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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Towards Feshbach Resonances in the S-WAVE Channel
(University of Waterloo, 2024-09-12) Del Franco, Paul
Ultracold molecules offer unique opportunities for studying quantum phenomena. This thesis presents work undertaken to repair and optimize an older experimental setup which last studied Feshbach resonances in the p-wave channel for ultracold Sodium-Lithium (NaLi) molecules. The primary focus of this work was on the process of repairing and reconfiguring the molecule machine back to a working state. This involved the repair of the optical systems which provide the laser light at the required frequencies. The characterization of the dual species atomic beam, leading to the replacement of the Sodium (Na) and Lithium (Li) sources. The re-optimization of the Zeeman slower current and light alignment. The alignment, configuration and optimization of the magneto optical trap, along with configuration of the transfer processes to the magnetic trap. Significant effort was dedicated to evaporative cooling, where we reached near quantum degeneracy temperatures for both species. With both ultracold gases, we optimized the transfer into an optical dipole trap where we could sweep a magnetic field to produce Feshbach molecules. Progress was made towards Stimulated Raman Adiabatic Passage (STIRAP) for efficient transfer into the triplet ground-state. Although the final step in the STIRAP process was not completed, the successful detection of Feshbach molecules was achieved. This work provides a solid foundation for future experiments, including the completion of the triplet ground-state molecule formation and enhanced stability which will allow for the exploration of s-wave Feshbach resonances in NaLi + NaLi collision complexes. This thesis contributes to the understanding of ultracold molecular interactions and offers insight into the problems encountered in the process of restoring and optimizing a complex experimental apparatus.
Towards Dipole Blockade Controlled-NOT Gate Using Ultracold Molecules
(University of Waterloo, 2024-09-10) Byres, Megan
Quantum computing is a promising field that aims to achieve large increases in computational speed by taking advantage of the unique properties of quantum physics. There are many proposals for how it can be implemented in the real world, one of these being the use of Rydberg atoms. Rydberg atoms are limited by the instability of the highly excited Rydberg states, resulting in lifetimes measured in the hundreds of microseconds. Molecules can be used to perform quantum gates with a similar method to Rydberg atoms, and their lifetimes can be several orders of magnitude longer than the lifetimes of Rydberg atoms. This thesis builds on a previous work in which the ideal fidelity of this method was calculated by investigating various real world factors and their implications for the feasibility of molecules as a platform for quantum computing. Additionally, it discusses many changes and improvements to the ovens and larger vacuum system designed to perform these experiments.
Novel Techniques for the Calibration of Systematics in Next Generation Galaxy Surveys
(University of Waterloo, 2024-08-29) Nguyen, Alan
Baryon Acoustic Oscillation (BAO) observations offer a robust method for measuring cosmological expansion. However, the BAO signal in a sample of galaxies can be diluted and shifted by interlopers - galaxies that have been assigned the wrong redshifts. Because of the slitless spectroscopic method adopted by the Roman and Euclid space telescopes, the galaxy samples resulting from single line detections will have relatively high fractions of interloper galaxies. Interlopers with a small displacement between true and false redshift have the strongest effect on the measured clustering. In order to model the BAO signal, the fraction of such interlopers and their clustering need to be accurately known. We introduce a new method to self-calibrate these quantities by shifting the contaminated sample towards or away from us along the line of sight by the interloper offset, and measuring the cross-correlations between these shifted samples. The contributions from the different components are shifted in scale in this cross-correlation compared to the auto-correlation of the contaminated sample, enabling the decomposition and extraction of the component terms. We demonstrate the application of the method using numerical simulations and show that an unbiased BAO measurement can be extracted. Unlike previous attempts to model the effects of contaminants, self-calibration allows us to make fewer assumptions about the form of the contaminants such as their bias. We also introduce a new statistical technique to cosmology, called the Leave One-Out Probability Integral Transform (LOO-PIT), as a complementary test to the standard best fit statistic χ2. This technique combines two concepts: LOO-CV (Leave One Out-Cross Validation), and the well known Probability Integral Transform (PIT). LOO-PIT primarily has the advantage of diagnosing the type of modelling failure as well as relaxing the constraint of assuming Gaussian likelihoods in one’s data analysis, paving the way for more general methods. While it is a general method, we apply LOO-PIT to the problem of diagnosing unknown interlopers in galaxy catalogues.
Branes and Relations in Holography
(University of Waterloo, 2024-08-28) Lee, Ji Hoon
In this thesis, we propose and study a holographic relation between the states of certain D-branes in anti-de Sitter space and trace relations in the dual gauge theory.
EFFECT OF TREHALOSE AND LITHIUM IN MOLECULAR MECHANISM OF NEUROPROTECTION IN ALZHEIMER’S DISEASE
(University of Waterloo, 2024-08-23) Xu, Yue
Alzheimer's Disease (AD) is still a challenging issue for humans since its first case was identified by Alois Alzheimer over one hundred years ago. Approximately thirty years ago, the "Amyloid cascade hypothesis" was proposed, which is a milestone that began to reveal the mystery of AD. The aggregation and deposition of endogenous amyloid-beta (A-beta) proteins in brains are known to be one of the main pathogenic factors of AD. One of the pathways to neurodegeneration driven by A-beta proteins involves A-beta damage to neuronal membranes, which may result in neuron impairment and death. On the other hand, A-beta proteins have antimicrobial properties, suggesting they may serve functionally in the brain. This could be one of the reasons to explain the severe side effects seen in clinical anti-A-beta treatment for AD. Instead of focusing on anti-A-beta, I aim to explore a therapeutic strategy that focuses on membrane protection. The goal of my work is to investigate the potential of membrane-targeted agents, trehalose and lithium, to protect lipid membranes against A-beta toxicity. Trehalose, a natural-derived sugar, is explored as a potential treatment for neurodegenerative Parkinson's Disease. Lithium, as a mood stabilizer, is commonly used for treating bipolar disorder. Both of the agents are investigated for neurological disorders and can interact with cellular membranes with distinct mechanisms. In this thesis, I ask whether their interaction with lipid membranes can protect membranes from A-beta-induced damage, thereby lowering A-beta neurotoxicity. Hence, the entire thesis addresses two main questions. 1. How does trehalose/lithium affect membrane properties? 2. Can trehalose/lithium protect membranes from A-beta toxicity? To explore the two questions for trehalose and lithium, respectively, the thesis is divided into two parts: Part 1 - trehalose (Chapters 3-8) and Part 2 - lithium (Chapters 9-11). In Part 1, I used Langmuir-Blodgett (LB) Trough, atomic force microscopy (AFM), and Kelvin probe force Microscopy (KPFM) to explore the influence of trehalose on the mechanical and electrostatic properties of model lipid monolayers composed of DPPC, POPC lipids, and cholesterol. The study found that trehalose can enhance the fluidity and alter the electrostatic properties of lipid monolayers, with modulation by NaCl. To assess whether trehalose can protect lipid membranes from A-beta damage, I utilized black lipid membrane (BLM) electrophysiological techniques to evaluate the quality and permeability of membranes exposed to trehalose and A-beta. Results from BLM experiments demonstrated trehalose alleviates A-beta-induced membrane disruption. Furthermore, I explored the binding of A-beta to lipid membranes in the presence of trehalose solutions by localized surface plasmon resonance (LSPR) spectroscopy and found that trehalose can reduce A-beta binding to lipid membranes. Finally, I confirmed the unique neuroprotection of trehalose in cell studies, where trehalose decreased the cell mortality rate caused by toxic A-beta proteins. Part 2 explored the potential of lithium in mitigating A-beta toxicity on lipid membranes. Similarly, I used LB trough, AFM, and KFPM to compare the influence of LiCl and KCl on lipid membranes. The results demonstrated the distinct contribution of Li+ and K+ on the mechanical and electrostatic properties of DPPC-POPC-Chol lipid monolayers. Li+ has a pronounced effect on reducing the lipid molecular area, increasing monolayer fluidity, and strongly competing with K+ in interacting with lipid monolayers. Lastly, BLM was employed to evaluate the membrane permeability in exposure to A-beta and LiCl. The membrane conductance results obtained by BLM suggested that the modulation of LiCl at the therapeutic level enhances membrane resilience to A-beta-induced damage. This research exposes the modulation of membrane-active trehalose and lithium on lipid membrane properties and their protective role in AD. It contributes to exploring a new therapeutic approach against A-beta toxicity that focuses on membrane protection, which may aid in developing prevention and treatment strategies for AD.
Constraining the quenching mechanisms of galaxy clusters through the evolution of stellar mass functions within GOGREEN and GCLASS
(University of Waterloo, 2024-08-21) Hewitt, Guillaume
We present an analysis of the stellar mass functions (SMFs) of 17 rich galaxy clusters within the GOGREEN and GCLASS surveys in the redshift range of 0.8 < z < 1.5, down to a stellar mass limit of log(M/M⊙) = 9.5. We fit all the data simultaneously with a model that allows the Schechter function parameters of the quiescent and star-forming populations to vary smoothly with radius and redshift. The model also fits for the concentration parameter of each population, and the quenched fraction is modeled as a smooth function of redshift and velocity dispersion. We fit the data in a Bayesian manner, using MCMC. We find no significant dependence of the shape of the star-forming SMF on radius nor redshift, and find it to be consistent with the field. We confirm previous results of a radial dependence on the quenched fraction. We find a moderately significant radial dependence on the α and M* parameters of the quiescent population SMF. The cluster core has a highly elevated quenched fraction, yet the core quiescent SMF is more similar in shape to the quiescent field. The cluster non-core has an moderately elevated quenched fraction, and its quiescent SMF is more similar to the shape of the star-forming field. We explore the contributions of ‘early mass quenching’ and mass-independent ‘environmental quenching’ models in each of these radial regimes. We find the core to be described primarily by early mass quenching, which we interpret as accelerated quenching of massive galaxies in protoclusters, possibly through merger-driven AGN feedback, and the non-core to be described by environmental-quenching, signifying the increase of mass-independent quenching mechanisms that dominate low redshift clusters.
New Constraints on the Halo Mass of Ultra-Diffuse Galaxies with UNIONS using Weak Gravitational Lensing
(University of Waterloo, 2024-08-19) Ducatel, Jordan
While a lot of progress has been made in detecting and measuring various properties of Ultra-diffuse Galaxies (UDGs) over the last decade, the dark matter halo mass of these extremely faint and large objects remains a mystery. A better constraint on the total halo mass of UDGs would disentangle the wide variety of proposed formation mechanisms. We detect a contaminated sample of 545 potential UDGs, of which we estimate 290 to be true UDGs, in the ongoing Ultraviolet Near Infrared Optical Northern Sky Survey (UNIONS) using the Canada-France Imaging Survey (CFIS) r-band imaging, limiting our search to within 66 galaxy clusters up to redshift z ≤ 0.1. From weak gravitational lensing measurement around our UDG sample corrected for interloper contamination, we find an excess surface density consistent with zero (no detection) and a 2σ upper limit on the average halo mass of m200 ≤ 10^12.51 M⊙. By combining our measurement with that of Sifón et al. (2018), the only other weak gravitational lensing measurement of UDGs, we are able to constrain the halo mass further with a 2σ upper limit of m200 ≤ 10^12.05 M⊙ when accounting for the potential low-biasing effect of interlopers in this combined sample. Our results do not disentangle whether UDGs tend to be, on average, more dark matter-dominated or dark matter-deficient galaxies and therefore does not allow us to put new constraints on their formation mechanism. This work on UDG detection in a wide field survey optimized for weak lensing helps pave the way for future direct halo mass measurements of UDGs in upcoming surveys such as the Euclid Wide Survey.
Nanoscale Dynamic Nuclear Polarization in Force-Detected Magnetic Resonance
(University of Waterloo, 2024-08-13) Singh, Namanish
Nuclear magnetic resonance (NMR) has played a pivotal role in modern science with its ability to perform non-destructive imaging and spectroscopy of various systems. Despite this, NMR has been plagued by low detection sensitivity when trying to study nanoscale ensembles of spins, primarily due to the small thermal polarization of nuclear spins. Extending the capabilities of NMR to address nanoscale sample volumes would present exciting opportunities for studying biological systems, enabling high-resolution imaging of single biomolecules and virus particles. Over the years, a great deal of techniques to improve the detection sensitivity of the measurement apparatus have been made. Force-detected magnetic resonance is one such technique, that has demonstrated the capability to detect nanoscale ensembles of spins, where it has been successfully used to achieve three dimensional images of virus particles. Nevertheless, further improvements are needed to achieve high resolution atomic scale imaging of nanoscale systems. Techniques such as dynamic nuclear polarization (DNP) have been widely implemented in traditional NMR experiments for boosting the signal, by transferring the comparatively larger polarization of electrons to surrounding nuclei . The use of DNP in force-detected magnetic resonance platforms however, has remained relatively limited though. Bringing DNP to nanoscale force-detected magnetic resonance setups would mark a significant next step in improving the detection sensitivity of nanoscale NMR experiments. In this thesis, we discuss the implementation of DNP in a force-detected magnetic resonance experiment in order to achieve sensitivities needed to realize high resolution imaging of nanoscale spin ensembles. In these experiments, we observed a 100 fold enhancement in the proton thermal signal in a nanoscale droplet composed of trityl-OX063 radicals suspended in a sugar-water glassy matrix. We also compare the signal-to-noise ratio (SNR) boost this provides over measurements that rely upon statistical polarization, where we demonstrate a reduction in averaging time by a factor of 204. This work explores various tunable parameters to optimize the enhancement such as the proton and radical relaxation times. This work also investigates the role fast-relaxing paramagnetic defect centers from the surrounding environment play in reducing the radicals spin-lattice relaxation time, a crucial component for efficient DNP.
Twisted Holography in B-model
(University of Waterloo, 2024-08-12) Budzik, Katarzyna
Supersymmetric quantum field theories contain protected subsectors which can be obtained by the procedure known as twisting. The idea of twisted holography is to study holographic duals of such twists. The main example of twisted holography in this thesis is the duality between the chiral algebra subsector of N = 4 super Yang-Mills and the B-model topological string theory on the complex manifold SL(2,C). In this thesis, we study two aspects of the duality: the correspondence between determinant operators in the chiral algebra and “Giant Graviton” branes in the dual geometry, and the extension to non-conformal vacua of the chiral algebra. The second BPS subsector studied in this thesis is the holomorphic twist of 4d N = 1 super Yang-Mills. The holomorphic twist is defined as the cohomology of one supercharge and captures the quarter-BPS operators that are counted by the supersymmetric index. The twisted theory is endowed with extra structures and symmetries which are a 4d analogue of a 2d chiral algebra. We observe that the differential in the holomorphic twist receives loop corrections which make the theory topological and can be interpreted as a sign of confinement of the original theory. Finally, we present a holographic realization of the holomorphic theory in the B-model topological string theory.
Aspects of Quantum Information in Quantum Field Theory: Particle detector models, entanglement, and complexity
(University of Waterloo, 2024-08-09) de Souza Leao Torres, Bruno
This thesis explores three themes in the interface between quantum information and quantum field theory (QFT). Part 1 is devoted to particle detector models, which are one of the key ingredients in formulations of a measurement theory for quantum fields. In tune with recent efforts to devise a fully local and relativistic measurement theory for quantum fields, we present a simple model of a local probe that is, itself, formulated in terms of a field theory. We then proceed to show how to systematically reduce this field-theoretic description of the probe to an effective theory restricted to a finite set of modes. The resulting dynamics at leading order in perturbation theory are given precisely by the widely adopted models for detectors based on nonrelativistic probe systems. These results pave the way to bridge the gap between the fully field-theoretic and the detector-based approach to measurements in QFT, and give particle detector models an effective field theory flavor. Part 2 then focuses on the concept of entanglement in quantum field theory. We start in the first half of Part 2 by studying a protocol known as entanglement harvesting, which allows two localized probes to extract entanglement from a quantum field even before they have time to exchange causal signals. Recent works on field-theoretic models for local probes in relativistic quantum information have raised objections against the possibility of entanglement harvesting at weak coupling between the probes and the field when the probes themselves consist of localized degrees of freedom of a field theory. We address the origins of these concerns and show that, for an appropriate choice of modes used as probes for the quantum field, it is indeed possible to harvest entanglement using localized probes described along the lines of the formalism presented in Part 1. In the second half of Part 2, we also show how to best couple to a quantum field in order to most accurately reproduce its entanglement structure. This helps to establish limits on how efficient entanglement harvesting between complementary subregions can be, and also suggests further directions for how to address the problem of probing the structure of entanglement in field theory in more general scenarios. Finally, in Part 3, we delve into the concept of computational complexity of Gaussian states, which are a special class of quantum states that is pervasive in many contexts in QFT. Following Nielsen's geometric approach to circuit complexity, we devise a general class of metrics on the space of Gaussian unitary circuits which allows the circuit complexity of any pure Gaussian state to be characterized in a unified fashion. This gives us a generalized framework that can accommodate additional physical constraints on the notion of complexity adopted; we comment on a few examples where these additional features can be of physical relevance.
Towards Gated Quantum Emitters from Undoped Nano-LEDs
(University of Waterloo, 2024-07-25) Sherlekar, Nachiket Sunil
Quantum light emitters have the potential to transform emerging quantum technologies and their applications, such as secure quantum communication, metrology, and quantum computing. Ideally, these light sources emit on-demand at high rates and efficiencies with high degrees of single-photon indistinguishability. Additionally, these emitters can emit entangled photon pairs, are position-controllable, and wavelength-tunable. Current state-of-the-art single- and entangled-photon sources based on spontaneous parametric down-conversion (SPDC) and on-demand (or deterministic) implementations suffer from various drawbacks that make them deviate from ideality. SPDC sources emit probabilistically, and increasing their brightness degrades their single photon purity, indistinguishability, and entanglement fidelity. Of the various deterministic sources, optically-driven semiconductor quantum dots have very high single-photon efficiencies (~ 71%), purities (> 99%) and indistinguishabilities (> 99%), are position-controllable and wavelength-tunable. However, complex synchronized optical routing between the pump laser, sources and detectors is required to scale their usage. This would occupy a large footprint, restricting them to a laboratory setting. Quantum dots may be current-injected instead, but while gigahertz (GHz) emission frequencies are possible, the electron injection number is not controllable. This inability to control the electron injection is akin to non-resonant optical excitation in which there are many charges in the environment around the quantum dot, thus making current-injected quantum dots inferior to optically-driven quantum dots. This thesis proposes a novel design for a high-rate, deterministic, electrically-driven quantum emitter that combines a gate-defined lateral planar p-n junction (or nano-light-emitting-diode or nano-LED) with a quantized charge pump along a quasi-one-dimensional channel in dopant-free GaAs/AlGaAs heterostructures. In contrast to other electrically-driven sources, our implementation allows for a precise control of the injected electron number via the quantized charge pump. In addition, by using gates to define the p-type and n-type regions of the junction instead of intentional dopants (as in conventional vertical p-n junctions), the charge carrier mobility in these heterostructures is much higher. The lack of dopants also allows p-type and n-type regions to exist simultaneously on both sides of the device (such devices are termed `ambipolar'), in turn allowing flexible operation. By operating the charge pump at GHz frequencies, this source could emit a billion photons per second. Integrating a cavity at the site of emission would boost the rate of emission and the efficiency, and could also increase the single-photon indistinguishability. The following research obstacles were identified over the course of developing our nano-LED (the prerequisite for our quantum emitter): - quenching of device electroluminescence (EL) and time-instability of emissions due to parasitic charge accumulation, necessitating thermal cycling to reset the device; - alternate current pathways (both radiative and non-radiative) through the device mesa that reduce both internal and external quantum efficiency; - delocalized emission at mesa edges due to minority currents under the topgate edges, affecting extraction efficiency and position-controllability; and - multimode emission and slow rate of spontaneous emission that reduce extraction and collection efficiencies. Descriptions of our nano-LEDs and their emissions as well as solutions to the above obstacles obtained through experiment are summarized below. The nano-LEDs discussed in this thesis are gate-induced either in GaAs rectangular quantum wells or at GaAs/AlGaAs single heterojunction interfaces. All nano-LEDs reported in literature are induced using the former and not the latter. In fact, a recent theoretical study concluded that radiative electron-hole recombination was impossible in nano-LEDs induced at single heterojunction interfaces. Our demonstration of EL from nano-LEDs induced at GaAs/AlGaAs single heterojunction interfaces is the first of its kind. Since the fabrication yield using single heterojunction wafers is higher than when using rectangular quantum wells, they offer an alternative for easier fabrication of the nano-LEDs. To understand how the EL quenches in our nano-LEDs, we propose a scenario of localized parasitic charging that results in enhanced non-radiative recombination and causes a gating of the p-n channel that suppresses the diode current. To address this issue, we have devised a gate voltage sequence that we call the `Set-Reset' protocol. This protocol clears away accumulated parasitic charge, extending the lifetime of device operation without the need for thermal cycling. Our nano-LEDs can be operated in four distinct circuit measurement configurations, depending on whether the left side is p-type or n-type (with the right side being n-type or p-type, respectively), and whether the left side is grounded or floating (with the right side being floated or grounded, respectively). EL from our nano-LEDs (induced at both quantum wells and single heterojunctions) is observed not only around the p-n junction interface, but also as far as the edges of the etched mesa, indicating the presence of unwanted radiative recombination pathways. The p-side is consistently brighter in the single heterojunction samples while the n-side was brighter for the quantum well devices. A neutral and a negatively charge exciton peak was observed in the spectra from the n-side of the nano-LEDs. Spectra from the p-side were measured only for the single heterojunction devices, and showed the neutral exciton peak as well as a lower energy peak. The narrowest neutral exciton emission linewidths (0.70 meV) from lateral p-n junctions to date were recorded from the quantum well nano-LEDs. Our nano-LEDs were also shown to be compatible with radio frequency operation, necessary for quantized charge pump integration to create a quantum emitter. To address the issue of delocalized emission and time-instability of EL, we fabricated and tested a nano-LED with a novel gate architecture that included two wide surface gates placed adjacent and perpendicular to the p-n channel. The extra gates add a degree of freedom that along with standard DC operation and the Set-Reset protocol opens up many measurement configurations. A downside is that these surface gates are prone to current leakage. Several measurement configurations were explored, with two standing out---one yielded localized emission at the junction interface while using the Set-Reset protocol; another yielded time-stability of emission in DC operation. A conceptual model has been laid out that is compatible with the results from these various operating configurations. From the time-stable measurements of EL intensity and p-n current, the internal and external quantum efficiencies were estimated to be ~ 1.19x10^(-3) and ~1.95x10^(-5), respectively. These values may be boosted in future designs by incorporating insulator-separated side gates, blocking gates, and a cavity around the emission region. The side gates and blocking gates will respectively time-stabilize and localize the EL emission during DC operation, and the cavity will increase the rate of spontaneous emission and shape the mode. A long-standing problem in the field of deterministic quantum emitters is the fact that they emit light omnidirectionally and into multiple modes. Various confining structures such as tapered nanowires, micropillar cavities, photonic crystals, solid immersion lenses and circular Bragg gratings have been proposed and implemented in literature. We identified the circular Bragg grating cavity etched into a heterostructure with a Bragg mirror grown below the rectangular quantum well as the optimum solution for our nano-LED. Through simulation, both the Bragg mirror and circular Bragg grating designs were tuned to match the quantum well emission wavelength (~ 807.5 nm). The circular Bragg cavity etched into the Bragg mirror wafer around the emission region enhances the rate of spontaneous emission via the Purcell effect, and simultaneously funnels the emission into a single elliptical Gaussian mode for efficient collection. A split was included in the circular Bragg grating to make it compatible with our proposed emitter design. Theoretically, for in-plane exciton dipoles oriented parallel to this split, the cavity enhances the spontaneous emission rate by a factor of 5.3 at a center wavelength of 807.4 nm and a bandwidth of ~ 3.7 nm or ~ 7.0 meV. The split in the cavity causes emission to be linearly polarized. This linear polarization is unfortunately incompatible with the emission of polarization entangled photon pairs. The effective collection efficiency (from simulation) is ~ 30%, which is ~ 52 times greater than that of a device without a cavity. The inclusion of our cavity also boosts the internal and external quantum efficiencies by factors of 4.5 and 89, yielding values of ~ 5.32x10^(-3) and ~ 1.74x10^(-3), respectively. Design validation of the Bragg mirror using reflection measurements yielded a Bragg stopband frequency and bandwidth that closely match simulation. Simulated and measured reflection spectra from the circular Bragg gratings indicated a linear relationship between the ring width of the grating and the cavity resonance wavelength, with a consistent wavelength offset between simulation and measurement of ~ 16.3 nm. From these results, a cavity with a ring width of ~ 94.8 nm would most closely match the emission wavelength of ~ 807.5 nm. Through our proposed and implemented solutions for the obstacles facing our nano-LEDs, we pave the way for the realization of a high-rate, electrically-driven quantum emitter.
New Experimental Observables for the QCD Axion
(University of Waterloo, 2024-07-08) Madden, Amalia
The QCD axion is one of the best motivated extensions to the Standard Model of particle physics that could also serve as the dark matter. The thesis will demonstrate new experimental observables that could be used to search for the axion. These observables are based on piezoelectric materials that spontaneously break parity symmetry, thereby enabling sensitivity to the axion's fundamental, model independent coupling to gluons. The first observable explores how axion dark matter could generate an oscillating mechanical stress in a piezoelectric crystal. We call this new phenomenon ``the piezoaxionic effect". When the frequency of axion DM matches the natural frequency of a bulk acoustic normal mode of the piezoelectric crystal, the piezoaxionic effect is resonantly enhanced and can be read out electrically via the piezoelectric effect. We also point out another, subdominant phenomenon present in all dielectrics, namely the ``electroaxionic effect". An axion background can produce an electric displacement field in a crystal which in turn will give rise to a voltage across the crystal. We find that this model independent coupling of the QCD axion may be probed through the combination of the piezoaxionic and electroaxionic effects in piezoelectric crystals with aligned nuclear spins, with near-future experimental setups applicable for axion masses between $ 10^{-11}\text{eV}$ to $10^{-7}\text{eV}$, a challenging range for most other detection concepts. The second observable, the ``piezoaxionic force" demonstrates how a piezoelectric crystal can be used to source virtual QCD axions in the laboratory, giving rise to a new axion-mediated force. The presence of parity violation in the piezoelectric crystal, combined with aligned nuclear spins, provides the necessary symmetry breaking to generate an effective in-medium scalar coupling of the axion to nucleons. We propose a detection scheme that uses the axion's model-dependent pseudoscalar coupling to nuclear spins, such that the new force can be detected by its effect on the precession of a sample of polarised nuclear spins. When the distance between the source crystal and the detector is modulated at the Larmor precession frequency of the nuclear spins, the signal is resonantly enhanced. We predict that near-future experimental setups should be sensitive to the axion in the unexplored mass range from $10^{-5} \text{eV}$ to $10^{-2} \text{eV}$.
Dark Screening of the Cosmic Microwave Background with Hidden-Sector Particles and New Dynamical Observables in First Order Phase Transitions
(University of Waterloo, 2024-07-02) Pirvu, Dalila
This thesis focuses on two research directions within the field of Cosmology. It comprises the main results of my work as a PhD student. Part~\ref{PartI} introduces new observables of false vacuum decay derived from real-time numerical simulations. Part~\ref{PartII} describes a new method to search for hidden-sector particles using information from Cosmic Microwave Background (CMB) and Large Scale Structure (LSS) data. The first part studies metastable `false' vacuum decay in relativistic first order phase transitions. It is a phenomenon with broad implications for Cosmology and is ubiquitous in theories beyond the Standard Model. Describing the dynamics of a phase transition out of a false vacuum via the nucleation of bubbles is essential for understanding vacuum decay and the full spectrum of observables. We study vacuum decay by numerically evolving stochastic ensembles of field theories in 1+1 dimensions from an initially metastable state. First, we demonstrate that bubble nucleation sites cluster by measuring correlation functions in simulations. Next, we show that bubbles form with a Gaussian spread of centre-of-mass velocities for a field with an initial thermal spectrum. Finally, we show that nucleation events are preceded by oscillons - long-lived, time-dependent, pseudo-stable field configurations. We provide theoretical tools to model and generalize our findings. In the second part, we introduce a new type of secondary CMB anisotropy: the patchy screening induced by resonant conversion of CMB photons into dark-sector massive scalar (axions) and vector (dark photons) bosons as they cross non-linear LSS. In two of the simplest low-energy extensions to the SM, CMB photons can resonantly convert into either dark photons or axions when their local plasma frequency matches the mass of the hidden sector particle. For the axion, the resonance also requires the presence of an ambient magnetic field. After the epoch of reionization, resonant conversion occurs in dark matter halos if the hidden-sector particles have masses in the range $10^{-13} {\rm \; eV} \lesssim m_{{\rm A^{\prime}}} \lesssim 10^{-11} {\rm \; eV}$. This phenomenon leads to new CMB anisotropies correlated with LSS, which we refer to as dark screening, in analogy with anisotropies from Thomson screening. Each process has a unique frequency dependence, distinguishing both from the blackbody CMB. In this thesis, we use a halo model-based approach to predict the imprint of dark screening on the CMB temperature and polarisation and their correlation with LSS. We then examine $n$-point correlation functions of the dark-screened CMB and correlation functions between CMB and LSS observables to project the sensitivity of future measurements to the dark photon and axion coupling parameters.
Variational Quantum Computing: Optimization and Geometry
(University of Waterloo, 2024-06-27) Roeland, Wiersema
Quantum computing potentially offers unprecedented computational capabilities that transcend the limitations of classical computing paradigms. Despite its conceptual inception over three decades ago, recent years have witnessed remarkable progress in the realization of physical quantum computers, spurring a surge of research activity in the field. Although fault-tolerance devices remain unrealized, modern quantum hardware is getting less noisy, which allows us to investigate quantum algorithms that require only short depth circuits. One particular class of algorithms that falls into this category are variational quantum algorithms, which treat a quantum computer as a black box with tunable parameters that can be optimized via a classical optimization routine. This thesis delves into the realm of variational quantum algorithms and explores their optimization properties, trainability and geometric properties. Through a blend of numerical experiments, geometric insights, and mathematical analysis, it provides a comprehensive exploration of variational quantum algorithms paving the way for future advancements in variational quantum computing.
Dynamics and phases of matter in open quantum many-body systems
(University of Waterloo, 2024-06-24) Sang, Shengqi
The thesis is divided into two parts, both focusing on the topic of open quantum many-body systems. The first part explores the properties of quantum circuits interspersed with measurements. Tuned by the frequency of measurements, the circuit exhibits two stable dynamical phases: a weakly-monitored phase and a strongly-monitored one. For the former case, we analyze its non-equilibrium properties and unveil that it exhibits physical length scales that grow super-linearly with time. For the latter case, we demonstrate that it can maintain non-trivial quantum order when symmetries are present. The second part addresses phases of matter for mixed many-body states. We propose a real-space renormalization group approach for mixed states and apply it to derive phase diagrams for various examples. For decohered topological codes, we establish a precise relationship between the decodability and the topological phase transitions. Lastly, we introduce the notion of 'Markov length', a length scale that measures the locality of correlation, as a diagnostic for the stability of mixed state phases.
Leveraging Polarization in the Era of Submillimeter VLBI
(University of Waterloo, 2024-06-14) Ni, Chunchong
With the advancement of technology, global very long baseline interferometry (VLBI) observations at millimeter wavelengths become possible. The Event Horizon Telescope (EHT) is the first such experiment, which makes observing accretion disk and jet launching regions near supermassive black holes and active galactic nuclei (AGN) possible, including polarimetry observations. Centaurus A (Cen A) is a nearby radio-loud AGN, with large jet structures of angular size measured in degrees. It was observed by the EHT, whose first total intensity image shows a fork-shaped edge brightening jet structure. Chapter 2 applies Bayesian imaging method to the Cen A data. We first construct the total intensity image of Cen A, which we directly compare with the previous publication. Second, the Bayesian method produces the first polarization studies of Cen A jet. Both the total intensity imaging and the polarization mapping feature a full image posteriors with access to the image uncertainty. This proves to be essential in the case of Cen A, where the data is very challenging for various reasons. With polarization image posterior of Cen A, we are able to study different regions of the jet separately, eventually producing a robust estimate of a collection of important physics quantities, including magnetic field strength, the electron number density and the jet velocity. In Chapter 3, we explore the origin and influence of the interstellar scattering on observations of Sgr A*, and propose a novel method to mitigate this scattering via EHT and next-generation EHT (ngEHT) polarimetry in the future. In EHT and other radio astronomical observations of Sgr A*, scattering contaminates the image with external small-scale structures, essentially preventing further studies of the turbulence in the accretion disk. However, for credible interstellar magnetic field strengths, the scattering is proved to be insensitive to polarization. Therefore, it is possible to distinguish intrinsic and scattered structures via the image power spectra constructed in different polarization components. Via numerical experiments, we demonstrate a method for reconstructing intrinsic structural information from the scattered power spectrum. We demonstrate that this is feasible through a series of numerical experiments with general relativistic magnetohydrodynamic (GRMHD) simulation images. Specifically, we show that the ratio of the power spectra, obtained independently for different polarization components, is independent of the scattering screen. Therefore, these power spectra ratios provide a window directly into the MHD turbulence believed to drive accretion onto black holes.
Effects of Noncommuting Charges in Quantum Information and Thermodynamics
(University of Waterloo, 2024-06-13) Majidy, Shayan
The advancement of quantum theory is rooted in challenging established assumptions. This trend persists as quantum theory extends into other fields, including thermodynamics. One such assumption in thermodynamics is that conserved quantities, known as charges, commute. Lifting this assumption has led to a new subfield, noncommuting charges, at the intersection of quantum information and quantum thermodynamics. The work presented in this thesis identifies various effects of noncommuting charges and extends the topic to many-body physics and experiments. Initially, the field’s findings were conveyed in abstract information-theoretic terms. To transition these findings to experimental practice and tie them to many-body physics, constructing relevant Hamiltonians is essential. We introduce a method for constructing Hamiltonians that globally conserve noncommuting quantities while facilitating their local transport. Having demonstrated the testability of noncommuting-charge physics, we aim to delineate its effects. To do so, we construct analogous models that differ in whether their charges commute. We find that noncommuting models exhibit higher entanglement entropies. Since entanglement accompanies thermalization, our result challenges previous assertions that charges’ noncommutation hinders thermalization. Motivated by understanding noncommuting charges’ effects on entanglement, we introduce them into monitored quantum circuits. Monitored quantum circuits typically transition from a highly entangled volume-law phase to a less entangled area-law phase as one increases the rate of measurements. This holds for monitored quantum circuits with no charges and commuting ones. We find that by introducing noncommuting charges into monitored quantum circuits, the area-law phase becomes replaced with a critical phase. Since critical phases are characterized by long-range entanglement, this result reinforces entanglement enhancement by noncommuting charges. Finally, we revisit the puzzle of whether noncommuting charges promote or hinder thermalization. Most quantum many-body systems thermalize; some don’t. In those that don’t, what effect do noncommuting charges have? One type of system that does not thermalize is a system whose Hamiltonian has so-called dynamical symmetries (or spectrum-generating algebras). We find that noncommuting charges promote thermalization by reducing the dynamical symmetries in a system.
Entangled photon source for a long-distance quantum key distribution
(University of Waterloo, 2024-06-11) Oh, Sungeun
Satellite-based Quantum Key Distribution (QKD) leverages quantum principles to offer unparalleled security and scalability for global quantum networks, making it a promising solution for next-generation secure communication systems. However, many technical challenges need to be overcome. This thesis focuses on theoretical modeling and experimental validation for long-distance QKD, as well as the development and testing of the quantum source necessary for its implementation, to take strides towards realization. While various approaches exist for demonstrating long-distance QKD, here we focus on discussing the approach of sending entangled photon pairs from an optical quantum ground station (OQGS), one through free-space on one end (uplink), and the other one through ground on the other end. In the thesis, we first discuss the considerations relevant to establishing a long-distance quantum link. Since a substantial amount of research has already been conducted on optical fiber communication through ground-based methods, our focus is specifically directed towards ground-to-space (i.e., free space) quantum links. One of the most concerning aspects in free-space quantum communication is signal attenuation caused by environmental factors. We particularly examine pointing errors that arise from satellite tracking systems. To investigate this further, we designed a tracking system employing a specific tracking algorithm and conducted tracking tests to validate its accuracy, using the International Space Station (ISS) as a test subject. Our findings illustrate the potentially significant impact of inaccurate ground station-to-satellite alignment on link attenuation, according to our theoretical model. Given that photons serve as the signals for the QKD, we also investigate the background light noise resulting from light pollution around our Optical Quantum Ground Station (OQGS), which is another concerning aspect, as it could worsen the link attenuation. Consequently, we estimate the minimum photon pair rate required for successful QKD, taking into account both the obtained values from these measurements and the expected level of link loss. Having determined the minimum photon pair rate and other requirements for the long-distance QKD, we proceed to fully elaborate on the development process of the Entangled Photon Source (EPS), which is one of the crucial devices for demonstrating entanglement-based QKD. Here, the thesis includes a detailed explanation for the customization of a crystal oven. It also explains the implementations of a beam displacer scheme and a Sagnac scheme to create a robust interferometer, responsible for creating quantum entanglement. In addition, we demonstrate a novel approach to effectively compensate for the major weaknesses of the interferometer, namely spatial and temporal walk-offs. Finally, we conduct the entanglement test and demonstrate its suitability for long-distance QKD. As a side project, we investigate the performance degradation of nonlinear crystals in response to proton radiation, exploring the potential of deploying the EPS in space for downlink QKD in the future. This thesis provides a comprehensive analysis and testing of elements required for long-distance QKD, contributing to the advancement of future global quantum networks.
Developing a robust quantum simulator with trapped ions
(University of Waterloo, 2024-05-27) Hahn, Lewis
Trapped ion systems have experienced significant growth in recent years as their potential for excelling as quantum simulators has become recognized. Ions make an excellent qubit due to their long coherence times with moderate gate times, high fidelity detection and state initialization, and their ability to create long range spin interactions. As the experimental demands of trapped ions increases, so too do the demands that sustain and control them. In this thesis, I will cover the design and implementation of robust systems for the trapped ions platform and describe the development of robust lab infrastructure, equipment, and optics required to perform high contrast entangling operations on an existing four-rod system. By redesigning the DC power distribution and grounding system I have been able to supply our quantum simulator with clean DC voltages while reducing ground loops that can introduce noise into the system. With delicate alignment of the 355 nm system, our four-rod system is able to entangle qubits together. By incorporating 3D printing and inexpensive DC motors, I was able to motorize the controls used to align our 355 nm beam in the vertical direction which has made alignment reliable and accessible. With future iterations of the motorized stage, I’ve been able to achieve a resolution of 70 nm in all three axes. We then look to the next generation ion trap in the form of a meticulously engineered blade trap. With the ability to perform simulations on systems of about 30 qubits, reaching very low vacuum pressures is essential to increase ion life times. By careful preparation of our Shapal blade holder I’ve been able to preserve the 9 × 10−13 mbar pressure of our blade trap vacuum chamber. I then discuss the design of the imaging system for the blade trap which utilizes dual 0.5 NA (numerical aperture) objectives to achieve high state detection fidelity (∼ 99.9%). By simulating the imaging system and taking into consideration the effects of the systems’ efficiencies, I find that high state detection fidelity should be achievable for detection times on the order of 20 µs. This offers potential for performing in-situ mid-circuit measurement on the blade trap system. By performing some initial tests, I compare the experimental results to the simulated performance of the imaging system and find they match reasonably well.
Random State Production for Quantum Key Distribution using Weak Coherent Pulse Source
(University of Waterloo, 2024-05-27) Piatt, Matthew
This thesis addresses two challenges involved in the culmination of the Quantum En- crYption and Science Satellite mission. This mission aims to demonstrate quantum key distribution (QKD) in space. The first part of the thesis will be dedicated to the intro- duction of a quantum random number generator. This generator will be based off of the arrival time of photons and subsequently used to produce random bits that will be turned into the random states necessary for the QKD protocol. The latter half of the thesis will address creating the random states for the protocol. This is non-trivial due to the finite resources involved in the physical apparatus that comprises the chosen weak coherent pulse source for the QEYSSat mission.

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