.
Amended Model of Large Scale Dark Matter Structure
(University of Waterloo, 2020-09-03) Chen, Alice
Dark matter makes up around a quarter of the total energy density in the universe, but its identity remains elusive. Current ways of studying dark matter have centered around its macroscopic properties, such as density distribution and large scale structure formation. The halo model of large scale structure is an important tool that cosmologists use to study the phenomenological behaviour and nonlinear evolution of structure in the universe. However, it is well known that there is no simple way to impose conservation laws in the halo model. This can severely impair the predictions on large scales for observables such as weak lensing or the kinematic Sunyaev-Zel’dovich effect, which should satisfy mass and momentum conservation, respectively. For example, the standard halo model overpredicts weak lensing power spectrum by > 8% on scales > 20 degrees. To address this problem, we present an Amended Halo Model, explicitly separating the linear perturbations from compensated halo profiles. This is guaranteed to respect conservation laws, as well as linear theory predictions on large scales. We also provide a simple fitting function for the compensated halo profiles, and discuss the modified predictions for 1- halo and 2-halo terms, as well as other cosmological observations such as weak lensing power spectrum. Similar to previous and recent works centered around the halo model, this work is physically motivated and matches simulation data to a greater degree of accuracy than the standard halo model currently does. We compare our results to previous work, and argue that the amended halo model provides a more efficient and accurate framework to capture physical effects that happen in the process of large scale cosmological structure formation.
Amplitudes in the Spin Foam Approach to Quantum Gravity
(University of Waterloo, 2017-07-31) Chen, Linqing
In this thesis, we study a Spin Foam model for 4D Riemannian quantum gravity, and propose a new way of imposing the simplicity constraints that uses the recently developed holomorphic representation. Rather than imposing the constraints on the boundary spin network, one can impose the constraints directly on the Spin Foam propagator. We find that the two approaches have the same leading asymptotic behaviour. This allows us to obtain a model that greatly simplifies calculations, but still has Regge Calculus as its semi-classical limit. Based on this newly developed model, we aim at answering the following questions that previously has never been properly addressed in the field: how to efficiently evaluate arbitrary Spin Foam amplitudes in 4D? Do we have residual diffeomorphism invariance of the model? What happens to the amplitudes under coarse graining? Can we learn the degree of divergence of an amplitude simply by its graphic properties? What type of geometry in the bulk has the dominant contribution to the partition function? Using the power of the holomorphic integration techniques, and with the introduction of new methods: the homogeneity map, the loop identity and a natural truncation scheme, for the first time we give the analytical expressions for the behaviour of the Spin Foam amplitudes under 4-dimensional Pachner moves. The model considered is not invariant under the 5--1 Pachner Move, as the configuration of five 4-simplices reduces to a single 4-simplex with an insertion of a nonlocal operator inside. Similar behaviour occurs also for the 4--2 move. The non-invariance under 5--1 move means that the vertex translation symmetry, the residual of diffeomorphism invariance for discrete gravity, is broken in this path integral formalism. We also developed a natural truncation scheme that captures the dominant contribution and preserves the geometrical structures, while at the same time efficiently reduces the complexity. We then push the result to be more general -- evaluating arbitrary amplitudes. We study the amplitudes on arbitrary connected 2-complexes and their degrees of divergence. First we derive a compact expression for a certain class of graphs, which allows us to write down the value of bulk amplitudes simply based on graph properties. We then generalize the result to arbitrary connected 2-complexes and extract a formula for the degree of divergence only in terms of combinatorial properties and topological invariants. By regulating the model, this result allows us to find the dominant contributions to the partition function, which gives us some valuable hints about the continuum limit. The distinct behaviors of the model in different regions of parameter space signal phase transitions. However, in the regime which is of physical interest for recovering diffeomorphism symmetry in the continuum limit, the most divergent contributions are from geometrically degenerate configurations. We finish with discussing possible resolutions, the physical implications for different scenarios of defining the continuum limit and the analytical insights we have gained into the behavior of Spin Foam amplitudes.
Analysis of the Thermal Fisher Information in 2+1 Dimensional Black Hole Spacetimes
(University of Waterloo, 2023-01-24) Patterson, Everett Avison
In the past decade, relativistic quantum information (RQI) has presented itself as a promising avenue for examining the interaction between relativistic and quantum effects. This is most commonly achieved by coupling Unruh-DeWitt detectors to quantum fields in curved spacetime, or by applying quantum properties to otherwise relativistic systems. A lot of attention is placed on what information we can extract from the vacuum state of a quantum field and what can that tell us about the underlying spacetime. While there has been a lot of theoretical progress made within the field, its experimental applications remain rather scarce. Relativistic quantum metrology (RQM), which concerns itself with the precision of measurements within systems that have both relativistic and quantum effects, is an example of an experimental flavour that RQI can take. One of the metrics of particular importance within RQM is the Fisher information. This form of `information' quantifies the knowledge that can be extracted about an underlying parameter based on the measurement of a dependent observable parameter. In this thesis, we consider the thermal Fisher information extracted by a UDW detector in (2+1)-dimensional spacetimes, including the first Fisher information analysis of a black hole spacetime. We provide a detailed analysis of the Fisher information for the BTZ black hole including the identification of the true black hole effects by contrasting our black hole results against those in anti-de Sitter (AdS) spacetime. We further characterize its dependence on various black hole and detector parameters, in addition to describe how the Fisher information might be used as a black hole probe. We find that the Fisher information is sensitive to the black hole parameters of mass and rotation. So much so in the case of the mass that based on the Fisher information for a given set up, we can identify the mass of the black hole. We also identify novel Fisher information behaviours unique to the BTZ black hole by contrasting these with AdS and previous results (which we actually correct). Beyond acting as a spacetime probe, the Fisher information analysis in this thesis can also enable the improvement of the estimation of the KMS temperature by a UDW detector. The majority of the work in this thesis can be found on the arXiv in a publicly accessible manuscript [Patterson and Mann, 2022], and a second manuscript is in progress covering the remainder of the work presented here.

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