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Search for Dark Matter and Vacuum Quantum Gravity Fluctuations using Gravitational Wave Experiments
Get PDFThe biggest physics discoveries of recent decades—the detection of the Higgs at the Large Hadron Collider (LHC) in 2012 and the observation of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015—are often celebrated as two monumental yet distinct discoveries. While the potential of gravitational wave experiments to illuminate particle physics has been acknowledged, its full scope has not been fully appreciated. In this dissertation, we explore various methods to utilize experiments designed for gravitational wave observations in the pursuit of understanding physics beyond the standard model. Specifically, we study two pressing aspects of particle physics: dark matter and quantum gravity, examining their potential signatures in these experiments.
Pulsars, due to their stable periods, are exceptionally suited for gravitational wave observations. Recently, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) reported positive evidence of a stochastic gravitational wave background in 2023. While the application of pulsar timing measurements in gravitational wave detection is well established, they also offer avenues to study various properties of dark matter, such as the small-scale power spectrum and gravitational wave signatures from a cosmological phase transition. We will discuss search strategies for dark matter using realistic pulsar timing array data, current constraints, and future prospects.
Laser interferometry-based gravitational wave detectors like LIGO also offer a potential pathway for dark matter detection. With their high precision in measuring laser phase fluctuations, even feeble interactions between dark matter and standard model particles can produce signals of potentially measurable size. This includes gravitational interactions as well as other long-range forces, such as scalar or vector mediated Yukawa interactions. We will explore the spectral shape of such signals and their detection prospects.
Finally, recent proposals suggest that vacuum quantum gravity effects may manifest as observable phenomena at low energies in laser interferometers. Planck-sized fluctuations arising from quantum gravity are amplified by the large number of degrees of freedom on the horizon of the causal diamond, corresponding to its entropy. Although LIGO is nominally sensitive to these signatures, its lack of sensitivity at the free-spectral range frequency of the cavities renders it ill-suited for detecting such phenomena, highlighting the need for additional experimental setups. We will discuss one approach to estimate the size of these fluctuations by drawing connections between a four-dimensional causal diamond and known solutions of two-dimensional Jackiw–Teitelboim (JT) gravity, as well as the experimental implications.</p
Neurocomputational Understanding of Decision-Making in Novel Environments
Get PDFThis thesis investigates the neural and computational mechanisms underlying human decision-making in unfamiliar environments through three interconnected studies. The first study demonstrates that aesthetic value computation for visual art can be systematically predicted from visual features, which are hierarchically represented along the brain's rostrocaudal axis, as revealed by combining deep neural networks with functional MRI data. The second study examines feature-based transfer learning, highlighting the importance of slow integration mechanisms, akin to glial cell functions, for effective knowledge transfer in humans. The third study explores how action affordance influences decision-making in novel environments, showing that action selection results from a competitive interaction between affordance-based and value-based systems, with meta-control exerted by the pre-supplementary motor area and anterior cingulate cortex. Taken together, these studies provide a comprehensive neuro-computational perspective for understanding how the brain navigates novel environments by doing feature-based value computation, transferring knowledge, and using affordance as a guide for action selection
Investigating the Dual Processes Underlying Human Recognition Memory
Get PDFThis thesis examines two aspects of human recognition memory by using two separate behavioral paradigms. Given the dual process hypothesis of recognition memory, the first chapter investigates the correlation between encoding and retrieval of recognition and source memory for images by using a cued retrieval paradigm. Participants were shown images in a particular judgment task (source context) and later asked to retrieve them in a cued retrieval task. Recording from the human brain, I found separate cell populations to be responsive to the source context during the encoding and recognition stages of the task, suggesting a lack of single-cell level reactivation during source retrieval. In the second chapter, I examined how recognition memory signals change over time using repeated longitudinal behavioral testing in an fMRI study. Through repetitive presentation and memory tests over a period of three months, face stimuli were introduced to three participants. The behavioral outcome of the task showed that as frequency of exposure to specific faces increases, the memory performance and judged confidence increases correspondingly, supporting the hypothesis of a continuous familiarity signal
From Photosynthesis to Detoxification: Microbial Metabolisms Shape Earth’s Surface Chemistry
Get PDFEarth’s chemistry, through geologic time and in the present, is inextricably linked with biologically mediated reactions. All major elemental cycles on Earth’s surface have arisen from two competing processes – life shaping its chemical environment through the evolution of key biochemical pathways, and the environment constraining metabolism by dictating which reactions will occur. Understanding this complicated interplay motivates the research presented in this thesis, which studies this phenomenon over two major elemental cycles – the modern Nitrogen (N) and ancient Carbon (C) cycle.
Chapters One and Two focus on the evolution of ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), the enzyme that catalyzes the key carbon fixation step in modern oxygenic photosynthesis. This reaction also imparts a large kinetic isotope effect (KIE) that causes the fixed carbon to be relatively depleted in natural abundance ¹³C compared to its substrate; this isotopic fingerprint can be seen in both the modern C cycle and in rock records recording the ancient C cycle. Therefore, this KIE has been used both in vitro (outside the cell) by biochemical models to rationalize rubisco’s reaction mechanism, and in vivo (in the cell) as a proxy for environmental CO₂ concentrations in the past and present. However, both the in vitro and in vivo measurements are calibrated using modern organisms even though rubisco and oxygenic photosynthesis have undergone profound evolution over geologic time. Therefore, we measured the KIE in vitro and in vivo of a reconstructed ancestral Form IB rubisco dating to >> 1 Ga, and the KIE in vitro of a recently discovered Form I’ rubisco that presents a modern analogue to ancestral Form I rubiscos prior to the evolution of the small subunit. Overall, we find that the KIEs of both rubiscos are smaller than their modern counterparts, which is surprising given that the rock record indicates overall carbon isotope fractionations in vivo are larger in the past. In addition, we find that models strictly based on modern organisms may not apply to the past, questioning the basic assumption that uniformitarianism can be readily applied to biological processes. However, these models can be rescued by accounting for other aspects of cell physiology.
Chapter Three focuses on disentangling the source of key metabolites, like nitrous oxide (N₂O) in the modern N cycle. Like Chapters 1 and 2, an isotopic fingerprint that measures the ‘preference’ of ¹⁵N for the central or outer nitrogen site in N₂O (“Site Preference” or “SP”) has primarily been calibrated using dissimilatory, or energy-generating, nitric oxide (NO) reductases (NORs). However, there exists a much larger and phylogenetically widespread class of NO-detoxifying enzymes; in particular, flavohemoglobin proteins (Fhp/Hmp) produce N₂O as a strategy to neutralize damaging NO-radicals in anoxic conditions. This enzyme, which generates N₂O in non-growing and anoxic conditions, may be more relevant to natural environments where N₂O production has been detected. Surprisingly, we found that Fhp imparts a distinct SP on N₂O that differs from both bacterial and eukaryotic NORs, and that this value better aligns with existing in situ measurements of N₂O from soils. In addition, we find that in strains with both Fhp and NOR, the Fhp signal dominates when cells are first exposed to high concentrations of NO in oxic conditions while growing before being shifted to an anoxic, non-growing state. Therefore, in addition to telling us ‘Who’s there,’ the SP fingerprint may also be able to tell us something about cell physiology in vivo. We propose a new framework for interpreting the source of N₂O based on SP values.</p
Characterization of a Novel Membrane Protein Insertase in the Mitochondrial Outer Membrane
No full textMitochondria are eukaryotic organelles derived from the endosymbiosis of an ancient bacteria. As a result of their endosymbiotic origin, the mitochondrial proteome is composed of a mixture of ancient bacterial derived genes and others which are unique to eukaryotes. This dual bacterial/eukaryotic protein origin results in a complicated landscape for biogenesis of mitochondrial proteins. This is particularly true for mitochondrial membrane proteins, since mitochondria have both an inner and outer membrane each with unique protein composition. Proteins localized to the outer mitochondrial membrane (OMM) are of particular interest due to their connection to many important physiological pathways in humans. OMM proteins are known to be inserted into the lipid bilayer by the MIM complex in yeast and by ATOM36 in trypanosomes, however it is not known how they are inserted in human cells. In my Ph.D. thesis, I describe the development of improved biochemical tools for protein purification and characterization, and then use them as part of an effort to characterize MTCH2, which we identify as the human gene responsible for OMM protein insertion. After identifying MTCH2 in a genome-wide screen, we use a variety of cell biology and biochemical experiments to show that MTCH2 is both necessary and sufficient for OMM protein insertion. We further show that endogenous OMM proteins are affected by MTCH2 depletion, and that apoptosis, a pathway relying on OMM proteins, is sensitive to MTCH2 modulation. Additional work in my thesis demonstrates that MTCH2 is a deeply conserved gene across metazoans, that other OMM insertases likely evolved independently in separate multi-cellular eukaryotic lineages
Dealing with Imperfections: From Aberration to Scattering
Get PDFOptical imaging has gained popularity in life science, biomedical imaging, fundamental physics research, and various other fields due to its non-invasive nature. In a carefully designed optical instrument operating in an ideal environment, the resolution of the optical imaging system is defined by its numerical aperture. However, practical manufacturing issues and inaccurate lens models make it challenging to achieve high resolution across a large area. High magnification lenses introduce aberrations that degrade image quality, prompting the use of complex lens systems dedicated to mitigating such aberrations. Furthermore, when a scattering medium is introduced into the imaging system, image formation becomes infeasible as light follows a complicated trajectory. These challenges pose great obstacles to the use of optical imaging methods in various scenarios. This thesis primarily consists of two parts, one aims to deal with aberration and the other tries to solve scattering induced imaging problems.
In the first part of my thesis, I will discuss a technique called APIC (Angular Ptychographic Imaging with Closed-form method), which enables high-resolution imaging across a large field of view. To make APIC applicable in many non-ideal cases where aberrations (such as defocus) degrade image quality, we equip APIC with a closed-form aberration correction algorithm. We will demonstrate that APIC is unprecedentedly robust against aberrations and can retrieve high-resolution complex light fields using low magnification objectives.
In the second part, we move on to dealing with scattering induced imaging problems. To form images where a scattering medium is present, we first explore the application of ultrasound modulation in optical imaging. We show that, by using ultrasound, we can image a hidden object in a highly scattering medium with ultrasonic resolution. Although this technique helps obtain clear images in the presence of a scattering medium, its resolution is limited. We then demonstrate a method in addressing another scattering problem, namely the non-line-of-sight (NLOS) imaging problem. In a general NLOS problem, modulation mechanisms such as the aforementioned ultrasound modulation are infeasible. We demonstrate that light can be directly focused on the hidden target with an optical diffraction-limited resolution by exploring the properties of the hidden target itself. We will show that this active focusing method possess remarkably improved resolution compared to existing methods and is able to image objects with large reflectance differences.</p
Characterization and Tuning of Quantum Emitters in Hexagonal Boron Nitride
Get PDFHexagonal boron nitride (h-BN) is a two-dimensional material hosting atomic defects that serve as single-photon emitters, attributed to its large bandgap. Its high stability at room temperature, substantial Debye-Waller factor, and integrability into 2D devices make h-BN a compelling choice for quantum applications involving single-photon emitters.
Initially, we investigate the properties of emitters in h-BN to comprehend the limitations of their spectral linewidth. This study includes examining the effects of the host crystal's growth method, the emitter's environment (the substrate), and temperature. As a result, we identify two primary broadening regimes: thermal broadening and spectral diffusion. Secondly, we address spectral diffusion, the predominant broadening mechanism at cryogenic temperatures, which depends on local electrical charges near the emitter. We propose a device structure comprising graphene - emitter h-BN - buffer h-BN - graphene, designed to apply a DC electric field and suppress spectral diffusion. This approach leads to a dramatic two orders of magnitude reduction in linewidth, achieving Fourier transform-limited linewidth.
Moreover, we explored the 3D dipole orientation and axial location of emitters within an h-BN crystal slab by coupling them to a phase change material. We discovered that the dipole orientation of some emitters is predominantly out-of-plane, and these emitters tend to exist close to the crystal's surfaces. This insight aids in the quest to determine the atomic structure of the emitters.
Finally, we examine the photon statistics of single-photon beams generated by h-BN emitters. We demonstrate that these beams exhibit sub-Poissonian statistics with both pulsed and continuous-wave excitation. Our findings reveal that excitation power can serve as a control to alter photon statistics, and we utilize this dependency to illustrate how photon statistics influence the use of quantum emitters in quantum random number generation applications.</p
General Domain FC-Based Shock Dynamics Solver
Get PDFThis thesis presents a novel FC-SDNN (Fourier Continuation Shock-detecting Neural Network) spectral scheme for the numerical solution of nonlinear conservation laws in general domains and under arbitrary boundary conditions, without the limiting CFL constraints inherent in other spectral schemes for general domains. The approach relies on the use of the Fourier Continuation (FC) method for spectral representation of non-periodic functions in conjunction with smooth artificial viscosity assignments localized in regions detected by means of a Shock-Detecting Neural Network (SDNN). Like previous shock capturing schemes and artificial viscosity techniques, the combined FC-SDNN strategy effectively controls spurious oscillations in the proximity of discontinuities. Thanks to its use of a localized but smooth artificial viscosity term, whose support is restricted to a vicinity of flow-discontinuity points, the algorithm enjoys spectral accuracy and low dissipation away from flow discontinuities, and, in such regions, it produces smooth numerical solutions—as evidenced by an essential absence of spurious oscillations in contour levels. The FC-SDNN viscosity assignment, which does not require use of problem-dependent algorithmic parameters, induces a significantly lower overall dissipation than other methods, including the Fourier-spectral versions of the previous entropy viscosity method, especially in the vicinity of contact discontinuities. The approach, which does not require the use of otherwise ubiquitous positivity-preserving limiters, enjoys a great geometrical flexibility on the basis of an overlapping-patch discretization. This allows its application for the simulation of supersonic and hypersonic flows and shocks, including Euler simulations at significantly higher speeds than previously achieved, such as e.g. Mach 25 re-entry flow speeds, impinging upon complex physical obstacles. This multi-domain approach is suitable for efficient parallelization on large computer clusters, and the MPI implementation proposed in this thesis enjoys high parallel scalability and in particular perfect weak scaling, as demonstrated by simulations on general complex domains. The character of the proposed algorithm is demonstrated through a variety of numerical tests for the linear advection, Burgers and Euler equations in one and two-dimensional non-periodic spatial domains, with results in accordance with physical theory and prior experimental and computational results up to and including both supersonic and hypersonic regimes
Molecular Tuning of Electrocatalysts for Generation of Commodity Chemicals
No full textImproving our understanding of electrocatalytic transformations is envisioned to facilitate society’s implementation of technologies that achieve a net zero carbon footprint. Carbon dioxide is one of the most emitted greenhouse gases, and improvement in CO₂ capture technologies along with decreasing costs of renewable energy provide an opportunity to convert this species to value-added chemicals using electrochemical processes. Tuning homogeneous and heterogeneous electrocatalyst performance with well-defined molecular species can render systems more selective and active while also allowing us to readily predict variables crucial in achieving these transformations. This thesis investigates 1) molecular and polymeric species as electrode coatings for enhanced generation of carbon-coupled products and 2) discrete electrocatalyst active sites for formation CO₂ reduction products at low overpotentials; generation of highly reduced liquid fuels is observed with molecular electrocatalysts supported on electrodes.
Chapter I provides context and background to the contents of this thesis.
Chapter II discusses novel, polyaromatic molecular additives utilized for low pH CO₂ reduction on Cu electrodes. N-phenyl isoquinolinium triflate film facilitates high selectivity for C₂+ products in 0.1 M H₃PO₄/KH₂PO₄, suggesting enhancement in CO₂ mass transport rather than limiting proton carrier diffusion. Improvement in long-term stability and tolerance to lower pH compared to previous films is observed.
Chapter III reports on a series of polystyrene-based ionomers to probe the effect of local [K⁺] in the Cu electrode microenvironment on CO₂R performance. Partial current density towards C₂₊ products (|jC₂₊|) increases monotonically with [K⁺] in ionomer, up to 225 mA cm⁻². Replacing K⁺ with [Me4N]⁺ lowers performance to the level of bare Cu, highlighting the crucial role of K⁺ in improving C₂₊ product selectivity. Molecular dynamics simulations and partial pressure CO₂ electrolysis experiments are consistent with enhanced CO₂ mass transport due to K⁺ in the film.
Chapter IV discusses variation of ionomer/polymer structures to maximize CO₂R performance. Incorporation of neutral comonomers bearing cross-linking units rich in biphenyl and terphenyl motifs result in high current densities (~270 mA cm⁻²) towards C₂₊ products with 82% Faradaic efficiency. The analogous neutral variants (i.e., those lacking the charged comonomer) show comparable |jC₂₊| to the K⁺-containing polymers, suggesting a non-innocent role of the aryl-rich polymers in boosting performance.
Chapter V presents novel four-coordinate, dicationic Co complexes supported on carbon nanotubes capable of generating MeOH from CO₂. Electrolysis with CO also leads to formation of MeOH, suggesting a CO-bound complex to be a crucial intermediate in CO₂R to MeOH. This work highlights rare examples of molecular systems facilitating multi-electron electrochemical transformations to highly demanded commodity chemicals.
Chapter VI presents work on molecular electrocatalysts bearing novel polyaromatic ligands that lower the electrocatalytic potential (Ecat) of CO₂R by ~310 mV compared to state-of-the-art complexes as determined via cyclic voltammetry. The extended π system motif is more proximal to the metal center relative to previously reported nanographene-containing electrocatalysts. Well-defined characterization was obtained via single-crystal X-ray diffraction in addition to solution-state techniques. Density functional theory calculations reveal significant ligand contributions in the frontier orbitals of relevant CO₂R intermediates.
Chapter VII highlights a polycyclic aromatic hydrocarbon (PAH) bearing twelve edge nitrogen atoms. Spectroscopy, electrochemistry, and computational results suggest a significant narrowing of the HOMO-LUMO gap compared to the N-free analogue owing to the electron-deficient extended π system imposed by the nitrogen dopants. Changes to absorption and emission spectra from titration of the PAH with metal salts suggest that coordination chemistry provides an additional degree of freedom towards tuning electronic structure. Dramatic changes from addition of trifluoromethanesulfonic acid suggest this material to be a possible pH sensor. This approach in judiciously tuning the band gap of bulk graphene materials via saturation of the nanographene edge sites with nitrogen atoms gives rise to a novel compound with intriguing electronic properties.
Appendix A describes systematic attempts in demonstrating cascade electrocatalysis between molecular CO₂-to-CO complexes and pyridinium film-modified Cu towards enhanced rates of C₂₊ products formation.
Appendix B provides results coupling electrodeposited imidazolium-derived films with pyridinium towards enhanced CO₂R to C₂₊ on Cu. While promising performance is achieved, the difficulty in characterizing the films limits the tractability of these systems with respect to their impacts on the microenvironment.
Appendix C discusses developing coordination complexes of heteroatom containing polyaromatic hydrocarbons. Several examples characterized via X-ray crystallography are reported.
Appendix D shows CO₂R data on K⁺ ionomer-coated Au. Elevation in |jCO| is demonstrated as a function of potassium content in the electrode-electrolyte interface provided by the film.
Appendix E discusses attempts to determine and CO₂ uptake by K⁺ ionomers via solid state NMR spectroscopy.</p
Many-Body Cavity Quantum Electrodynamics and Spin Dynamics with an Ensemble of Rare-Earth Ions
Get PDFStudying and controlling light-matter and matter-matter interactions is a central theme in quantum physics and provides the foundation for quantum applications. Rare-earth ions (REIs) doped in solids are promising candidates for engineering scalable quantum technologies, such as quantum memories and quantum transducers, and for exploring emerging fundamental phenomena. This is because REIs have highly stable optical and spin transitions at cryogenic temperatures, and as a solid-state platform, they are compatible for integrating with quantum devices using well-established semiconductor manufacturing techniques.
This thesis is centered on nanophotonic devices coupling to an ensemble of REIs. To explore the light-matter interaction, we build a light-matter interface by coupling an inhomogeneously broadened ensemble of ytterbium-171 doped in yttrium orthovanadate to a nanophotonic cavity with high cooperativity. In this many-body cavity quantum electrodynamics (cavity QED) system, we observe the appearance of a narrow transparency window in the cavity reflection spectrum under optical driving (collectively induced transparency, CIT). This phenomenon results from the destructive interference between pairs of two-level emitters across the inhomogeneous line and the saturation of resonant ions. Furthermore, coherent excitation of the system within this transparency window enables us to observe highly nonlinear optical emission, spanning from fast superradiance to slow subradiance. To study matter-matter interactions, we shift the focus to the strongly interacting spins. These spins feature clock transitions and pure spin exchange interactions, leading to comparable magnitudes of interaction strength and on-site disorder. We characterize and control the many-body dynamics via Hamiltonian engineering and population initialization. Furthermore, we observe the emergence of robust subharmonic oscillations under Floquet driving, providing evidence for the presence of a discrete time crystal.
The discoveries in many-body cavity QED enable new mechanisms for achieving slow light and frequency referencing, and they provide potential for superradiant lasers. Meanwhile, our studies on spin dynamics showcase REIs as a promising platform for the study of many-body physics, with potential applications in quantum sensing and quantum simulations. In general, our findings deepen the understanding for a disordered quantum system and offer valuable insights for development of quantum applications.</p