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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Analysis of Neural Networks with Physics Applications
(University of Waterloo, 2026-03-30) Mohamed, Ahmed
This thesis investigates core aspects of machine learning, spanning foundational studies on generalization phenomena in neural networks, novel architectural strategies for enhancing representation learning and classification performance, and high accuracy predictive and inverse modeling of emerging nanoelectronic devices. Together, these studies highlight the significance of data and model structure, the impact of nonlinearity, and the potential of interpretable, generalizable machine learning methods for scientific and engineering applications. For generalization in neural networks, the thesis focuses on the phenomenon of grokking, a delayed generalization effect where models initially overfit but eventually learn to generalize well after extended training. Through a series of interconnected studies, this work proposes insights and practical tools to diagnose, forecast, and enhance generalization in modern machine learning systems. The first part of the thesis examines grokking in modular arithmetic tasks, revealing how dropout induced variance, embedding similarity, activation sparsity, and weight entropy evolve across training, and hence introduces diagnostic metrics to capture phase transitions between memorization and generalization. Further analysis shows that nonlinearity, network depth, and symmetry in data collectively modulate grokking behavior, linking model architecture to its capacity for structured generalization. Next, the thesis introduces a Branched Variational Autoencoder (BVAE), a hybrid architecture that integrates generative and discriminative objectives. By shaping latent representations through a supervised branch, the BVAE achieves improved class separability and interpretability on benchmark datasets, illustrating the potential of structured latent shaping for semi-supervised learning. Finally, the research extends to scientific machine learning, demonstrating how neural and ensemble models as Random Forests can accelerate the modeling and inverse design of Carbon Nanotube Tunnel Field-Effect Transistors (CNT TFETs). By coupling physical insights with machine learning interpretability techniques, this work bridges the gap between theoretical ML and real-world scientific applications.
From Asymptotic to Finite-Size Security in Decoy-State Quantum Key Distribution
(University of Waterloo, 2026-03-24) Kamin, Lars
Quantum Key Distribution (QKD) promises information-theoretic security, yet bridging the gap between theoretical proofs and practical implementations, specifically those operating with finite resources and imperfect devices against general coherent attacks, remains a critical challenge. This thesis develops a spectrum of efficient security proof techniques within the composable security framework, calculating key rates for both fixed- and variable-length protocols while accounting for realistic imperfections. We begin by addressing detection setups through an extension of a squashing map, the flag-state squasher, used for reducing the infinite-dimensional Hilbert spaces of optical elements to finite dimensions. This extension accommodates arbitrary passive linear optical setups while allowing for the inclusion of detection inefficiencies and dark counts in the security analysis. Subsequently, we advance the analysis of decoy-state protocols and introduce two major improvements. First, we reformulate the decoy-state analysis to recover no-decoy key rates, tightening the optimization. Second, we derive a unified framework that performs the key rate optimization and decoy analysis in a single step. This enables the bounding of the relevant entropies with arbitrary precision in the finite-size regime and successfully recovers the Devetak-Winter formula in the asymptotic limit. Furthermore, we improve the security analysis for generic QKD protocols against independent and identically distributed (IID) collective attacks. Our refined analysis yields finite-size corrections proportional to detected rather than transmitted signals and, by developing sharper concentration inequalities, achieves significantly improved finite-size scaling. Finally, leveraging the marginal constrained entropy accumulation theorem (MEAT), we establish a flexible numerical Rényi security framework against coherent attacks for both fixed- and variable-length protocols. This approach consistently outperforms existing reference proof techniques, including those based on entropic uncertainty relations, providing significantly higher key rates for both qubit and practically relevant decoy-state protocols. Moreover, we present finite-size key rates for generic QKD protocols accounting for realistic intensity and phase imperfections. Overall, this thesis provides the necessary theoretical framework to bridge the gap between idealized models and experimental reality, offering a scalable path toward secure quantum communication under realistic conditions, as demonstrated by the application of these techniques in experimental collaborations.
Measuring the Weak Gravitational Lensing Signal from Cosmic Voids
(University of Waterloo, 2026-03-23) Martin, Hunter
The field of cosmology is currently in an era principally focused on statistical precision. To achieve greater precision, deeper and wider surveys are actively being developed and conducted to gather more and more information about the surrounding cosmos. Alternatively, there are efforts to develop new statistical probes to better utilize currently-existing data to obtain tighter constraints. Cosmic voids, as vast underdense regions, then represent an ideal candidate to complement the statistical information already extracted from the opposite density extremes: massive luminous galaxies and galaxy clusters. Voids have already seen some success in constraining cosmology through probes like the void size function and void-galaxy cross-correlations. This thesis introduces the matter distribution within cosmic voids as measured by weak gravitational lensing as a new probe that is significantly detectable within current and future data sets. The goal of this demonstration is to justify future efforts in extracting the cosmological information from this newfound signal. For data currently available, we make use of the large overlap of the Sloan Digital Sky Survey (SDSS) Baryon Oscillation Spectroscopic Survey (BOSS) and the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). We measure void lensing around BOSS voids and find that we can detect the signal at 6.2 sigma significance, the most significant detection from spectroscopically-identified voids to date. We additionally are able to significantly detect differences in the void profile with void size between the larger half and smaller half of the void catalogue at 2.3 sigma. To help perform this measurement, we present and validate a novel method for computing the Gaussian component of the conventional weak lensing covariance, adapted for use with void studies. Comparing the void profile to a measurement of the void-galaxy cross-correlation to test the linearity of the relationship between mass and light, we find good visual agreement between the two, and a galaxy bias factor of 2.45 pm 0.36, consistent with other works. We additionally assist in future developments of the UNIONS data by running quality control tests for the future photometric redshift data sets. These future releases will provide additional data to make these detections stronger. For future data, we use the Flagship simulation of the Euclid survey to simulate the expected data from the Euclid VISible imager (VIS) and Near-Infrared Spectrometer and Photometer (NISP) instruments across an octant of the sky. This octant then roughly corresponds to an equivalent area of the planned second data release of the survey. We extend the methodology and covariance model to a lensing tomography setup by binning voids and sources along the line of sight. We then stack information along source bins. From this, we are able to detect the void lensing profiles with 12 sigma, 11 sigma, 7.1 sigma, and 4.7 sigma significance across the four different void redshift bins. Scaling the most significant result to the expected areas for the first and final data releases, we get 6.9 sigma and 21 sigma respectively. We additionally find a 4.4 sigma difference between the lensing profiles of the smallest voids and the largest voids.
Modeling Logical Error Rates in Surface-Code Quantum Computing based on Lattice Surgery and Applications
(University of Waterloo, 2026-02-23) Dagnew, Gebremedhin
Utility-scale quantum computation requires choosing fault-tolerant error-correcting codes and logical operations in a manner that guarantees a target program execution accuracy. In surface-code-based quantum computers, this requirement translates into the ability to predict the logical error rates of all lattice-surgery operations executed during computation. These logical error rates determine the required code distances, circuit depth, physical-qubit overhead, and, ultimately, the feasibility of large-scale quantum algorithms. In this thesis, we develop an operational benchmarking framework for predicting the logical error rates of surface-code primitives under realistic circuit-level hardware noise. The framework is based on numerical evaluation of experimentally implementable protocols, including single-patch logical memory and two-patch lattice-surgery protocols such as logical teleportation. Rather than attempting to simulate full fault-tolerant algorithms–which is computationally intractable–we extract geometric error scaling models that relate logical failure probabilities to the space–time structure of each protocol and to underlying physical noise parameters. Using these models, we quantify how improvements in hardware performance translate into enhanced logical error suppression and identify the dominant mechanisms limiting fault-tolerant performance. The resulting logical error models are compact, reusable, and directly applicable to resource estimation and architecture-level studies. Together, these results provide a practical bridge between low-level hardware characteristics and high-level fault-tolerant quantum computation, enabling more informed design and evaluation of surface code-based quantum computing architectures.
Electroluminescence in the Classical and Quantum Regime in Undoped GaAs/AlGaAs Heterostructures
(University of Waterloo, 2026-01-26) Harrigan, Stephen
Quantum information processing holds the promise to radically change the way we perform computations and transmit information. In the realm of quantum computing, there has been enormous progress in the last few decades in a huge variety of quantum systems and it is unclear which platform will be the leading system to execute quantum computations. Conversely, photons have always remained the front-runner for the long distance transfer of quantum information since photons travel at the speed of light and have limited mechanisms of decoherence (as compared to other carriers of quantum information) when traveling over long distances. The method used to generate single photons remains the pertinent open question. Current state-of-the-art single-photon sources (SPSs) are optically-active quantum dots driven by an external laser source. For laboratory-scale experiments, they have proven fruitful in order to demonstrate key components of a quantum network, as well as performing fundamental tests on the nature of quantum mechanics. However, one challenge associated with these optically active quantum dots is two-qubit interactions since the quantum dots are usually spatially isolated. Conversely, two-qubit interactions for spin qubits in gate-defined quantum dots is routinely achieved via the Heisenberg exchange interaction. Thus, it would be highly desirable to have a way to convert the quantum information of the spin state of gate-defined quantum dots to photon polarization. Furthermore, for the prospects scaling of the technology, it would be highly desirable for this quantum information transfer to be all-electrical in order to leverage conventional multiplexing techniques. In the first part of this thesis, we outline our proposal for an all-electrical SPS where single-photon emission is driven by electroluminescence (EL) at the single-charge to single-photon level. In order to control carriers at the single-charge level, we propose using non-adiabatic single-electron pumps (SEPs) previously investigated as quantized current sources for metrology. We have also previously developed a lateral p—n junction whose geometry allows direct integration with a SEP. We compare our proposed SPS to existing electrically-driven SPS in the literature, highlighting anticipated strengths of our proposed device, including a fabrication process compatible with standard semiconductor fabrication techniques. Given the key role SEPs play in our proposed SPS, we describe the established theory underpinning the high fidelity operation of SEPs. We also highlight practical considerations for the operation of SEPs, including device fabrication challenges faced during the course of this research, and demonstrate how to measure and characterize a SEP. A secondary focus of this thesis has been investigating EL from lateral p—n junctions in regimes where there was no attempt to control carriers at the single-charge level. While measuring lateral p—n junctions, we noticed an unconventional form of EL that did not require a forward bias to be applied. By swapping the polarity of the top gate voltage of our ambipolar induced devices, existing carriers recombine radiatively with incoming carriers of the opposite charge. Due to the flow of carriers in and out of the device, we called this form of luminescence the tidal effect. We develop a model to explain the non-monotonic frequency-dependent EL intensity and perform temperature-dependent measurements to identify the species responsible for the observed EL. We also further investigate a similar phenomenon when two adjacent top gates are periodically swapped with a phase difference between the two signals. We demonstrate that this form of EL is more efficient over larger areas than the tidal effect, and therefore may be more suitable for general illumination purposes. Lastly, we also performed the first EL measurements from lateral p—n junctions in single heterojunction interfaces. Despite the lack of a bottom barrier in these devices, our measurements suggest that carrier recombination is occurring near the interface. We characterize the EL spectra and observed the so-called H-band, a type of space-indirect exciton created in proximity to a populated single heterojunction interface, which has only previously been observed in photoluminescence experiments. Time-resolved EL experiments suggest reduced dimensionality of neutral excitons. We show that the lifetime of the H-band can be tuned electrically. We also demonstrate that the tidal effect can also be observed in these single heterojunction interfaces.
Field-Theoretic Simulations of Binary Blends of Complementary Diblock Copolymers
(University of Waterloo, 2026-01-21) Willis, James
The phase behavior of binary blends of AB diblock copolymers of compositions f and 1 − f is examined using field-theoretic simulations. Highly asymmetric compositions (i.e., f ≈ 0) behave like homopolymer blends macrophase separating into coexisting A- and B- rich phases as the segregation is increased, whereas more symmetric diblocks (i.e., f ≈ 0.5) microphase separate into an ordered lamellar phase. In self-consistent field theory, these behaviors are separated by a Lifshitz critical point at f = 0.2113. However, its lower critical dimension is believed to be four, which implies that the Lifshitz point should be destroyed by fluctuations. Consistent with this, it is found to transform into a tricritical point. Furthermore, the highly swollen lamellar phase near the mean-field Lifshitz point disorders into a bicontinuous microemulsion (BμE), consisting of large, interpenetrating A- and B-rich microdomains. A BμE has been previously reported in ternary blends of AB diblock copolymers with its parent A- and B-type homopolymers, but in that system the homopolymers have a tendency to macrophase separate. Our alternative system for creating BμE is free of this macrophase separation.
Progress towards FrAg molecules for nuclear CP violation
(University of Waterloo, 2026-01-21) Lagno, Andrew
Ultracold francium silver is a promising experiment that has the potential to set a new upper bound on nucleon electric dipole moments. In working towards making francium sil- ver molecules, our short term goal is to develop the knowledge and ability to evaporatively cool francium and silver. This entails finding the scattering properties of francium and silver using photoassociation spectroscopy and developing the ability to sub-Doppler cool silver. In this thesis, I talk about my work towards this goal, including attempting pho- toassociation at TRIUMF during francium beam time, work at the University of Chicago towards photoassociation and gray molasses in silver. Even though these efforts weren’t successful, the next steps are clear. Additionally, I talk about what I’ve accomplished at Waterloo when I’m not working on the francium silver project. This includes working to- wards better control over and stabilization of lasers and experimental optics and beginning optimization of the Cs Zeeman slower.
A proof-technique-independent framework for detector imperfections in QKD
(University of Waterloo, 2026-01-14) Nahar, Shlok
The security of Quantum Key Distribution (QKD) protocols is theoretically established using idealised device models. However, the physical implementations upon which practical security relies inevitably deviate from these ideals. This thesis develops a rigorous and versatile framework to address a subclass of such deviations: detector imperfections. This framework, termed ’noise channels’, is independent of security proof technique. This approach recasts imperfections as a quantum channel preceding an idealised measurement process. By granting the eavesdropper control over this channel, the security analysis is simplified to an ideal scenario, with the effects of the imperfections mathematically contained within a well-defined parameter. The utility and versatility of the framework are demonstrated through applying it to the postselection technique, and for phase error estimation. The application to phase error estimation is an improvement over past analyses which either assumed qubit detection setups, IID attacks, or required hardware modifications. We observe a remarkably high tolerance to imperfections when using the postselection technique. Finally, we extend the framework to address cross-round correlations, providing a methodology to prove security against detector memory effects such as afterpulsing and dead times. This work thus establishes a structured and powerful toolkit for analysing detector imperfections in practical QKD systems, unifying their treatment across different security proof techniques and advancing the development of robust implementation security.
Systems and Control Protocols for Neutral-Atom-Array Quantum Processors
(University of Waterloo, 2026-01-12) Zhutov, Artem
Neutral atom arrays are a leading platform for programmable quantum processors, offering individual qubit addressability, long-lived hyperfine ground states, and strong Rydberg interactions. Recent progress has demonstrated coherent control over thousands of atoms. However, achieving scalable control requires precise mitigation of environmental and hardware imperfections that degrade gate performance. This thesis presents an integrated neutral-atom array platform built from the ground up that incorporates quantum sensing directly into the processor. Each atom functions both as a qubit and a local magnetometer. We design, build, and characterize from first principles three subsystems: 1) a microwave control system for driving hyperfine transitions in ground-state rubidium atoms; 2) a Raman laser system for site-selective single-qubit gates; and 3) a Rydberg laser system with quantum optimal control for robust two-qubit gates. This work provides a universal gate set and quantifies which error sources limit performance. First, we develop an in-situ magnetic field imaging technique using the atom array as a quantum sensor. Through site-resolved Ramsey spectroscopy, we image magnetic fields across a 260 μm × 160 μm region with 3 μm spatial resolution. We then apply computed corrections that compensate for the bias magnetic fields, producing uniform global microwave single-qubit rotations. Second, we introduce a hardware-aware simulation framework to evaluate Raman laser systems for hyperfine qubit manipulation. Simulations predict a single-qubit gate infidelity of 4.4 × 10⁻⁴ using BB1 composite pulses to mitigate thermal motion errors. We validate the Raman laser system by building and characterizing its phase noise. Third, we develop a Rydberg laser system for high-fidelity entangling gates. We apply linear response theory to map laser phase noise to single-atom Rydberg excitation fidelity. We then demonstrate fast phase-noise engineering by optimizing laser servo parameters. We employ hardware-aware quantum optimal control to design both Rydberg excitation and two-qubit gate pulses with built-in robustness against physical and control parameter fluctuations, outperforming analytical benchmarks. This integrated platform demonstrates high-fidelity universal control of neutral-atom registers with hundreds of qubits. By systematically addressing environmental inhomogeneities through integrated sensing and hardware-aware control design, this work provides a validated path for scaling quantum processors while maintaining gate fidelity.
Micro-Calorimeter X-Ray Spectroscopy of Galaxy Clusters using the XRISM X-Ray Observatory
(University of Waterloo, 2026-01-07) Meunier, Julian
Galaxy clusters are some of the most massive objects in the universe, yet the evolution of these objects, particularly with regards to the heating and cooling of the intracluster medium, still has many unknowns. With the launch of the new X-ray imaging and spectroscopy mission, XRISM, galaxy clusters can now be studied with higher precision. This data can reveal a plethora of new information about the dynamics of the atmospheres of these clusters, which can be used to develop a better understanding of the evolution of galaxy clusters. In this thesis, I present analysis of X-ray spectroscopic data of the Perseus, Hydra A, and Cygnus A clusters obtained with XRISM Resolve. We apply spectral modeling techniques to the data to derive gas temperature, metal abundance, velocity dispersion, and bulk velocity to develop a further understanding of the dynamics of the intracluster medium in these galaxy clusters. I present spectral analysis of the five XRISM Resolve pointings of the Perseus cluster, binned into a radial profile. We measure radial profiles of temperature, metal abundances, velocity dispersion, and bulk velocities up to $\sim250$ kpc from the cluster center with single temperature models. While the temperature and abundance profiles are consistent with typical cool core clusters, the velocity dispersions suggest a relatively quiescent state for the intracluster medium, with only up to $\sim175$ km s$^{-1}$ dispersion in the central $\sim 60$ kpc. We interpret this velocity dispersion to be due to turbulence. The dispersion profile suggests that the jets and bubbles may be driving turbulence in the core, but also that the core may be under-heated. We find evidence for a second temperature component in the inner $\sim60$ kpc, that is cooler ($\sim2-2.4$ keV) and has a significantly higher velocity dispersion of $\sim300-400$ km s$^{-1}$. We interpret the cooler component to be sloshing gas from a merger or gas being churned by the jets and bubbles. I present spectral analysis of the full-FOV XRISM Resolve data of the Hydra A cluster, measuring temperature, velocity dispersion, and bulk velocity with a single temperature model. Despite Hydra A's high jet power, we find a remarkably low velocity dispersion of $164^{+10}_{-10}$ km s$^{-1}$, and a small velocity offset of $-37^{+20}_{-17}$ km s$^{-1}$ between the gas and the central galaxy. We interpret this velocity dispersion to be due to turbulence, which may suggest that the relationship between the jet power and the velocity structure of the intracluster medium is less significant than expected. However, further analysis of the outer regions of the cluster is needed to fully understand the dynamics of the gas in Hydra A. Finally, I present spectral analysis of the full-FOV XRISM Resolve data of the Cygnus A cluster, measuring temperature, metal abundances, velocity dispersion, and bulk velocity with a single temperature model. We measure a relatively higher velocity dispersion of $272^{+14}_{-13}$ km s$^{-1}$ with a bulk velocity of $101\pm26$ km s$^{-1}$ with respect to the central galaxy. These velocities likely reflect a combination of both turbulence in the gas and motion of the cocoon shock. We find some evidence for a second temperature component, that is cooler ($2.06^{+0.43}_{-0.20}$ keV) and broader ($333^{+127}_{-129}$ km s$^{-1}$), with a bulk velocity of $-311\pm118$ km s$^{-1}$. The second component may be necessary for fitting asymmetric features in the prominent emission lines of the spectrum. However, the large uncertainties of the model fit along with other uncertainties suggest that this component may not be significant.
Studies on the Characterization and Measurement Optimization of Superconducting Microwave Resonators
(University of Waterloo, 2025-12-22) Chen, Mengyang
This work presents a study aimed at improving the accuracy and efficiency of low-temperature loss-tangent measurements in superconducting resonators. Measurements were performed on aluminum and niobium devices, where a plateau in the internal quality factor at the single-photon level was observed, consistent with prior reports. By truncating the acquired resonance data, it was shown that the loss tangent experienced no systematic shift even when only four points spanned the resonance linewidth; the resulting increase in uncertainty was attributed to reduced effective averaging. Based on this result, an optimized data acquisition scheme was developed, reducing measurement time by a factor of four while maintaining approximately 1% accuracy. Further improvements were achieved through the use of a lower-noise HEMT amplifier, which reduced measurement noise and decreased acquisition time to 60% of the original. Additional circuit modifications showed that improved infrared shielding reduced total resonator loss, while the nonlinear behavior at high RF power was attributed to intrinsic device nonlinearity rather than external circuitry. Finally, crossover temperature measurements showed agreement with BCS theory at high temperatures, although its accuracy could be limited by not fully saturated TLS-loss, indicating the need for improved device designs.
Revisiting the ‘Lensing is Low’ Problem With UNIONS
(University of Waterloo, 2025-12-22) Campbell, Martine
In this thesis, we present new measurements of the galaxy–galaxy lensing (GGL) signal around Baryon Oscillation Spectroscopic Survey (BOSS) CMASS galaxies using background sources from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). With an overlap of approximately 2650 square degrees between CMASS lenses and background source galaxies—the largest to date—we obtain precise large-scale GGL measurements. With these new measurements, we revisit the so-called ‘lensing is low’ problem, wherein galaxy–halo connection models calibrated on clustering data over-predict the GGL signal by 20–40% under cosmic microwave background (CMB)-based cosmologies. We model the galaxy–halo connection using a halo occupation distribution (HOD), and perform joint fits to both GGL and clustering signals across a wide range of scales, as well as a clustering-only fit. Similar to previous work, we find a lensing–is–low effect in the CMASS sample, although our GGL and clustering predictions are less inconsistent with each other. The best joint fits are achieved by lowering the amplitude of the matter power spectrum relative to Planck 2018, driven by the precision of our large-scale GGL measurements. Once a lower matter power spectrum amplitude is adopted, feedback is the only HOD extension that further improves the joint fit. Our feedback model redistributes matter within a halo, modifying the halo–matter cross–power spectrum. Overall, we find that two models describe our observables equally well: one where HOD and cosmological parameters are free, and one where HOD, cosmological, and feedback parameters are free. Importantly, we emphasize the role of large scales in driving the lensing–is–low effect, shifting the narrative away from a purely small-scale issue.
A Neural-network-based Solver for the Three Dimensional Shape of Vesicle Membranes
(University of Waterloo, 2025-12-17) Rohanizadegan, Yousef
A neural-network-based numerical solver is developed for computing three-dimensional (3D) equilibrium shapes of deformable biomembranes, specifically phospholipid vesicles modeled by Helfrich's curvature elasticity theory. The solver represents vesicle morphology using a phase-field formulation, in which a scalar field distinguishes the interior and exterior of the vesicle through a diffuse interface. The phase field is parameterized by a compact feedforward neural network, and the equilibrium shape is obtained by direct minimization of the Helfrich bending energy subject to global surface-area and volume constraints, enforced via Lagrange multipliers. Automatic differentiation is used to evaluate all spatial derivatives, thereby avoiding finite-difference truncation errors and explicit surface discretization. This framework produces both axisymmetric and fully non-axisymmetric vesicle shapes without imposing symmetry assumptions. Canonical free-space branches, namely prolates, oblates, and stomatocytes, are reproduced, and the classical bending-energy–reduced-volume diagram is recovered in close quantitative agreement with established results in the literature. In addition, a phase-field expression for the bilayer area-difference constraint is derived and incorporated into the solver, providing a numerical setting for the computation of non-axisymmetric equilibrium morphologies in free space. A major contribution of this work is a systematic investigation of vesicle morphology under confinement. Vesicles are studied within a range of hard-wall geometries, including cylindrical (tube), slit, spherical, and cubic confinements. By varying confinement size and reduced volume, the solver captures a rich spectrum of deformations, including biaxial squeezed states, bent prolates, squeezed stomatocytes, and cubic and clam-like morphologies. Stability diagrams, bending-energy curves, and phase diagrams are constructed for each confinement, revealing both discontinuous (first-order) and continuous (second-order) shape transitions, as well as hysteresis and metastable branches. These results extend existing confinement studies by providing fully three-dimensional, non-axisymmetric solutions across multiple geometries and different regimes of confinement (free space to weak to strong) within a unified computational framework. Overall, this work establishes a versatile and scalable neural-network-based phase-field approach for vesicle shape modeling. By unifying classical membrane elasticity theory with modern machine-learning optimization, the solver facilitates a structured exploration of equilibrium morphologies, phase transitions, and confinement effects beyond the reach of traditional axisymmetric or surface-discretization methods. The framework provides a foundation for future extensions to more complex membrane models, dynamic processes, and biologically relevant geometries in soft-matter and biophysical systems.
Quantum Fields in Curved Spacetimes: From Detector Entanglement to Black Hole Thermodynamics
(University of Waterloo, 2025-12-05) Bhattacharya, Dyuman
This thesis presents two independent investigations into quantum field theory in curved spacetime. The first concerns relativistic quantum information, with a focus on entanglement harvesting and detector-based probes of quantum fields in curved spacetimes. The second addresses semiclassical aspects of black hole thermodynamics in AdS braneworld settings, incorporating the backreaction of quantum fields to all orders of perturbation theory, and extending previous studies of quantum black holes to include both charge and spin. In Part I, we study the entanglement of quantum fields in curved spacetime, using localized particle detectors interacting with a scalar field. We analyze scenarios involving both flat and curved backgrounds, including gravitational shock waves, the BTZ black hole, and general dimensional anti–de Sitter and de Sitter spacetimes. For the case of initially entangled detectors, we find that interactions with the field can lead to either degradation or amplification of entanglement, depending on the initial state and spacetime geometry. We further derive exact expressions for density matrix elements, at the lowest perturbative order, in the form of infinite analytical series, for detectors on static worldlines in various spacetimes. The transition rate of an in-falling detector in the BTZ black hole spacetime is also derived as an infinite series. These analytic results allow for exact evaluation of quantities, namely the entanglement measures of concurrence and negativity, which are typically computed numerically. In addition, we provide a new example of the ability of detectors to distinguish topologically distinct spacetimes which are locally identical outside of horizons, focusing on the ℝP² and Swedish geons built from the BTZ spacetime. Our results show that localized measurements are sensitive not only to curvature but also to topological features of the underlying geometry. Part II is concerned with the construction and thermodynamic analysis of quantum-corrected black holes in a doubly holographic braneworld model. We obtain a charged and rotating solution localized on an AdS₃ brane embedded in an AdS₄ bulk, incorporating the full backreaction from conformal fields to all orders of perturbation theory. We compute the thermodynamic properties of these black holes, and examine their behavior in extended thermodynamic phase space where the cosmological constant is a variable. We find that the inclusion of charge or spin removes re-entrant phase transitions present in the neutral-static case, and that the critical exponents of these objects match those predicted by classical mean-field theory. The re-entrant phase transitions of the neutral-static quantum black hole has critical exponents which differ from the mean-field values
Control and Characterization of the Central Spin System
(University of Waterloo, 2025-11-18) Chen, Jiahui
Precise, coherent, robust quantum control and characterization of quantum systems play important roles in the development of applications of quantum technologies. In particular, advancing the quality of control requires precise characterization, which, in turn, depends on the quality of control. In the first part of the thesis, we introduce a general framework for designing efficient, precise, and robust quantum control strategies using effective Hamiltonian engineering. The methods enable designs that are robust to systematic control errors and variations in the Hamiltonian. The efficiency benefit of achieving control at zeroth order in the Magnus expansion is highlighted. Design tools, such as methods that identify the space of achievable effective Hamiltonians at each order from the Magnus expansion, are introduced. Objective functions for engineering arbitrary effective Hamiltonians are provided and can be used by numerical optimizers for control sequence design. The second part of the thesis explores the characterization of general noise models based on experiments on a central spin system. The noise is probed through stimulated echo experiments, multi-dimensional correlation spectroscopy, and multi-quantum experiments to characterize system/environment correlation and environmental memory effects. Combined with Bayesian inference, these experiments provide quantitative measures of correlation growth, environmental mixing, and deviations from stochastic noise models. Measures that influence the choice of control schemes include non-Gaussianity, non-stationarity, and non-Markovianity. The multi-quantum experiments can also reveal an extended environment and show how the environmental mixing propagates quantum information throughout the environment.
Machine learning for quantum sensing
(University of Waterloo, 2025-11-18) MacLellan, Benjamin
Precisely measuring the natural world around us underpins new scientific discoveries and technological innovation. Quantum sensors, which harness quantum effects such as superposition and entanglement, represent the frontier of precision measurement and are capable of surpassing conventional limits on measurement sensitivity, precision, and resolution. Such instruments have myriad applications in, e.g., astronomical observations, biological imaging, material science, and geophysical surveys, among many others, and provide new opportunities in the search for new physics, including in gravitational wave detection, searches for dark matter and physics beyond the Standard Model, and probing many-body phenomena such as superconductivity. In recent years, artificial intelligence and machine learning have emerged as a promising paradigm for quantum physics, providing computational tools to extract insights from large scientific datasets, discover structure in complex models, and automate scientific processes. In this thesis, we present novel numerical techniques for the study, design, and implementation of quantum sensing protocols by leveraging machine learning and optimization techniques. First, we propose and demonstrate an end-to-end variational quantum sensing framework, in which parameterized quantum circuits and neural networks form adaptive, trainable ansätze for the quantum dynamics and estimator, respectively. Extending this machine-learning design perspective, we study quantum-enhanced very-long baseline imaging, which uses entanglement distributed through a quantum network to increase the achievable angular resolution of optical telescope arrays. We develop differentiable simulation and optimization techniques to identify optimal resource states and measurements in realistic regimes. Next, we propose and demonstrate a simulation-based inference technique for quantum sensing protocols, which maps observed data to estimated values without the need for explicit likelihood functions. Finally, we investigate the generation of quantum graph states using hybrid photon-emitter platforms, and present a framework for optimizing the generation of large, noise-robust entangled probe states for quantum sensing protocols.
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.

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