Physics

Permanent URI for this collectionhttps://digital.lib.washington.edu/handle/1773/4956

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Discovering plasticity rules for learning and resilience in neural circuits
(2026-04-20) Bell, David G; Fairhall, Adrienne L
While modern supervised and reinforcement learning algorithms can train neural networksto solve a wide range of tasks, the brain often operates in data-sparse regimes where such extensive supervision is unavailable. This thesis argues that the brain succeeds in these settings by leveraging inductive biases about the tasks it is likely to encounter. These biases are embedded in initial connectivity, cell-type structure, and critically, in synaptic plasticity rules. Here, we investigate how unsupervised synaptic plasticity can shape neural circuits prior to extensive behavioral experience. In the first part of this thesis, we study plasticity in the zebra finch song system. In collaboration with researchers at the California Institute of Technology, we examine the restoration of singing behavior following viral perturbation of nucleus HVC, a premotor region essential for song production. Adult male zebra finches transiently lose song after viral manipulation but recover within approximately two weeks. Strikingly, birds prevented from practicing during early recovery subsequently require less practice to regain song, suggesting that recovery is partially unsupervised. We model this process using several unsupervised plasticity mechanisms, including spike timing-dependent and homeostatic plasticity. While standard homeostatic rules restore regular spiking activity in a network model of HVC, they fail to reproduce experimentally observed synaptic reorganization. We therefore propose a local population-level homeostatic rule that recruits previously silent neurons, accounting for both activity recovery and synaptic changes. In the second chapter, we employ meta-learning, a technique by which biologically plausible learning rules are learned via a supervised procedure, to discover biologically plausible plasticity rules that organize robust sequential dynamics in HVC-like networks. In this framework, candidate unsupervised plasticity rules are optimized by a supervised outer loop to maximize a task objective. Starting from disordered connectivity, the learned rules reliably self-organize networks into sequence-generating circuits resembling those observed in vivo. Analysis of resulting rules reveals that plasticity on recurrent excitatory synapses generalizes Oja’s rule, replacing the classical Hebbian term with a spike timing-dependent component. We further show that learned plasticity rules can compensate for continual synaptic turnover and that learned inhibitory plasticity enhances the precision and robustness of sequential dynamics. In the final chapter, we apply meta-learning to the self-organization of neural inte- grators—circuits that generate long timescales via carefully tuned structure to maintain representations of sensory inputs. Such integrators underlie functions including head di- rection coding and oculomotor control. We hypothesize that unsupervised plasticity can shape these circuits from weak structural priors. Using meta-learning, we identify plasticity rules that reliably organize integration dynamics without requiring previously hypothesized anti-Hebbian mechanisms. Instead, the learned rules rely heavily on three-factor plasticity. In a simplified model, we demonstrate how such three-factor mechanisms can tune integrator circuitry and stabilize persistent dynamics.
The Krypton String Test Cryostat: Characterizing Surface Charge-Induced Dead Layers on High Purity Germanium Detectors
(2026-04-20) Nave, Christian John; Detwiler, Jason
The LEGEND experiment is searching for neutrinoless double beta decay, a proposed nuclear decay that, if detected, can help explain some of the most fundamental questions in physics. LEGEND is using germanium detectors, enriched in the double beta-decaying isotope $^{76}$Ge to search for this rare process. Background reduction is crucially important to rare event searches like LEGEND and understanding these germanium detectors is a necessary guard against misunderstood backgrounds. This work describes the findings of a germanium test-stand, the Krypton String Test Cryostat (KrSTC). KrSTC uses low energy ($< 40$~keV) photons and electrons to probe the passivated surface of these detectors. Analysis of the KrSTC data shows different low energy spectra depending on detector and cryostat conditions and suggests a dead layer model investigated through simulation work. This dead layer model, accounting for surface charge collecting on the passivated surface, can be a useful tool in understanding LEGEND backgrounds in the search for neutrinoless double beta decay.
Electronic Properties and Measurement Techniques of 2D Heterostructures
(2026-04-20) Lester, Eric; Cobden, David H.
For the last fifty years, two-dimensional (2D) systems have stood out as model platforms for exploring electronic phenomena. Reduced dimensionality, electrostatic tunability, and enhanced electron-electron interactions enable effects not seen in bulk 3D conductors. Following the isolation of graphene, research on van der Waals (vdW) materials has exploded, revealing a vast landscape of correlated and topological physics. The ability to stack dissimilar materials into heterostructures has enabled control of the carrier density and displacement field of 2D samples while also opening the door for less conventional measurements. The limitless combinations and permutations of 2D heterostructures make for a rich and rewarding field of study. This thesis investigates the niche of 2D semimetals, specifically the electronic transport properties of the hybrid semimetal graphene-WTe2. While graphene hosts massless Dirac fermions that have high mobility and weak intrinsic interactions, WTe2 is dominated by correlated physics. This includesa putative excitonic insulator ground state, a likely unconventional superconducting state, and topological phenomena like the quantum spin Hall effect. By combining these complementary materials, we discover details of interlayer coupling in 2D semimetals as well as measurement and analysis techniques for other 2D heterostructures. I will begin this thesis by presenting historical context for the study of 2D systems, including techniques for the fabrication and measurement of 2D vdW heterostructures. I will then introduce both graphene and WTe2. Through careful analysis of the transport properties of the resulting hybrid, I will recover parameters of physical interest for both constituent materials. I will also present a sensing technique utilizing a remote graphene sensor to probe the chemical potential of the hybrid, shedding further light on its character. It will be shown that the properties of the hybrid can be largely understood from those of the individual monolayers with modifications due to interlayer tunneling and electron-electron interactions. Next, I will present my studies of the 2D dielectrics hexagonal boron nitride (BN) and Bi2SeO5. This work will show a hitherto unreported inverted temperature dependence in the breakdown of BN as well as the potential uses of Bi2SeO5 in 2D devices. Finally, I will discuss the problem of sample rotation and how it can be used to further characterize the electronic properties of matter. I will present my work towards the development of an in-situ two-axis rotator for use in a dilution refrigerator, including some preliminary testing results.
Numerically Exact Configuration Interaction at Quadrillion-Determinant Scale
(2026-04-20) Shayit, Agam; Li, Xiaosong
Quantum chemistry is the field of study that uses quantum mechanics to make chemical predictions. The chemistry of the systems of interest (atoms and molecules) is faithfully described by the electronic Dirac equation. The equation describes the relativistic dynamics of the electrons in a molecule subject to electric forces from the nuclei and other electrons. For virtually all systems of practical interest, the equation is impossible to solve by means of exact analytical techniques. Thus, the use of numerical methods becomes inevitable. The most accurate such method is configuration interaction (CI), which yields the exact formal solution of the Dirac equation in the complete-basis-set limit. Due to the factorial growth in the resource requirements of CI calculations, they have historically been applied to the smallest of chemical systems. In this dissertation, we introduce categorical compression of CI vectors within the small tensor product distributed active space (STP-DAS) CI framework. We demonstrate its capabilities by conducting a CI calculation consisting of over one quadrillion determinants, the largest CI calculation to date. We then explore strategies to accelerate STP-DAS CI calculations by adapting the STP-DAS σ-build step to graphics-processing units (GPUs). Finally, we use the developed framework to conduct previously untenable benchmark studies of two state-of-the-art quantum chemical methods, namely coupled cluster (CC) and density matrix renormalization group (DMRG).
TopmetalSe: Development of a CMOS Imager for Selena
(2026-02-05) Ni, Xiaochen; Chavarria, Alvaro
The Selena experiment couples an amorphous selenium (aSe) ionization target to a custom complementary metal–oxide–semiconductor (CMOS) pixel array to create an imaging detector for next-generation neutrino physics. Hybrid aSe/CMOS devices combine the high-Z stopping power and room-temperature operation of aSe with the fine-pitch, low-noise readout of modern CMOS sensors, enabling precise reconstruction of ionization tracks for advanced background discrimination. This work presents the comprehensive simulated modeling and hardware development of Selena as a candidate search for neutrinoless double-beta decay in 82Se. A detailed simulation framework is developed to model double-beta events in a Selena detector, incorporating charge generation, drift, and collection processes, along with dedicated reconstruction techniques for event topology and energy response. In parallel, the Topmetal-Se prototype, fabricated in the open-source SkyWater 130 nm CMOS process, demonstrates direct charge sensing, low noise, and fine pixel pitch ionization track imaging when coupled to amorphous selenium. Together, these developments establish Selena as a viable platform for high-resolution ionization imaging and rare-event detection.
Exploiting Geometric Constraints for Parameter Quantification in Balanced Steady-State Free Precession MRI
(2026-02-05) Dong, Yiyun; Taulu, Samu
Balanced Steady-State Free Precession (bSSFP) is a magnetic resonance imaging (MRI) sequence well known for its high signal-to-noise ratio efficiency and T2/T1 contrast that suffers from banding artifacts caused by its dependence on static magnetic field inhomogeneities. While phase-cycling is a common remedy to banding, the complex, biphasic nature of the bSSFP signal has historically made its rich phase information difficult to exploit for quantitative tissue and field mapping. This work overcomes this challenge by introducing novel techniques for parameter quantification in bSSFP MRI, and exploiting the geometric constraints of its unique signal profile, which forms a parameterized ellipse in the complex plane. First, a parameter quantification method using four phase-cycled bSSFP acquisitions is established via the geometric property of the signal ellipse's cross-point. An auxiliary circle with a one-to-one correspondence to the bSSFP signal ellipse is considered, facilitating the elucidation of ESM parameters and leading to an analytic ''ellipse unlocking'' method. This cross-point formalism allows for the direct extraction of ellipse parameters, from which quantitative maps of the transverse relaxation time T2 and field-related phase components are generated. Building on the identified information redundancy within the signal ellipse, the second project develops an analytical solution requiring only three phase-cycled acquisitions, further exploiting the inherent constraints. This Direct Analytic Solution (DAS) is derived from a linear system during the ellipse-to-circle transformation, formally reducing the data redundancy. However, analysis reveals that DAS is highly noise- and banding-sensitive and fails in specific scenarios. This investigation indicates the limitations of a purely analytical approach with minimal data, thereby motivating the search for a more robust method. To overcome these limitations, a robust numerical method was developed using three acquisitions. This approach imposes a new geometric constraint—that the signal ellipse passes through the origin—to create a simplified model. A regularized joint optimization procedure then solves the resulting nonlinear least-squares problem, yielding artifact-free images and quantitative B0 maps. A practical challenge common to all these quantification methods is the effective, phase-preserving combination of solutions from multi-channel coil data. An accurate, phase-preserving solution combination strategy is therefore critical for robust quantification performance. The Optimal Weighted Average (OWA) method is implemented for this purpose, which uses regional variance weighting to combine these solutions effectively. The optimality of the weighting scheme is confirmed through mathematical derivation, and efficacy in reducing noise is demonstrated with experiments. Collectively, this thesis demonstrates how geometric constraints can be leveraged for robust and highly efficient parameter quantification from phase-cycled bSSFP MRI, achieving accurate results with a minimal number of acquisitions.
Quasi-Background-Free Neutrinoless Double-Beta Decay Searches with LEGEND: Statistical Methods and Cryogenic SiPM Characterization
(2026-02-05) Borden, Samuel; Detwiler, Jason
As an electrically neutral and massive fermion, the neutrino is the only Standard Model particle whose Lagrangian mass term could include a Majorana term. The nature of the neutrino mass is exciting because it is inherently Beyond the Standard Model: neutrino oscillations show that the neutrino has a mass and that it violates conservation of individual lepton numbers, in direct contradiction to predictions of the Standard Model. The most sensitive probe of the Majorana nature of the neutrino is the search for a hypothetical second-order weak decay called neutrinoless double-beta decay. Observation of this decay would prove that conservation of total baryon minus lepton number is violated and that the neutrino has a Majorana mass term. The Large Enriched Germanium Experiment for Neutrinoless-double beta Decay (LEGEND) collaboration is searching for the neutrinoless double-beta decay of 76Ge by deploying an array of highly enriched germanium detectors inside of a liquid argon cryostat. In order to fully cover the allowed parameter space for inverted ordered light Majorana neutrinos, the LEGEND collaboration is pursuing a phased approach. The already-built LEGEND-200 experiment aims for a half-life discovery sensitivity of 1E+27 years using 200 kg of enriched detectors, and the proposed tonne-scale follow-on experiment LEGEND-1000 aims for a discovery sensitivity of 1E+28 years. The success of these experiments rests in their great ability to reduce external background through techniques such as pulse shape discrimination and liquid argon scintillation anti-coincidence vetoing. The LEGEND-200 experiment has completed its first year of searching for neutrinoless double-beta decay with an accumulated exposure of 61 kg·yr. This thesis reports the first frequentist statistical analysis performed on data from the LEGEND experiment. A new Python-based framework, freqfit, is introduced to robustly and rapidly produce frequentist statistical inference on unbinned data. Using this framework, no evidence of neutrinoless double-beta decay is found (p = 0.1), and a lower limit on the half-life is placed at T_1/2 > 5 × 1E+25 years (90% confidence level). The background index for the group of detectors mainly comprised of the geometry to be used in LEGEND-1000 is computed to be 5^+3_-2 × 1E−4 counts/keV/kg/yr. A frequentist joint analysis incorporating LEGEND-200 data and the data from two recent 76Ge experiments, the MAJORANA DEMONSTRATOR (MJD) and Germanium Detector Array (GERDA), is also presented. The combined analysis also observes no evidence for a signal (p = 0.29) and results in the strongest lower limit, and highest half-life sensitivity, on the half-life in 76 Ge to-date: T_1/2 > 1.9 × 1E+26 years (90% confidence level). This half-life limit can be converted into a limit of the effective Majorana mass of the neutrino, assuming mediation by a light Majorana neutrino and a range of phenomenological nuclear matrix elements, and yields mββ < 75 − 200 meV (90% confidence level). With fewer than one expected count due to accidental radioactive background near the Q-value of neutrinoless double-beta decay, LEGEND-1000 will be a quasi-background-free experiment. The behavior of the frequentist profile likelihood ratio treatment for statistical inference on unbinned data from quasi-background-free experiments is investigated. It is found that test statistic distributions in this regime deviate from the asymptotic form predicted by Wilks’ theorem — it is required to generate pseudo-experiments for these statistical analyses. The coverage is computed for ensembles of pseudo-experiments generated with a known Poisson-distributed background index. We show that the profile likelihood ratio treatment guarantees coverage very close to the nominal value for these ensembles. In order for LEGEND-1000 to reach its nominal background index goal of 1E−5 counts/keV/kg/yr, it is necessary that its liquid argon detector collects as much scintillation light in its silicon photomultiplier (SiPM) readout as possible. The detection efficiency of the liquid argon system is directly proportional to the photon detection efficiency (PDE) of the SiPMs. The PDE is a well-known quantity reported by the manufacturer, at room temperature; however, LEGEND operates its SiPMs at cryogenic temperature. This work describes the operation and results of a cryogenic SiPM characterization test stand built at the University of Washington. We report that two SiPMs exhibit a nearly 20% drop in their PDE at liquid nitrogen temperature relative to their room temperature values. This drop was measured for the green wavelengths (562 nm) of light that match the optical emission spectra of light collection technology for LEGEND-1000. This drop in the PDE represents an important input for forecasting the background index for LEGEND-1000 in order to guarantee that it reaches its discovery sensitivity goal.
Towards integration of AI-enabled rationally designed de novo protein into solid-state chips
(2026-02-05) Pfeffer, Akira Mihara; Gundlach, Jens H
For my dissertation in the UW Nanopore Laboratory, we developed methods aimed at creating permanent, robust, and integrated de novo protein sensors within an electronic SiNx chip. We identified critical design parameters needed for ideal de novo nanopore sensors with atomistic engineerability and reproducibility. This work integrates a wide array of de novo protein, with varying levels of stability, selectivity, and sequencing applicability, culminating in demonstrating ssDNA translocation using designed protein integrated into SiNx. Chapter 1 provides an introduction into nanopore sequencing, biological and solid-state nanopore systems, protein design through deep learning diffusion, and surface chemistry. Chapter 2 summarizes the work done in creating custom 3D printed microfluidics, control measurements for the hybrid system, CPD and ICR applications for surface coating analysis. Chapter 3 discusses the selection of protein from the early iterations of our design campaigns and identifies the primary parameters that are needed for stable insertions. Chapter 4 details improvements build off of the preliminary work in chapter 3 to demonstrate the translocation of small charged molecules of PAA and ssDNA. These results will provide insights towards rational design for hybrid sequencing platforms with stable integration of AI-enabled designer protein with solid-state platforms. With time, this work paves the way for atomically reproducible, purposefully engineered, and highly parallelized multi-omic hybrid nanopore system.
The CAGE Scanner: Development of a Novel Surface Event Rejection Technique in High Purity Germanium Detectors
(2026-02-05) Song, Grace; Detwiler, Jason
The search for neutrinoless double beta (0νββ) decay aims to answer far-reaching questions about thenature of the neutrino mass, violation of lepton number conservation in the Standard Model, and the matterantimatter asymmetry of the universe. The LEGEND experiment is pursuing this rare event search with the use of semiconductor detectors constructed of high-purity germanium, enriched in the candidate isotope 76Ge. Precise understanding of detector response to radiation is crucial to the success of this experimental effort. This work describes the operation and improvement of the Collimated Alphas, Gammas, and Electrons (CAGE) Scanner, a test cryostat dedicated to the study of surface event behavior in different germanium detector geometries. Analysis of CAGE scans led to the development of a new pulse shape parameter, Early Charge (EQ), which is then applied to a subset of LEGEND-200 data to investigate its impact on LEGEND-200 backgrounds.
New Developments in Fermionic Many-Body Physics within Time-Dependent Density Functional Theory
(2026-02-05) Kafker, Matthew; Bulgac, Aurel
This thesis documents various recent developments in the physics of systems composedof many strongly interacting fermions, obtained within time-dependent density functional theory, which is a leading microscopic approach for the treatment of the non-equilibrium dynamics of such systems. Various aspects of the problem of nuclear fission are addressed, in particular focusing on the stage of evolution from the outer saddle of the nuclear potential energy surface, through the scission point, and beyond until full separation between the fragments is achieved, for a variety of actinide nuclei. The dynamics of neck rupture during nuclear fission are presented, and it is argued that scission neutrons are expected to be released during this stage. The saddle-to-scission dynamics of nuclei with an odd number of neutrons, and in some cases also an odd number of protons, are presented, which reveal a highly complex nuclear shape evolution and enhanced effects of time-reversal symmetry breaking in such odd systems. The intrinsic spins of the fission fragments are evaluated after scission, and it is found that, with significant probability, the spins are not oriented perpendicular to the fission axis and that their directions in space are correlated. Various aspects of the dynamics of the single-particle occupation numbers, which change in time due to the pairing interaction, are addressed, and these are used to define a notion of complexity for quantum many-body systems. It is also demonstrated that the occupation numbers evolve in time as a non-Markovian stochastic process, both in the case of fission dynamics and also in the case of quantum turbulence and subsequent thermalization of the unitary Fermi gas. Various aspects of the problem of restoring translational invariance of the many- body wave function within density functional theory are presented. Finally, preliminary results are presented concerning the problem of multi-nucleon transfer reactions, treated within time-dependent density functional theory.
Investigating the Electronic Properties of Next Generation Moiré Materials
(2026-02-05) Thompson, Ellis; Yankowitz, Matthew
Since the discovery of monolayer graphene in 2004, van der Waals (vdW) materials have become a mainstay in condensed matter physics. Their layered nature enables us to isolate atomically thin sheets and stack them together with virtually limitless possibility. These designer 2D heterostructures also introduce a new twisting degree of freedom, in which layers are stacked with a rotational offset. The beating pattern formed between their crystalline lattices is known as a moiré superlattice and often leads to new and unexpected properties absent in the parent materials. In this dissertation, I will push the boundaries of this ever-growing parameter space through electronic characterization of various novel moiré materials. The prototypical example of a moiré heterostructure consists of two graphene layers stacked with a small interlayer twist and has been shown to exhibit a wide variety of novel electronic properties driven by strong electron-electron interactions. In the first part of this thesis, we show that adding more layers to this basic framework preserves much of the same physics and in some cases leads to entirely new electronic phenomena. Furthermore, we find that signatures of the moiré superlattice survive into bulk graphite structures, composed of up to ~40 layers of graphene. The second part of this thesis focuses on moiré systems composed of transition metal dichalcogenides, a class of vdW materials composed of alternating layers of transition metal and chalcogen atoms. In particular, twisted molybdenum ditelluride (tMoTe2) was recently found to exhibit the long-sought fractional quantum anomalous Hall (FQAH) effect, a consequence of strong electron interactions and robust band topology. We use scanning tunneling microscopy to image the energy-dependent nanoscale wave function of tMoTe2, illustrating a connection between its microscopic structural properties and band topology.
Exploring Quantum Phenomena in 2D Materials: From Valley Topology and Exciton Selection Rules to Tuning Magnetic Correlations
(2026-02-05) Fernando, Tharindu; Cao, Ting
This thesis presents two independent parts under the common theme of quantum phenomena in 2D materials: Part I introduces a gauge-invariant, quantized interband index for multiband 2D systems and uses it in two applications: A) to analyze valley topology, and B) to derive gauge-invariant exciton selection rules. Part II investigates monolayer Nb₃Cl₈, quantifies magnetic anisotropy, and shows that biaxial strain tunes antiferromagnetic, paramagnetic, and ferromagnetic behavior. Part I, Application A: Interband index and valley topology. We introduce a novel gauge-invariant, quantized interband index in two-dimensional (2D) multiband systems. It provides a bulk topological classification of a submanifold of parameter space (e.g., an electron valley in a Brillouin zone), and therefore overcomes difficulties in characterizing topology of submanifolds. We confirm its topological nature by numerically demonstrating a one-to-one correspondence to the valley Chern number in k·p models (e.g., gapped Dirac fermion model), and the first Chern number in lattice models (e.g., Haldane model). Furthermore, we derive a band-resolved topological charge and demonstrate that it can be used to investigate the nature of edge states due to band inversion in valley systems like multilayer graphene. Part I, Application B: Gauge-invariant optical selection rules for excitons. Excitons are central to the photophysics of 2D semiconductors and photonic devices. Prior circular selection rules for excitons in 2D are successful but gauge-dependent due to assumptions that exclude singular gauge behavior at band edges. By developing a chiral form of the interband index from Application A above, we obtain selection rules that are manifestly gauge-invariant. This framework is directly compatible with numerical workflows used in device modeling, and strengthens the theory of quantum materials, especially two-dimensional semiconductor photophysics. Part II. Strain-tunable magnetism in Nb₃Cl₈. Recent research suggests the possibility of the two-dimensional breathing-Kagome magnet Nb₃Cl₈ hosting a quantum spin liquid state, warranting further study into its magnetic properties. Using ab initio calculations, we show that monolayer Nb₃Cl₈ has short-range antiferromagnetic correlations among Nb₃ trimers with S = 1/2, and becomes magnetically frustrated due to the underlying effective triangular lattice geometry, and is evidenced by a frustration index of f > 1. The high-temperature susceptibility shows a negative Weiss temperature from Monte Carlo calculations. Considering spin-orbit coupling, we investigate the magnetic anisotropy, including anisotropic exchange, single-ion anisotropy and the Dzyaloshinskii–Moriya interaction using the four-state energy mapping formalism. Although the elements have relatively small atomic numbers, the Dzyaloshinskii–Moriya interaction is comparable in magnitude to the anisotropic exchange. Additionally, we show that biaxial strain tunes the short-range correlations between antiferromagnetic, paramagnetic and ferromagnetic. These findings strengthen our understanding of Nb₃Cl₈ and advance its applications in current condensed matter physics and materials science research, including nanoscale mechanical and spintronics applications. Summary. Part I supplies a valley-focused topological index and a gauge-invariant theory of exciton selection rules. Part II elucidates the magnetic anisotropy and tunability of magnetism in monolayer Nb₃Cl₈. Collectively, these findings fulfill the thesis aim of exploring quantum phenomena in 2D materials.
Engineering Photons for Quantum Networks: Temporal-Mode Control, Optimization and Post-Selection Demonstrated in Trapped Ions
(2026-02-05) Thomas, Carl Jacob; Blinov, Boris
Control of the temporal waveform and Fock basis statistics of photons produced during spontaneous emission provides a crucial tool in the establishment of hybrid systems, optimal state transfer, interferometric stability and optimization of entanglement generation protocols for quantum networks. We describe novel methods to generate photons of any temporal waveform from emitters of any lifetime. Our broadly applicable approach has only two requirements for a candidate qudit: (1) control of the phase-parity and (2) modulation of the amplitude of a field coupling a ground state to an excited manifold which produces a photon during relaxation. We describe an approach to find optimal excitation pulse shapes, both numerically and experimentally, by employing variational algorithms to feedback on atomic populations. Additionally, we develop a quantum trajectory theoretical approach to determine emission statistics and establish tools for optimal post-selection to ensure maximum fidelity of photon generation protocols. We situate our work in the context of other prior research on bespoke single photon sources and networking including post-emission pulse shaping, temporal gating and cavity-based methods. In comparison, our free-space process has greater flexibility in producing any waveform, requires less infrastructure and can be readily applied across a wide domain of emitters of any frequency or lifetime. We validate our approach in a singly trapped $^{174}$Yb+ ion and provide experimental results demonstrating photon temporal waveform control.
Improving the Methodology and Instrumentation for On-Scalp Magnetoencephalography (MEG)
(2026-02-05) McPherson, Alexandria Nicole; Taulu, Samu
The recent implementation of novel on-scalp magnetoencephalography (MEG) sensors, specifically optically pumped magnetometers (OPM), has brought about exciting prospects for more precise measurements of natural human brain activity. In order to leverage the full potential of on-scalp systems, certain challenges must be overcome, requiring improvements in both the methodology and instrumentation of these MEG systems. First, traditional signal space separation (SSS) methods for isolating the nano-Tesla (nT) magnetic fields generated form neuronal activity fail when the MEG sensors are on the scalp, as opposed to elevated above the head in a liquid helium Dewar as with traditional, cryogenic MEG systems made of Superconducting Quantum Interference Devices (SQUID). Next, due to the increased proximity of sensors to the brain, on-scalp systems can in principle capture higher spatial frequencies of magnetic signal topology, but current inverse methods may fail with the increased noise that comes with higher frequency components. Finally, the OPM sensors themselves are more sensitive to low-frequency and DC fields than SQUID MEG systems, so new hardware and magnetic field compensation techniques are needed to reduce the remnant magnetic field around the sensor systems. In this dissertation, we first present the novel multi-SSS (mSSS) method, a straightforward mathematical adaptation to the SSS method to account for the on-scalp sensor geometry with various OPM systems. Next, we explore the applications of a matrix regularization method, Foster's Inverse, on SSS to reduce the detrimental impacts of sensor noise on the reconstruction of the internal brain activity, specifically when focusing on higher order components of the magnetic field. Finally, we discuss challenges and current solutions for reducing the remnant magnetic field in the presence of OPM sensors low enough for desired operation and present the coil compensation system designed for use at the Institute for Learning and Brain Sciences (I-LABS) MEG Center, University of Washington. All three of these projects culminate to an advancement of the methodology and instrumentation needed for successful studies of human brain activity with on-scalp MEG systems.
Ytterbium Atom Interferometry Within an Optical Lattice
(2026-02-05) Rahman, Tahiyat; Gupta, Subhadeep
Matterwave interferometers utilizing atoms and optical lattices are subject to theinstabilities and systematics associated with lattice dynamics. We apply a Bloch band approach to atom optics to understand the systematic effects on interferometric phases. In particular, we examine the effects of the coherent quantum passage of atoms accelerating in different lattice bands—also known as Bloch oscillations—in a vertically oriented optical lattice for atom interferometry. This work details observations of multi-path Landau-Zener-Stückelberg-Majorana interference effects, used to measure phases within an optical lattice due to Bloch oscillations. We expound on their relevance towards next-generation atom interferometers employing many Bloch oscillations for improved sensitivity. Optical lattices are also a promising tool for trapped atom interferometry, the matterwave analog for optical interferometry with fiber optics. We demonstrate the first lattice-trapped atom interferometer with a Bose-Einstein condensate. The effect of the choice of band on the visibility of lattice-trapped interferometers has been hitherto unexplored. We show improvements in the visibility of the interferometer fringes by trapping at the so-called “magic depths” of excited bands, where lattice-induced phases are first-order insensitive to variations in lattice depth. We showcase excited-band lattice-trapped interferometers and trapped interferometers for ytterbium for the first time and use them for gravitational sensing.
Functionalized nano-optics for studying optical and mechanical properties of low-dimensional materials
(2026-02-05) Manna, Arnab; Majumdar, Arka
A central challenge in nanoscience is the development of new tools to non-invasively probe the rich physics of low-dimensional materials whose properties are dominated by quantum confinement and surface effects. Conventional characterization techniques often lack the required sensitivity or can perturb the fragile systems they aim to measure. To address this metrological gap, we introduce and develop "functionalized nano-optics" where we transform static photonic crystal cavities into dynamic, reconfigurable instruments by endowing them with active degrees of freedom. Two principal functionalities are explored: in-situ strain tuning for precise spectral control, and spatial mobility, which recasts the nanocavity as a scanning probe tool. The first functionality is demonstrated through the development of a cryo-compatible, in-situ strain-tuning platform for hybrid photonic systems. By integrating a Gallium Phosphide (GaP) photonic crystal cavity with a monolayer of the 2D semiconductor WSe2 on a strain cell, we overcome the common problem of spectral mismatch between emitters and cavities. This system achieves a continuous and reversible tuning of the cavity resonance by 5.5 nm at 5 K, an order of magnitude larger than the cavity linewidth, without degrading the Q-factor. This spectral control is used to modulate the cavity-enhanced exciton photoluminescence, establishing a robust method for systematically studying light-matter interactions in 2D materials. As a complementary approach and followup to hybrid integration, we also explore the use of the van der Waals material itself as the core photonic component. By fabricating waveguides and integrated devices directly from bulk MoS2, we demonstrate deeply subwavelength light confinement, with guided modes in structures as thin as $\lambda$/16. These devices are characterized using a combination of far-field spectroscopy and scattering-type near-field optical microscopy (SNOM) to reveal the properties of highly confined exciton-polaritons. The second core functionality of spatial mobility is realized through the development of a novel scanning optomechanical probe. A high-Q Silicon Nitride nanobeam cavity is integrated onto the tip of a tapered optical fiber, creating a versatile instrument for studying nanomechanical motion. This platform is applied to perform the first direct, high-bandwidth measurements of the thermally-driven mechanical vibrations of suspended DNA bundles, which are prepared via self-assembly on super-hydrophobic micropillar arrays. Additionally, we observe significant optomechanical back-action, first of its kind in a DNA resonator, which manifests as a symmetric frequency softening characteristic of a dissipative-like coupling mechanism. Together, these results establish functionalized nano-optics as a powerful and versatile platform for exploring the complex optical and mechanical properties of diverse low-dimensional material systems.
Designing, Operating and Analyzing: The Quest for Axion Dark Matter with ADMX
(2025-10-02) Guzzetti, Michaela; Rybka, Gray
The Axion Dark Matter eXperiment (ADMX), located at the University of Washington, is a world-renowned experiment and is at the forefront of the hunt for the elusive QCD axion. The QCD axion, originally proposed in the late 1970s to solve a problem in particle physics, was quickly identified as a promising dark matter candidate. While it was initially thought to be impossible to detect due to its extremely weak coupling to standard model particles, over the past couple of decades major advancements in experimental technology have allowed experiments, like ADMX, to become sensitive enough to detect such a particle. In this thesis I will begin by providing context on the history of and evidence for dark matter, the origin of the axion as a theoretical particle, and what makes the axion a good dark matter candidate. Next, I will explain how experiments like ADMX search for axions today as well as situate ADMX in the landscape of existing and proposed axion searches. The remainder of the thesis will cover the details of the most recent data taking run with ADMX, Run 1D. More specifically I will detail the hardware changes made since the last run, the most thorough noise calibration campaign done with ADMX to date, the operations and procedures involved in taking data, as well as the full analysis process. I will finish by reporting results including a 90% confidence level upper limit on axion-photon coupling between 1.088-1.315 GHz, as well as a discussion about the discovery ability of this data set.
The Fractional Quantum Anomalous Hall Effect in Twisted MoTe2
(2025-10-02) Park, Heonjoon; Xu, Xiaodong
Emergent quantum phenomena in two-dimensional moire superlattices, particularly twisted bilayerMoTe2 (tMoTe2), reveals a rich interplay between electronic correlations and band topology. Leveraging device fabrication, optical spectroscopy, local imaging techniques, and low-temperature electrical transport measurements, we have experimentally demonstrated robust integer and fractional quantum anomalous Hall (QAH) states without external magnetic fields. Fractionally quantized Hall conductance plateaus at fillings such as ν = −2/3 and −3/5, accompanied by vanishing longitudinal resistance, provide definitive evidence for fractional Chern insulating (FCI) phases driven purely by electron-electron interactions. Additionally, local visualization of fractional edge states through microwave impedance microscopy has directly confirmed bulk-edge correspondence. Further exploration into higher Chern bands and dissipationless transport has expanded understanding of correlation-driven phenomena, uncovering potential pathways to non-Abelian fractional states relevant for quantum computing. These results collectively establish twisted MoTe2 as an exceptional platform for exploring novel quantum states and highlight their potential for future topological quantum technologies.
Electromechanical Manipulation of Transition Metal Dichalcogenides for Quantum Information Applications
(2025-10-02) Ripin, Adina; Li, Mo
Two-dimensional semiconductors, particularly transition metal dichalcogenides (TMDs), have garnered significant attention in recent years due to their strong light–matter interactions and unique quantum properties, making them promising candidates for quantum information science. These properties are highly tunable using external electric and strain fields. In this thesis, we explore both static and dynamic approaches to tuning TMDs with these fields to develop quantum information devices. In Chapter 1, we introduce the fundamental properties of TMDs and the mechanisms by which electric and strain fields modulate their behavior. We also review prior work in this area and lay the groundwork for our experiments by introducing surface acoustic waves (SAWs) as a method for dynamic field control. Chapter 2 presents our work demonstrating long-range, directional transport of interlayer excitons in bilayer WSe2 using SAWs. The excitons' intrinsic out-of-plane dipole moment allows them to be trapped in the dynamic potential wells created by the SAW electric field and carried along the propagation direction. We examine this transport across varying SAW powers and temperatures, and introduce an ITO capping layer that suppresses in-plane electric fields to prevent exciton dissociation. In Chapter 3, we investigate quantum emitters in bilayer WSe2 and demonstrate energy tunability via out-of-plane electric fields applied through graphite gates, addressing the challenge of inhomogeneous emission energies. We also report the discovery of phonon sidebands arising from coupling between individual excitons and localized interlayer breathing mode phonons. We quantify this exciton–phonon coupling and explore its tunability with electric field, highlighting opportunities for encoding our quantum emitters with fundamental information from the 2D phonon modes. Chapter 4 details our ongoing work using SAW resonators to dynamically manipulate excitonic states in bilayer WSe2. This platform enables spatial modulation of excitonic properties through standing SAW fields and time-averaged tuning of exciton energies and linewidths. We also investigate the possibility of exciton trapping at SAW anti-nodes and discuss its potential for enhancing exciton–exciton interactions, which could open a path toward excitonic quantum simulations. Finally, in Chapter 5, we synthesize the key results of each project, reflect on their implications, and outline future research directions that build on the integration of dynamic and static control in 2D material systems for scalable quantum technologies.}
Dynamical Quantum Phase Transitions, Scrambling and Quantum Simulations of Many-Body Neutrino Systems
(2025-10-02) Bhaskar, Ramya; Savage, Martin J
This dissertation covers three main topics: dynamical quantum phase transitions, the far-from-equilibrium phenomenon of quantum information scrambling, and quantum simulations on both trapped ion and superconducting qubit devices, all in the context of many-body neutrino systems.For the dynamical quantum phase transitions study in the first chapter, the analysis of Loschmidt echos within dense neutrino systems yields insight into a system's initial state requirements needed to achieve a dynamical quantum phase transition (DQPT). Focus is paid to Loschmidt echo crossing distributions, which confirm the presence of two distinct classes of DQPTs in two-flavor neutrino systems. Further analysis reveals a nontrivial dependence on the coupling angle distributions chosen for the two-body interaction term. The results establish two distinct classes of DQPT's in two flavor neutrino systems, verfied via robust statistics. Scrambling as diagnosed by the Out-of-Time-Ordered Correlator (OTOC) has been demonstrated in the Sachdev-Ye-Kitaev model, Transverse Field Ising Model, the transverse axial next nearest neighbor Ising model among many others. However they have yet to be characterized in many-body neutrino systems. Such systems are often modeled as all-to-all connected random-Heisenberg spin chains. In the second chapter, this work demonstrates numerical evidence for scrambling's occurrence in two-flavor many-body neutrino systems. The results demonstrate dynamical quantum phase transitions (DQPTs) potential role as a witness for scrambling in many- body neutrino systems. We see what appears to be discreet modes of scrambling times corresponding to a system's first DQPT occurrence. We attempt to formulate an analytical argument resting on the concept of weak measurement schemes to explain how the DQPTs can serve as a witness for OTOCs in families of random-coupled two-flavor many-body neutrino systems in the forward scattering limit. In the last chapter, quantum circuits for three flavor many body neutrino systems are constructed, for both qubit and qutrit devices. The qubit-based circuits are run on super- conducting qubit devices with heavy-hex connectivity and trapped ion devices with all-to-all connectivity, demonstrating the one of the first quantum simulations of three flavor neutrino systems on two level devices. This work demonstrates a proof of principle for simulating three-flavor neutrino systems on two-level devices, the performance of the qubit circuits on each device, and calculation of physical observables off of the device.

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