Physics and Astronomy

Permanent URI for this collectionhttps://uwspace.uwaterloo.ca/handle/10012/9949

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

Research outputs are organized by type (eg. Master Thesis, Article, Conference Paper).

Waterloo faculty, students, and staff can contact us or visit the UWSpace guide to learn more about depositing their research.

Browse

Now showing 1 - 20 of 841

Structured Wavefunctions for Precision Quantum Metrology
(University of Waterloo, 2025-10-20) Kapahi, Connor
In this thesis, several projects from biomedical optics measurements of the retina to precision gravimetric designs with neutron interferometers are presented, united by the common theme of applied quantum information techniques to develop next-generation precision metrological instruments. In particular, we introduce theoretical tools for analyzing neutron optical experiments and highlight parallels between neutron and light optics. These tools are applied to a new neutron prism design, demonstrating significantly higher transmission than traditional designs. Designs for devices applying these techniques, including a neutron Fresnel prism, spectrum analyzer, and spin collimator, are discussed. Potential advantages in neutron flux and spectrum resolution are quantified for these designs. The isometry between neutron spin and the polarization of light is exploited to validate the neutron spin collimator experimentally. Applications of structured states of light and experiments applying spin-orbit states to create patterns in the human visual system are described. Results demonstrate an increase in the perceived extent of these patterns, from 3° for Haidinger's Brush to 10° for a spin-orbit state. Work demonstrating a new method of generating a lattice of spin-orbit states in light is applied to neutron optics. Throughout the preceding experiments, methods of modeling neutron optics experiments with light and a semi-classical path-integral approximation have been developed. These methods are then applied to design an experiment that measures the gravitational constant using a neutron interferometer. A three-phase grating moiré interferometer (3-PGMI) design is first tested with infrared light. The deflection caused by a wafer sample is measured with the 3-PGMI and found to match direct measurements. The path-integral model is then applied to determine the uncertainty in the gravitational constant that can be achieved with a near-term measurement with a neutron 3-PGMI. An experiment to measure the gravitational constant is described, with an uncertainty budget, resulting in a measurement to 150 ppm. Potential corrections to previous experiments measuring the gravitational constant, due to lunar gravitational forces are quantified. Future applications of the tools and techniques described in this thesis are then discussed.
Translocation-Induced Shape Transitions in Vesicles using a Neural Network-Based Solver for the Helfrich Model
(University of Waterloo, 2025-10-16) Choheili, Soorna
This thesis discusses our efforts to model the translocation of an enclosed lipid bilayer membrane (vesicle) through a circular pore. First, we will discuss the study of lipid bilayers, introduce the standard model for representing the energy of a membrane, and provide background on the many theoretical and experimental efforts in the field of membrane modeling. We then review the relevant theoretical and practical considerations regarding the simulation of vesicles and translocation, and implement a neural network-based solver for a scalar phase field. We will proceed to detail our efforts to characterize each constraint imposed on the vesicle throughout the translocation and model them within the context of the solver. Following this, we provide a variety of visual snapshots of the translocation process showing different classes of translocation and the resulting behavior of each. Equally important is the quantitative analysis of the energy landscape traversed by the vesicle, where we chart the induced bending energy imposed upon it by the narrow pore. Additionally, we introduce two types of external effects that modify the energy landscape and illustrate their impact on the total vesicle energy throughout its passage. We then map the results out onto the relevant parameter space to give a picture of where the thresholds between qualitatively different behaviors lie. As a final demonstration of our model’s capabilities, we estimate the time of passage of the vesicle by modeling it diffusively using the energy landscape to calculate the effect of narrower pores on the time to translocate. This model successfully demonstrates explicit phase transitions between stable vesicle states and maps out the energy landscape throughout the unstable regime under the effects of translocation.
The Dependence of Halo Mass on Galaxy Size at Fixed Stellar Mass and Colour using Galaxy-Galaxy Lensing
(University of Waterloo, 2025-10-16) Patel, Darshak
To advance our holistic understanding of galaxy formation physics, we must examine the relationship between baryonic matter and dark matter (DM) within the universe. In this thesis, we investigate the correlation between dark matter halo mass and galaxy size at fixed stellar mass and colour. Using galaxy-galaxy lensing, we measure excess surface density (ESD) profiles for red, early-type galaxies from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging survey, with shape measurements from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). We model the lensing signal using a conditional stellar mass function (CSMF) halo occupation distribution (HOD) calibrated on the AbacusSummit simulations and adopt a power-law relation between halo mass and galaxy size at fixed stellar mass: Mh ∝ r^ηeff . To fit the HOD parameters to the observationally measured ESD vectors, we train and utilize neural-network emulators, a form of non-linear interpolators. The analysis performed in this work follows a two-step process. First, we fit the HOD parameters to the ESD vectors corresponding to three stellar mass bins, not split further into two size bins. We freeze the best-fit HOD parameters and generate the corresponding best-fit mock catalogue. Using this best-fit mock catalogue, we apply our halo mass-size model and fit to the size-split observational ESD measurements. For central galaxies with stellar mass 10.5 ≤ log10(M⋆/M⊙) < 11.2, we find no significant correlation. At higher stellar masses, 11.2 ≤ log10(M⋆/M⊙) < 11.78, we detect a positive correlation with ηcent = 0.51 ± 0.14. A linear fit as a function of the logarithm of stellar mass yields a slope of sηcent = 0.66 ± 0.28, indicating that the halo mass-size correlation strengthens with increasing stellar mass. For satellite galaxies, we observe a negative correlation at low and intermediate stellar masses for the host halo mass-galaxy size relation of ηsat = −0.19 ± 0.05 and ηsat = −0.09 ± 0.05, respectively. The correlation is consistent with zero within the stellar mass range of 11.2 ≤ log10(M⋆/M⊙) < 11.78.
From Spin Vorticity Models to Spin Liquids on the Octochlore Lattice
(University of Waterloo, 2025-09-23) Burke, Michael
Nearest-neighbour spin ice has been central to the study of frustrated magnetism for nearly three decades, providing a framework that reveals emergent gauge fields and monopole excitations within geometrically frustrated spins on the pyrochlore lattice. The geometry of corner-sharing tetrahedra admits only a single symmetry-equivalent nearest-neighbour bond, strongly constraining the range of allowed interactions. Recently, a new frustrated lattice of corner sharing octahedra, dubbed the octochlore lattice, has emerged as a promising platform for novel spin liquid phases. Unlike the pyrochlore, the octahedra permit distinct intra-octahedral interactions, greatly expanding the variety of realizable models. Building on the work of Szabó et al., where the spin-ice analogue was studied in a restricted region of parameter space, this thesis pursues two complementary directions. First, we investigate the spin vorticity model, in which the monopole excitations of spin ice are replaced with string-like excitations analogous to closed current loops. Second, we identify all the long-range ordered phases at the second nearest-neighbour level, fully elucidating the intra-octahedra model of Szabó et al. through an irreducible representation analysis. In doing so, we discover a novel classical U(1) analog to the celebrated X-cube model of fracton topological order. Overall, this work demonstrates that the octochlore lattice of corner-sharing octahedra constitutes a next-generation platform for three dimensional frustrated magnetism, uniquely capable of hosting exotic spin liquid phases with potential realizations in rare-earth based antiperovskites and potassium rare-earth fluorides.
Information-Theoretic Tools for Analyzing Non-IID Structures in Quantum Key Distribution
(University of Waterloo, 2025-09-23) Arqand, Amir
Quantum key distribution (QKD) is the task of establishing an information-theoretically secure key between two parties (Alice and Bob) by exploiting the rules of quantum mechanics. A central goal in QKD is to provide finite-size security proofs against the most general class of attacks—namely, coherent attacks. One approach for obtaining such proofs is through the original entropy accumulation theorem (EAT) and the generalized entropy accumulation theorem (GEAT). In this thesis, we improve and extend these EAT-style techniques in several aspects. We begin by presenting techniques for applying the GEAT in the finite-size analysis of generic prepare-and-measure protocols. We then improve the statistical analysis in these EAT-style techniques, yielding a significant improvement in the key rates obtained by these methods. In addition, these improvements can be applied directly at the level of Rényi entropies if desired, yielding fully-Rényi security proofs. Beyond this, we develop an EAT-style framework that can accommodate specific forms of marginal-state constraints ``compatible'' with the source-replacement scheme. This allows us to overcome the repetition-rate restriction previously imposed on the GEAT when analyzing prepare-and-measure protocols. Furthermore, it yields ``fully adaptive'' protocols that can, in principle, update the entropy estimation procedure during the protocol itself. Finally, we investigate the effect of imperfections in QKD by deriving a number of chain rules for mutual information suited for analyzing protocols involving information leakage from imperfect devices. In particular, we derive chain rules between smooth min-entropy and smooth max-information for characterizing one-shot information leakage caused by an additional conditioning register. Furthermore, we introduce an EAT-style theorem for mutual information to simplify the evaluation of leakage measured by smooth max-information. In addition, we derive suitable chain rules to incorporate the leakage process into the GEAT framework.
Vacuum assembly, atomic source development, and micromotion studies for a trapped Ba+ based quantum processor
(University of Waterloo, 2025-09-22) Khatai, Ali
Trapped-ions are one of the leading platforms for quantum information processing, due to their long coherence times and high-fidelity State Preparation and Measurement (SPAM) and gate operations. However, implementing a trapped-ion system with many desirable features such as Ultra-High-Vacuum (UHV), High-Optical-Access (HOA), and a high trapping probability presents significant technical challenges. These features are critical for enabling complex quantum simulation experiments, including simulations of spin-1/2 systems and beyond, by leveraging the multiple internal states of barium and tunable spin–spin interactions. High optical access is required to deliver tightly focused beams for site-resolved control and inter-ion coupling, while UHV conditions are essential for achieving long ion lifetimes necessary for stable, large-scale simulations. Achieving such optical access, which is vital for both coherent gate operations and high numerical aperture fluorescence collection, whilst maintaining vacuum integrity presents substantial engineering challenges. This thesis presents the construction of a vacuum chamber system intended for an individually addressable 16-qubit quantum processor based on barium-133 ions confined in a microfabricated surface-electrode ion trap. Although the complete processor has not yet been realized, the chamber has been assembled to support its future implementation. The microfabricated surface-electrode ion trap features ∼ 100 electrodes that require independent control of static voltages for precise tailoring of the confining electric potential. The experimental setup was designed to provide high optical access and to reach base pressures in the low 10^(−11) mbar range, thereby minimizing ion–background gas collisions. This is achieved by centrally mounting the trap within the vacuum chamber, a departure from conventional flange-mounted designs. To bring in-vacuum electrical connections to ∼ 100 trap electrodes, a custom implementation of an in-vacuum wire harness is described that enables reliable electrical connectivity to the centrally mounted surface electrode ion trap. The development of this wire harness enabled us to reach UHV with a pressure of ≈ 3 × 10^(−11) mbar. The successful development of this wire harness demonstrates that it is possible to create a UHV compatible wiring solution using commercially available components for a surface-electrode ion trap suspended at the center of the vacuum chamber. This centrally mounted design provides both high optical access and high conductance to vacuum pumps, enabling individual ion addressing and long ion lifetimes, which in turn facilitate more complex quantum simulation experiments. In addition, a multispecies ablation target was developed that incorporates both radioactive barium-133 and metallic barium to enable ion loading at lower trap depths, addressing a common challenge in surface-electrode ion traps. The velocity of the atomic plume generated from barium metal ablation targets was measured and compared to that of BaCl_2 targets. Although we were unable to definitively measure the effective temperature and peak velocity of the atomic plume generated from barium metal, the neutral fluorescence measurements from the barium metal targets indicate a larger number of slower moving atoms compared to BaCl_2 targets. The lower speeds of the atoms generated from metal targets is also plausible due to the low bond energy of metallic bonds compared to covalent bonds in salts. Barium has recently become a popular choice for high-fidelity trapped-ion experiments due to its favorable atomic structure. The larger number of slower moving atoms from barium metal demonstrates that metallic barium could be a much more promising atomic source for barium-based trapped-ion experiments, as the slower plume enables higher loading rates in surface-electrode ion trap systems. Finally, excess micromotion, caused by stray electric fields in RF traps, can lead to undesirable effects such as heating and frequency modulation of atomic transitions. This work evaluates the micromotion compensation capabilities of the Phoenix ion trap used in our system through numerical simulations of single ion trajectories. Through these simulations, it was determined that stray fields of Ex ≈ 3000 V/m, Ey ≈ 3800 V/m, and Ez ≈ 600 V/m can be effectively compensated for with the Phoenix ion trap. With typical stray electric fields in trapped-ion systems ranging from tens to a few hundred V/m, this demonstrates that the Phoenix trap should be well capable of compensating any stray fields expected in our system.
Temperature Dependence of 1/f Charge Noise Coupled to a Quantum Dot Confined in a 2DHG
(University of Waterloo, 2025-09-22) Nademi, Cody
Reliable operation of semiconductor quantum dot-based qubits relies on a stable electrostatic environment. However, charge noise stemming from the randomized motion of trapped charge carriers poses a significant challenge to qubit coherence. This thesis presents experimental results acquired from a quantum dot confined within a 2DHG formed at the interface of a cs-GoS heterostructure, with the motivation of characterizing the 1/f charge noise coupled to the dot. Charge stability diagrams reveal irregularities in transition between adjacent stable charge states, motivating the investigation into the behavior of charge noise in cs-GoS lateral quantum dot devices. Charge noise likely originates within the amorphous dielectric layer or at the dielectric-semiconductor interface where bonding mismatch serves as charge trapping sites, each with their own characteristic activation energy. To characterize the spectral and temperature-dependent behavior of the charge noise, current fluctuations across the quantum dot sensor were measured and analyzed to compare with the Dutta-Horn model for 1/f noise. During the process, a novel technique was developed to mitigate the effects of charge noise induced device drift and account for the quantum dot’s changing transfer function. To precisely characterize the behavior of the charge noise, the quantum dot sensor needs to maintain a constant sensitivity to surrounding electrostatic fluctuations. However, while studying charge noise, the operating point of the quantum dot sensor is inherently prone to changes. Thus, the technique involved implementing an AC modulation on a nearby gate to determine a time-averaged transfer function. Using the AC modulation technique, an ensemble of current spectra revealed an average frequency exponent value of 1.06±0.19, consistent with Dutta-Horn’s model for 1/f noise arising from a uniform distribution of two-level systems. Additionally, analysis incorporating the time-averaged transfer function showed that the charge noise increased nearly linearly with temperature. Across the ensemble of experiments, the average temperature exponent value was 1.01±0.19, also consistent with the model’s prediction for thermally activated two-level systems with a uniform distribution of activation energies. Furthermore, at 1 Hz and 100 mK, the average noise magnitude was determined to be √(S_E )=0.94 μeV/√Hz, with a significant spread likely stemming from thermal cycling and abrupt gate voltage changes, leading to a redistribution of trapped charges within the quantum dot’s environment. These results highlight the sensitivity of charge noise to device history and operating conditions, suggesting it is not entirely intrinsic. While the exact source of charge noise has not been identified, future work focused on dielectric engineering and improved gate control may open pathways towards reduced chare noise and eventually improved qubit coherence.
Explorations in Generalized Quasi-topological Gravities
(University of Waterloo, 2025-09-22) Mengqi, Lu
This thesis presents a comprehensive study of Generalized Quasi-Topological Gravities (GQTGs), a broad class of higher-curvature extensions of Einstein gravity in arbitrary spacetime dimensions. These theories are distinguished by possessing second-order lin- earized field equations around maximally symmetric backgrounds and admitting static, spherically symmetric black hole solutions characterized by a single metric function f (r) obeying a second-order differential equation. We rigorously demonstrate that, at order n in curvature, exactly n − 1 inequivalent GQTG densities exist in dimensions D ≥ 5, among which only one belongs to the Quasi-topological subclass for which the field equa- tions reduce to an algebraic equation for f (r). In contrast, we find strong evidence that only a unique such density exists for each order in four dimensions, which is not of the Quasi-topological kind. We analyze the thermodynamic properties of black holes in these theories and confirm that the first law of thermodynamics is satisfied. Moreover, we provide evidence that the black hole thermodynamics is fully encoded in the same embedding function that determines the maximally symmetric vacua of the theory, offering a unified and simplified framework for studying solutions with arbitrary higher-curvature corrections. Building on this foundation, we explore the multi-critical thermodynamic behavior of uncharged AdS black holes in GQTGs. Utilizing a reformulated version of Maxwell’s equal area law, we identify a geometric criterion for the existence of multicritical (or N -tuple) points in the black hole phase diagram. Applying this criterion, we explicitly construct black hole solutions exhibiting quadruple and quintuple critical points supported by genuine GQTG densities. Finally, we uncover a novel class of traversable wormhole solutions in four-dimensional Einsteinian Cubic Gravity—a specific GQTG—including examples that are entirely vac- uum configurations with no need for exotic matter. These wormholes connect asymptoti- cally AdS regions with a geometric deficit at infinity, interpretable as a global monopole. We demonstrate that these configurations satisfy standard traversability conditions and exhibit a variety of geometries depending on the parameter space.
Towards satellite-assisted quantum communication with reconfigurable networks and frequency-bin qubits
(University of Waterloo, 2025-09-22) Vinet, Stéphane
The development of quantum networks will enable advanced applications in quantum cryptography, quantum-enhanced metrology, and quantum computing. Central to this vision is the ability to reliably distribute quantum entanglement between distant nodes. To date, quantum networks have been largely confined to metropolitan scales and a small number of nodes with predominantly static implementations, constraining their scalability and adaptability for broader deployment. Satellites provide a promising platform for distributing entanglement over global distances. However, the integration of satellites into terrestrial quantum networks poses significant challenges, including intermittent connectivity and high link attenuation during orbital passes. In this thesis, we first propose a reconfigurable quantum network architecture that dynamically adapts its topology according to satellite availability. This reconfiguration ensures efficient and continuous operation while accommodating satellite links within a metropolitan quantum network. We demonstrate this network architecture, using a correlated photon source specifically designed for Canada’s Quantum Encryption and Science Satellite (QEYSSat) mission. Using both frequency and time multiplexing, we demonstrate a linear performance improvement with minimal resource overhead. Moreover, we propose an integrated source design to facilitate deployment in realistic scenarios. Second, we investigate frequency-encoded quantum communication over free-space channels. We propose a novel approach that leverages linear interferometry and time-resolved detectors to decode frequency-bins without any adaptive optics or modal filtering. Furthermore, we investigate the phase stability requirements so that frequency-bin encoding could be feasible for satellite to ground quantum links. A proof-of-concept experiment is conducted over a turbulent free-space channel. Third, we distribute frequency-bin entangled photons over multi-mode channels and test their non-local correlations. We report, to the best of our knowledge, the first measurement of the joint temporal intensity between frequency-bin entangled photons revealing a rich temporal structure. By combining time-resolved detection with energy-correlation measurements, we perform full quantum state tomography and further certify our source's non-classicality via a violation of the time-energy entropic uncertainty relations. We extend this scheme to higher-dimensional frequency-bin states, opening new possibilities for high-capacity robust quantum communication and quantum information processing. The concepts, protocols, and experimental demonstrations presented in this thesis establish new approaches to utilize frequency-bin entanglement and contribute to the development of satellite-assisted quantum networks thus paving the way towards the realization of a global quantum network.
Mitigating Optical Crosstalk for In-Situ Mid-Circuit Measurement and Reset in a Trapped-Ion Quantum Simulator
(University of Waterloo, 2025-09-22) Mahato, Shilpa
Trapped-ion qubits have emerged as a leading architecture for building both digital and analog quantum computers. Their long coherence times, simple state preparation and measurement procedures, and laser-based qubit manipulation make them a promising platform for quantum information processing. An important feature that can make these systems more fault-tolerant and expand their capabilities to perform different classes of simulation is high-fidelity Mid-circuit Measurement and Reset (MCMR). Several techniques have been proposed for implementing MCMR in trapped-ion systems. Our group has taken a bold approach by relying on sophisticated optical engineering to generate a low-crosstalk individual addressing beam for performing MCMR. A Digital Micromirror Device (DMD), which is a 2D array of micro mirrors, is used to engineer an incoming wavefront to generate individual addressing beams at the ions using Fourier holography. The technique has been optimized with in-situ aberration compensation and the use of Iterative Fourier Transform Algorithm (IFTA) for hologram generation, forming the current state-of-the-art individual addressing system. In this thesis, two methods have been proposed that further strengthen our addressing system, making it more robust against measurement errors introduced by intensity crosstalk. The proposed methods, secondary grating method, and using the DMD in a double pass configuration have been successful in minimizing the absolute intensity crosstalk at the nearest neighbor and at the asymptotic limit. These improvements will have a significant impact on the current standing of MCMR fidelities in trapped-ion qubits.
Black hole perturbations beyond the leading order
(University of Waterloo, 2025-09-22) May, Taillte
This thesis employs numerical methods to study black hole perturbations beyond the leading order. This includes vacuum gravitational perturbations as well as superradiant scalar, vector, and tensor boson perturbations. Understanding and quantifying beyond leading order effects enables more accurate tests of General Relativity and of physics beyond the Standard Model. First, we explore the nonlinear behavior of gravitational perturbations on a Kerr black hole. The ringdown gravitational wave signal, during the final stage of binary black hole mergers, contains important information about the properties of the remnant black hole, and can be used to perform clean tests of general relativity. However, interpreting the loudest portion of the ringdown signal requires understanding the role of nonlinearities and their potential impact on modeling this phase using quasinormal modes. Here, we focus on a particular nonlinear effect arising from the change in the black hole's mass and spin due to the partial absorption of a quasinormal perturbation. We estimate the size and characteristics of this third-order effect using numerical techniques. Quantifying these effects, we find that they may be relevant in analyzing the ringdown in black hole mergers. Next, we discuss self-gravity corrections to beyond the standard model particle dynamics around black holes. Specifically, for scalar and vector bosons forming superradiant clouds. Oscillating clouds of ultralight bosons can grow around spinning black holes through superradiance, extracting energy and angular momentum, and eventually dissipating through gravitational radiation. This makes gravitational wave detectors powerful probes of ultralight bosonic fields. Here, we use fully general-relativistic solutions of the black hole-boson cloud systems to study the self-gravity effects of scalar and vector boson clouds. We calculate the self-gravity shift in the cloud oscillation frequency, which determines the frequency evolution of the gravitational wave signal, improving the accuracy of gravitational wave searches for physics beyond the Standard Model. We also perform an analysis of the spacetime geometry of these systems, we compute how the presence of the cloud changes the innermost stable circular orbit and light ring. Lastly, we investigate the nonlinear phenomenology of the spin-2 boson's superradiant instability. Models that result in a massive spin-2 boson at low energies have been proposed as solutions to the dark matter problem or as modifications to general relativity. The existence of ultralight scalar and vector bosons is constrained using measurements of black hole spins, due to the mechanism of black hole superradiance, and attempts have been made to place constraints on the existence of spin-2 bosons using the same approach. However, those constraints so far have relied on the assumption that the spin-2 superradiant behavior matches that of lower-spin fields. Here we consider a particular nonlinear theory, quadratic gravity, to study the behavior of spin-2 particles that undergo superradiance. We find that the phenomenology is different from that of spin zero or one bosons, increasing the spin of the central black hole and resulting in an extremal black hole horizon.
Probing Micromotion in a Multi-Segmented Blade-Style Ion Trap
(University of Waterloo, 2025-09-19) Lefebvre, Fabien
Trapped ions are a leading candidate in quantum computing platforms. Their all-to-all connectivity and high-fidelity multi-qubit interactions serve as an essential pillar for scaling up quantum computing. Trapping linear chains of 171Yb+ ions has applications ranging from digital quantum computation to analog quantum simulation of physically relevant models. Whilst these applications seem attractive, many experimental challenges prevent trapped ions from easily scaling up. The quadrupole multi-segmented blade-style trap is a leading trap architecture for quantum simulation because of its deep and quadratic confining potential, high optical access and capability to hold long chains of ions. Although, blade-style traps face challenges such as complex electronics control and ion heating caused by micromotion. Blade-style traps are traditionally hand assembled, and therefore are prone to misalignment, leading to increased levels of micromotion. This limitation causes many adverse effects, all negatively contributing to the quality of the quantum simulations performable with the blade-style trap. In this thesis, I will describe my work in building the electronic infrastructure to confine long chains of 171Yb+ ions in a multi-segmented blade-style trap. The bulk of my work will focus on probing micromotion in the trap by using a repump transition with a narrow enough linewidth. I will first present some background on ion dynamics, micromotion and fundamentals of ion trapping. Afterwards, I will discuss the multi-segmented blade-style trap used in this work, along with the electronics that I designed to drive the confining electromagnetic fields. A novel approach of using a balanced radio-frequency drive along with completely out-of-vacuum electronics allows us to reach high secular frequencies. Using this approach, we demonstrate long chains of up to 25 ions with qubit phase coherence exceeding 0.9 s, demonstrating good control over the magnetic field level. The rest of the thesis will present our approach to probe and minimize micromotion in the segmented blade-style trap. Here, I will demonstrate that our results show inherently low micromotion without any compensation fields, indicating that the assembly of the blades is quite optimal. Displacements below 1 μm are required radially to find the micromotion null. Additionally, I will demonstrate that there is low micromotion at the center and edge of a long chain of ions, showing that for large-scale quantum simulations, we can expect low axial and radial micromotion across a long chain of up to 25 ions. These results demonstrate that hand-assembled blade-style traps can exhibit inherently low excess micromotion, such that the inherent micromotion is at the limit of resolvability for the sideband spectroscopic method. This work will is crucial for obtaining low micromotion for when we eventually run large-scale quantum simulations in this trap.
The Dynamical States and Mass Accretion Histories of Galaxy Clusters in IllustrisTNG
(University of Waterloo, 2025-09-19) Reid, Rashaad
The concordance cosmological model describes the history and large-scale structure of the universe using a few key parameters. Two of these parameters, σ8 and Ωm, determine the clustering of matter due to the growth of density fluctuations in the early universe. Current constraints on these parameters measured from nearby large structures and from the early universe are in statistical tension. Our ability to resolve this tension is limited by the degeneracy between the parameters when measured from observations of nearby structure. Since galaxy clusters are the most massive gravitationally bound objects in the universe, their formation is sensitive to σ8 and Ωm. An improved understanding of the formation histories of galaxy clusters can break the measurement degeneracy, thus providing new insights into this tension in our cosmological model. Since the formation time scales of galaxy clusters are unobservable, we must use the structure of clusters to probe their formation histories. In this thesis, we relate the observable structural properties of galaxy clusters to the mass accretion histories of their surrounding dark matter halos in the IllustrisTNG cosmo- logical simulations. Structure formation in the universe is hierarchical, so recently formed galaxy clusters will have experienced recent mergers with other systems. We examine a set of structural properties that are related to the dynamical states of clusters as indications of recent mergers to relate the structures of clusters to their formation histories. Using the cluster formation history information that is available in IllustrisTNG, we classify clusters as dynamically relaxed or unrelaxed based on their structural properties and compare the mean mass accretion histories of the resulting groups. We establish in this work that the stellar mass asymmetry and magnitude gaps of galaxy clusters are readily observable structural parameters that most effectively predict the mass accretion histories of halos. By comparing the gravitational lensing profiles of dynamically relaxed and unrelaxed clusters classified using different structural parameters, we demonstrate that the stellar mass asymmetry most reliably distinguishes between halos in different dynamical states with different density profiles. We also show that line of sight galaxy projection does not significantly affect IllustrisTNG cluster samples and that differences between 3D-identified clusters and optically selected clusters can be accounted for with accurate cluster mass estimates. However, we find that the density profiles traced by the weak gravitational lensing around relaxed and unrelaxed clusters in IllustrisTNG simulations and DLIS x UNIONS observations are discrepant. The structural differences between the simulated and observed galaxy clusters will be further explored in future work to better relate this work’s findings to real astrophysical systems. Overall, we find through cosmological simulations that the structural properties of galaxy clusters can be used to effectively trace their mass accretion histories. The findings of this thesis establish which observable properties of clusters can be targeted in both observations and simulations to grant us insight into the formation histories of the largest structures in the universe.
Multiphase Competition and Quantum Moment Fragmentation in Dipolar-Octupolar Pyrochlore Materials
(University of Waterloo, 2025-09-18) Howson, Griffin
In the field frustrated magnetism, magnetic pyrochlore oxides are one of the most widely explored platforms for realizing exotic phenomena in three dimensions, including classical, fragmented and quantum spin liquids. In this thesis, we elaborate on the existing understanding of the low-temperature properties of a subclass of magnetic pyrochlore oxides -- the so-called dipolar-octupolar (DO) pyrochlores -- using various analytical and numerical techniques. To begin, we explore the `XZ' line of the DO phase diagram, which exhibits a strong competition between neighbouring quantum spin liquid and long-range ordered phases using large-scale quantum Monte Carlo (QMC). In particular, we examine how this competition can explain the low-temperature behaviours observed in moment fragmentation candidate Nd_2Zr_2O_7. We proceed by examining the thermodynamics and correlations exhibited by another DO pyrochlore Ce_2Sn_2O_7, arguing that it too harbours moment fragmentation in the T=0 ordered state by comparing and contrasting its behaviour with Nd_2Zr_2O_7. To further elaborate the extent of this phenomenon, we scan the XZ line discussing the evolution of fragmentation as one varies the microscopic exchange parameters between neighbouring ordered and disordered phases in the DO phase diagram. Extending beyond the nearest-neighbour (NN) model, we present a derivation of the symmetry-allowed exchange interactions for the DO model up to third nearest-neighbours. Taking a subset of these new exchange parameters, we construct a mean-field phase diagram centered at the NN exchange parameters presented for quantum spin ice candidate Ce_2Zr_2O_7. In particular, we identify the ordering wavevectors characterizing the long-range order stabilized by lifting the degeneracy exhibited by the NNx model. Further, we present estimates of the critical temperatures associated with the ordering transitions presented, defending the lack of order observed in experiments.
Path Integral Monte Carlo Simulations of Flexible Water Clusters with Normal-Mode Sampling in Jacobi Coordinates
(University of Waterloo, 2025-09-18) Aaron, Nithin
The primary focus of this thesis is on developing the normal-mode sampling algorithm, an importance sampling method specifically designed for path-integral quantum Monte Carlo simulations of flexible molecular systems. This work is motivated by the desire to include vibrational degrees of freedom in numerical studies of confined molecular lattices. Describing the dynamics of a molecular system using its normal modes naturally allows for the inclusion of molecular rotations, translations, and even vibrations. We first introduce a novel density matrix factorization that arises from decomposing our system Hamiltonian into its harmonic and anharmonic terms. The normal-mode sampling algorithm is then constructed using this factorization. We also integrate Jacobi coordinates with our normal-mode sampling algorithm to allow for separability between translational and ro-vibrational degrees of freedom. Finally, we validate our normal-mode sampling algorithm in the context of path-integral quantum Monte Carlo simulations of several flexible molecular systems, namely the water monomer, dimer, hexamer cage, and hexamer prism. For each of these systems, we calculate the ground-state energy and various structural properties, benchmarking our results against exact diagonalization, path integral molecular dynamics, and diffusion Monte Carlo studies from the literature.
Fluctuation-induced order and thermal transport in frustrated quantum magnets
(University of Waterloo, 2025-09-18) Hickey, Alexander
Order-by-disorder is a mechanism of "fluctuation-induced" ordering that occurs in many frustrated magnetic systems where magnetic moments, or spins, are subject to competing interactions. So far, this phenomenon has been discussed in systems where the quantum ground state is not a "classical" product state. In such a case, both thermal and quantum fluctuations act to lift the accidental classical degeneracy, raising the question of whether one mechanism of order-by-disorder is possible without the other. In this thesis, we present results exposing a novel route to order-by-disorder, one without quantum zero-point fluctuations, in the ferromagnetic pyrochlore Heisenberg model with the Dzyaloshinskii-Moriya (DM) interaction as the leading perturbation. We show that any collinear ferromagnetic state is an exact eigenstate even in the presence of the anisotropic DM interaction, while thermal fluctuations give rise to a preferred magnetization direction. Using linear spin wave theory, we find that the anisotropy appears at lowest order as a sub-leading term in the low-temperature expansion of the free energy. Our results thus show that the phenomenon of thermal order-by-disorder can, in principle, occur even in the absence of quantum zero-point fluctuations driving quantum order-by-disorder. By extending our calculations to non-linear spin wave theory, we find that the ferromagnetic ground state becomes unstable for a spin-1/2 system when the DM interaction is large. Next, we ask the question of how to adequately characterize order-by-disorder in real materials, and how to distinguish it from conventional energetic ordering. Currently, the only clear and universal signature that has been proposed is a characteristic temperature dependence of the fluctuation-induced pseudo-Goldstone gap. Thus far, this temperature dependence of the pseudo-Goldstone gap has only been characterized in the classical limit. Here, we use non-linear spin wave theory to characterize the pseudo-Goldstone gap in quantum magnets at low temperature, to leading order in 1/S. Using exact sum-rules for the magnon spectral functions, we find that the gap exhibits a distinct power-law temperature dependence. We examine the implications of our results for several candidate materials. The final part of this thesis examines the thermodynamic and transport properties of the ferromagnetic pyrochlore Lu₂V₂O₇. Over the last decade, there has been immense interest in magnetic materials that host topologically non-trivial excitations. In ordered magnetic insulators, features analogous to those of topological insulators and semimetals can arise in the magnon band structure, and the associated Berry phases can manifest in observable heat and spin transport phenomena. This was unambiguously observed in Lu₂V₂O₇ in the form of a magnon thermal Hall signal, and proposed to arise from the DM interaction. A precise value of the DM interaction is not known, as the values obtained from fitting both thermal transport and inelastic neutron scattering data, as well as from density functional theory, are all mutually inconsistent. Motivated by this, we investigate the effect of additional symmetry allowed perturbations to the spin Hamiltonian of Lu₂V₂O₇ in an attempt to reconcile the different experimental probes of this material. We find that the thermal transport and neutron scattering measurements are consistent with the addition of a small second-nearest-neighbour DM interaction to the model. Conversely, we argue that existing specific heat measurements are inconsistent with neutron scattering experiments and cannot be reconciled with any additional exchange couplings to the bilinear spin-model in the perturbative regime. Our results motivate future thermal transport and specific heat measurements of this material.
Asymptotic Higher Spin Symmetries: Noether Realization & Algebraic Structure in Einstein-Yang-Mills Theory
(University of Waterloo, 2025-09-17) Cresto, Nicolas
This thesis deals with the phase space realization of asymptotic higher spin symmetries, in 4-dimensional asymptotically flat spacetimes. These symmetries live on the conformal null boundary, namely null infinity, and were first revealed few years ago via the study of conformally soft gluons and gravitons operator product expansions, in the context of celestial holography. Connections with twistor theory and phase space realization followed soon after. In the gravitational case for instance, these symmetries generalize the BMS algebra to include an infinite tower of symmetry generators constructed from tensors on the sphere of arbitrary high rank s. The bulk interpretation of these transformation parameters is still largely under investigation. Building on a first series of results on their canonical representation, we develop the necessary framework to define Noether charges for all degrees s. Importantly, we construct these charges out of a `holomorphic' asymptotic symplectic potential, such that we obtain an infinite collection of charges conserved in the absence of radiation. The classical symmetry is then realized non-perturbatively and non-linearly in the so-called holomorphic coupling constant, generalizing the perturbative linear and quadratic approach known so far. The infinitesimal action defines a symmetry algebroid which reduces to a symmetry algebra at non-radiative cuts of null infinity. The key ingredient for our construction is to consider field and time dependent symmetry parameters constrained to evolve according to equations of motion dual to (a truncation of) the asymptotic equations of motion in vacuum. We expose our results for Yang-Mills, General Relativity, and Einstein-Yang-Mills theories. This canonical analysis comes hand in hand with an in-depth study of the algebraic structure underlying the symmetry. We reveal several Lie algebroid and Lie algebra brackets, which connect the Carrollian, celestial and twistorial realizations. We show that these brackets are a deformation of the soft celestial algebra, where the deformation parameters are the radiative asymptotic data. On the one hand they allow us to define the symmetry algebra at non-radiative cuts of null infinity, for arbitrary values of the asymptotic shear and gauge potential. On the other hand, we can accommodate for radiation using the algebroid framework. For the specific case of non-abelian gauge theory, we also investigate how the asymptotic expansion around null infinity of the full Yang-Mills equations of motion in vacuum can be recast in terms of the higher spin charge aspects. Since the analysis of higher spin symmetries is for now inherent to a truncation of the latter equations of motion, this paves the way towards the understanding of the relevance of these symmetries in the full theory.
Theory and phenomenology of area-metric gravity
(University of Waterloo, 2025-09-12) Borissova, Johanna N.
Area metrics are generalised geometric structures to describe spacetime. They feature additional non-metric degrees of freedom beyond the metric degrees of freedom of classical gravity at low energies. As such, area-metric gravity is a candidate effective field theory for the continuum limit of loop quantum gravity and spin foams which accounts for the extended gravitational configuration space of these approaches in their semiclassical regime. On these grounds, following a bottom-up approach, we construct area-metric gravity perturbatively guided by the principle of general covariance. The most general local and diffeomorphism-invariant action quadratic in area-metric perturbations and of second order in derivatives contains four free parameters. These are the two masses of the right-handed and left-handed non-metric degrees of freedom, and the two interaction couplings between these and the metric degrees of freedom of the area metric. Linearised area-metric gravity violates parity for generic values of these parameters. The effective metric actions obtained after integrating out the non-metric degrees of freedom are quasi-local linearised Einstein-Weyl actions. For special choices of couplings, the spin-2 propagator is ghostfree and exhibits only the pole associated with the massless graviton. The corresponding two-parameter subclass of linearised area-metric actions is characterised by a shift symmetry in the kinetic term. The physical spectrum of these theories consists of two massless transverse-traceless modes and five additional massive modes. The Hamiltonian dynamics mixes the two massless transverse-traceless modes in the linear polarisation basis as a result of parity violation in the original area-metric Lagrangian. Extending the analysis of area-metric actions, we show that modified Plebanski theories provide a natural framework for non-linear area-metric gravity. In these theories, a subset of the simplicity constraints on the bivector field in the original Plebanski action is replaced by a potential. This mechanism may be viewed as a continuum analogue of the weak imposition of second-class constraints in the spin-foam path integral. The Immirzi parameter γ, defined as the inverse coupling in front of the Holst action, and appearing in the commutator between second-class constraints in the quantum theory, is identified as a parity-violating coupling in area-metric gravity. Different from classical metric gravity, γ enters the dynamical equations of motion in area-metric gravity. Based on these results, we proceed to analyse aspects concerning the phenomenological viability of area-metric gravity as a quantum and classical effective field theory. Considering area-metric gravity as a local quantum effective field theory in a regime below the cutoff scale of a fundamental theory of quantum gravity, we evaluate its renormalisation-group flow towards the infrared regime. The non-metric degrees of freedom generically decouple as a result of their heavy masses at low energies. However, simultaneously growing interaction couplings between these and the metric degrees of freedom may leave an imprint in the low-energy effective action for the metric. In addition, parity violation at high energies is dynamically enhanced at low energies. The flow of the Immirzi parameter exhibits fixed points and zero and infinite γ. Finally, we consider non-linear quasi-local Einstein-Weyl gravity as a classical effective field theory for the metric degrees of freedom in area-metric gravity. After localising the action and restricting to static spherical symmetry, we show that solutions in the weak-field regime are characterised by an effective mass parameter which reduces to the mass of the spin-2 ghost in local Einstein-Weyl gravity when the non-locality in the original action is taken to zero. Additionally, we derive a regular Frobenius solution family at the radial center as the first step towards a future classification of Frobenius solutions around a generic expansion point.
Systems and Methods for Generating Large Arrays of Optical Traps in Neutral Atom Array Quantum Processors
(University of Waterloo, 2025-09-08) Khoubyarian, Soroush
Neutral-atom array quantum processors provide a scalable and controllable platform for quantum simulation, computation, sensing, and metrology. Strong Rydberg interactions enable entanglement and high-fidelity gates, while the ability to rearrange atoms allows for multi-qubit operations between spatially distant qubits and the implementation of quantum error-correcting codes with nonlocal stabilizers. These arrays also support electromagnetic field sensing and form the basis for next-generation atomic clocks with improved stability and precision. Recent progress has demonstrated platforms with thousands of qubits, the implementation of error-correction codes, and experimental realizations of quantum spin models, including studies of quantum phase transitions. Current efforts now focus on scaling to even larger arrays and reducing quantum gate errors, with the goal of achieving computations and simulations beyond the reach of classical devices. However, increasing the scale and controllability of neutral-atom array platforms to address problems where quantum advantage can have practical impact remains challenging. Key difficulties include scaling to larger numbers of qubits, implementing fault-tolerant computation, and performing gates between spatially distant qubits. A critical experimental challenge is ensuring that qubits form an indistinguishable spatiotemporal ensemble, so that atoms across the array and over multiple experimental shots can be treated as identical. Achieving this uniformity becomes more difficult as the array size increases, due to the added complexity of ensuring that all atoms experience consistent control parameters, such as trap depth and magnetic fields. Without sufficient spatiotemporal uniformity, it is impossible to meaningfully average measurements across different qubits and experimental runs, undermining scalability and limiting the performance of quantum simulations, computations, and sensing applications. In this thesis, I propose, design, and implement methods to scale up the number of qubits and improve the spatiotemporal uniformity of qubit properties in acousto-optically generated trap arrays for neutral-atom quantum processors. These methods enable magnetic-field imaging across an array of 1,305 optical traps containing 690 qubits, as well as high-fidelity fluorescence imaging of single atoms in optical tweezers. First, I demonstrate a novel method for circularizing and de-astigmatizing the trapping beam using three cylindrical lenses. This approach is cost-effective, power-efficient, and broadly applicable. By improving the beam shape, we reduce the per-trap laser power, enabling the use of larger qubit arrays in neutral-atom quantum processors. Applied to a Ti:sapphire laser with an initial circularity of 0.69, this method achieves a circularity of 0.97 and a beam waist separation of 0.8 percent of the Rayleigh range, reducing the optical power required per trap by 5 percent. After beam circularization, I develop a real-time closed-loop feedback system for an optical trap array generated by two orthogonal acousto-optical deflectors to enhance the spatiotemporal uniformity of qubit properties. The system stabilizes trap depths using power measurements from a fast CMOS camera and in-situ depth estimates from an EMCCD camera based on the atomic signal. Noise is decomposed into uncorrelated temporal modes, each regulated by an independent PID loop. Unlike approaches that regulate only total laser power, our method stabilizes the depth of individual traps, eliminating the need for frequent recalibration of cooling-beam parameters and enabling higher duty cycles. In a 45 by 29 array, the generated traps exhibit a standard deviation of 6 percent relative to the mean trap depth, limited by the number of independent control parameters actuating the acousto-optical deflectors. Having established these capabilities, we demonstrate the controllability and practical utility of the platform by performing magnetic-field sensing using microwave horn spectroscopy on a 45 by 29 trap array, mapping field gradients across the array. This establishes the foundation for advanced quantum sensing protocols, such as quantum lock-in amplification, and paves the way toward entanglement-based quantum sensors capable of offering a quantum advantage in sensing. Lastly, to improve readout fidelity, I describe the optimization of imaging beam parameters in neutral-atom array quantum processors. This optimization maximizes atom classification accuracy while minimizing the probability of ejecting atoms from the traps. Applied to a 45 by 29 trap array, the method achieves a classification fidelity of 99.98 percent and a loss probability of 0.12 percent with a 75 millisecond imaging time. These improvements reduce measurement errors, enhance quantum state readout, and increase the sensitivity of neutral-atom-based quantum sensors.
Statistical Mechanics of Finite Length Semiflexible Wormlike Polymers
(University of Waterloo, 2025-09-03) Andersen, Nigel
The wormlike chain is a fundamental model in polymer physics used for describing the statistics of semiflexible polymers. It is a general model applicable to many polymers, with double-stranded DNA being a notable example. Previous theoretical work has focused on long polymers, where subtle differences in the choice of statistical ensemble are not relevant. This worked well in explaining early data on polymer stretching, but as more modern techniques have allowed for the stretching of shorter, more rodlike polymers, finite length ensemble effects have become visible in experimental measurements and computer simulations. Here, these effects are computed, and a full picture of wormlike chain statistical mechanics is presented. The new results are compared against several decades of polymer stretching literature, and are found to describe previously unexplained behaviour. Finally, the broad use of the model is demonstrated by applying it to DNA wrapped carbon nanotubes, a system that requires considering polymer flexibility, and predicting the pitch of the helical structure formed by these complexes.

Browse

Recent Submissions