Physics
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Item UNDERSTANDING NANOSCALE PHYSICS BY ADVANCEMENT OF NOVEL NITROGEN-VACANCY CENTERS BASED SCANNING PROBES AND SIMULATIONS(2024) Zhang, Huayu; Ouyang, Min; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis consists of two parts centering on understanding nanoscale material physics by technology development and simulation. In the first part, we briefly introduce the properties and applications of NV centers, then we present a cost-effective method for fabricating and characterizing a new class of durable, highly integrated scanning quantum sensors using both fiber-based and cantilever-based NV center scanning probes using nanodiamonds. The nitrogen-vacancy (NV) center is a photostable fluorescent atomic defect in diamond, exhibiting unique magneto-optic effects. Its Hamiltonian is sensitive to the local magnetic, electric field, temperature, and strain, making NV centers ideal for atomic-scale quantum sensing and various scientific and technological applications.The fiber-based NV center scanning probe is compatible with any tuning fork-based scanning probe microscope and supports multiple operational modes, including near-field excitation with far-field detection, far-field excitation with near-field detection, near-field excitation with near-field detection and far-field excitation with far-field detection. These diverse functional modes enable high-sensitivity and high-resolution quantum imaging and sensing applications, such as magnetic field imaging, electric field imaging, and thermal imaging at the nanoscale. The cantilever-based NV center scanning probe is suitable for use with any commercial cantilever-based scanning probe microscopes and it allows highly integrated microwave manipulation through metal coating. Additionally, we have developed a methodology to accurately determine the orientation of NV centers beneath the probe, establishing a foundation for utilizing these unique NV scanning probes for quantum sensing. Compared to existing fabrication methods, our method is reproducible, low-cost, and robust, offering significant advancements in NV center scanning probe technology. For the second part of thesis, we also investigated the phonon modes and manipulation methods of phonons in nanoparticles. The phonons are quasiparticles representing quantized sound waves in materials, they play a crucial role in defining the thermal, electrical, optical, and mechanical properties of materials. At the nanoscale, phonons exhibit unique behaviors due to quantum confinement effects and interactions with other quasiparticles. We investigated phonons in dumbbell-shaped Au-CdSe nanoparticles using time domain, frequency domain and eigenfrequency analyses, demonstrated an all-optical phonon mode manipulation method and explored potential chiral phonon modes in chiral gold nanocubes through FEM simulations. This research opens new avenues for manipulating material properties and enhancing device performance.Item QUANTUM APPLICATIONS, PARALLEL OPERATIONS, AND NOISE CHARACTERIZATION ON A TRAPPED ION QUANTUM COMPUTER(2024) Zhu, Yingyue; Linke, Norbert M.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum computing holds vast potential for solving classically hard problems ranging from optimization to simulations critical in material science research and drug discovery. While large-scale fault-tolerant quantum computers capable of these tasks are yet to come, small and noisy prototypes have been demonstrated on several candidate platforms. Among these, trapped-ion qubits have been at the forefront of quantum computing hardware because of their long coherence times, high-fidelity quantum gates, and all-to-all connectivity. This dissertation investigates new methods for efficient quantum computing at the interface of quantum information theory and trapped-ion experiments, and advances both the control of physical trapped-ion hardware and the characterization of their decoherence processes. We present a number of proof-of-principle experiments for early quantum applications on a trapped-ion quantum computer (TIQC). First, we experimentally show that the results of the Quantum Approximate Optimization Algorithm (QAOA)---a method to solve graph combinatorial optimization problems by applying multiple rounds of variational circuits---improve with deeper circuits for multiple graph-theoretic problems on several arbitrary graphs. We also demonstrate a modified version of QAOA that allows sampling of all optimal solutions with predetermined weights. Additionally, we implement the real-time evolution of a one-dimensional scattering process and demonstrate a more efficient and accurate method to extract the phase shift, forming a tentative first step toward the goal of lattice quantum chromodynamics (QCD) simulation. Furthermore, we demonstrate two Bell-type nonlocal games that can be used to prove quantum computational advantage as well as offer a set of practical and scalable benchmarks for quantum computers in the pre-fault-tolerant regime. Our experimental results indicate that the performance of quantum strategies for the non-local games exceeds basic classical bounds, and is on the cusp of demonstrating quantum advantage against more complicated classical strategies. We propose and demonstrate a high-fidelity and resource-efficient scheme for driving simultaneous entangling gates on different sets of orthogonal motional modes of a trapped-ion chain. We show the advantage of parallel operation with a simple digital quantum simulation where parallel implementation improves the overall fidelity significantly. We test and improve the performance of an ancilla-assisted protocol for learning Pauli noise in Clifford gates on a TIQC. With N ancilla, Pauli noise in an N-qubit Clifford gate can be learned with a sample size linear to N. We also design and demonstrate a way to improve the protocol's performance by reducing ancilla noise in post-processing.Item Proton Energization during Magnetic Reconnection in Macroscale Systems(2024) Yin, Zhiyu; Drake, James; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic reconnection is a widespread process in plasma physics that is crucial for the rapid release of magnetic energy and is believed to be a key factor in generating non-thermal particles in space and various astrophysical systems. In this dissertation, a set of equations are developed that extend the macroscale magnetic reconnection simulation model kglobal to include particle ions. The extension from earlier versions of kglobal, which included only particle electrons, requires the inclusion of the inertia of particle ions in the fluid momentum equation. The new equations will facilitate the exploration of the simultaneous non-thermal energization of ions and electrons during magnetic reconnection in macroscale systems. Numerical tests of the propagation of Alfvén waves and the growth of firehose modes in a plasma with anisotropic electron and ion pressure are presented to benchmark the new model. The results of simulations of magnetic reconnection accompanied by electron and proton heating and energization in a macroscale system are presented. Both species form extended powerlaw distributions that extend nearly three decades in energy. The primary drive mechanism for the production of these nonthermal particles is Fermi reflection within evolving and coalescing magnetic flux ropes. While the powerlaw indices of the two species are comparable, the protons overall gain more energy than electrons and their powerlaw extends to higher energy. The power laws roll into a hot thermal distribution at low energy with the transition energy occurring at lower energy for electrons compared with protons. A strong guide field diminishes the production of non-thermal particles by reducing the Fermi drive mechanism. In solar flares, proton power laws should extend down to 10's of keV, far below the energies that can be directly probed via gamma-ray emission. Thus, protons should carry much more of the released magnetic energy than expected from direct observations. In Encounter 14 (E14), the Parker Solar Probe encountered a reconnection event in the heliospheric current sheet (HCS) that revealed strong ion energization with power law distributions of protons extending to 500keV. Because the energetic particles were streaming sunward from an x-line that was anti-sunward of PSP, the reconnection source of the energetic ions was unambiguous. Using upstream parameters based on the data observed by PSP, we simulate the dynamics of reconnection applying kglobal and analyze the resulting spectra of energetic electrons and protons. Power law distributions extending nearly three decades in energy develop with proton energies extending to 500keV, consistent with observations. The significance of these results for particle energization in the HCS will be discussed.Item UNDERSTANDING AND CONTROLLING NANOSCALE CHIRALITY: MATERIALS SYNTHESIS, CHARACTERIZATIONS, MODELING AND APPLICATIONS(2024) Liu, Hanyu; Ouyang, Min; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chirality, the property of objects possessing non-superimposable mirror images, initially identified and explored in organic and biological molecules, has gained growing interests in the realm of inorganic nanomaterials due to its foreseeable applications in the fields such as Enantiochemistry, Nanophotonics, Spintronics. In the first segment of this dissertation, we demonstrate a bottom-up synthetic strategy to induce chirality in plasmonic nanoparticles and hybrid plasmonic-semiconductor nanostructures. Subsequently, we detail a simplified analytical coupled-oscillators model to facilitate the understanding of plasmonic-chiral coupling and predict various chiroptical responses based on different coupling strengths, validated through finite element method simulations. Furthermore, advancements in characterizing nanoscale chirality with high spatial resolution at the single nanoparticle level are explored using a novel polarization-dependent optical atomic force microscopy technique, overcoming resolution limits in far field measurements. Finally, we demonstrate the employment of nanoscale chirality to induce spin polarization and enable unique nanoscale chiral Floquet engineering.Item Characterization of Gap-Engineered Josephson Junctions and Gate Fidelities for a Superconducting Qubit(2024) Steffen, Zachary Andrew; Kollár, Alicia; Palmer, Benjamin S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum computing promises applications in physics, cryptography, material science, pharmaceuticals, and a wide range of other science. Superconducting qubits offer a possible platform for developing a quantum computer. To perform useful quantum computations, the coherence and control of present day superconducting qubits must be greatly improved. In this dissertation, I present two main results to improve the performance of transmon qubits. For the first project, I fabricated and characterized the coherence of transmon devices with asymmetric superconducting gaps. Previous models suggested that devices with asymmetric superconducting gaps on either side of the Josephson junction can be designed to be less subject to loss from quasiparticle tunneling. To gap-engineer the Josephson junctions, I used Ti metal to proximitize and lower the superconducting gap of the Al counter-electrode. Unfortunately, the energy relaxation time constant for an Al/AlOx/Al/Ti 3D transmon I fabricated and tested was T1 = 1 us, over two orders of magnitude shorter than the measured T1 = 134 us of an Al/AlOx/Al 3D transmon with Al capacitor pads and the measured T1 = 143 us of an Al/AlOx/Al 3D transmon with Ta capacitor pads. DC IV measurements of proximitized Josephson junctions showed a reduced superconducting gap, demonstrating that the gap-engineering in the Al/Ti layer was successful. However, these same IV measurements showed greatly increased excess current for voltage biases below the superconducting gap compared to my Al/AlOx/Al junctions. This suggests the addition of Ti caused the junction quality to worsen, potentially being a source of tunneling loss in the transmon devices. Intentionally adding oxygen disorder between the Al and Ti layers reduced the proximity effect and subgap current in DC measurements while increasing the relaxation time of a 3D transmon to T1 = 32 us. Additionally, I designed an Al/AlOx/Al SQUID device to perform DC IV measurements of junctions with tunable total critical current. In a single junction, subgap tunneling features can be due to the critical current interacting with the environment, subgap quasiparticle processes, or other sources. Reducing the critical current allows these features to be differentiated and more accurately measure the effects from quasiparticle tunneling alone. Characterizing this device showed subgap tunneling features consistent with inelastic Cooper pair tunneling and quasiparticle transport via multiple Andreev reflection in a low transparency junction. This measurement technique could be used to further study gap-engineered junctions. For the second project, I characterized an Al/AlOx/Al 2D transmon device with Ta features and performed high-fidelity single qubit gates. First, I used error amplifying pulse sequences to fine-tune the qubit gate pulses. I evaluated the gate error with randomized benchmarking. I characterized gates with Gaussian and cosine shaped pulses at a variety of pulse lengths. Analyzing the pulse envelopes in the frequency domain and directly measuring leakage to the transmon's second excited state revealed that leakage from driving higher qubit transitions was a major source of gate error. Next, I characterized gates using a pulse shape designed by a physics informed neural network designed by Güngördü and Kestner and found improved gate error for 16~ns pulses achieving an average error per gate of (3.36 +/- 0.03) x 10^-4. This outperformed errors of (5.54 +/- 0.24) x10^-4 for a cosine shaped pulse and (3.93 +/- 0.12) x10^-4 for a Gaussian shaped pulse of the same length. Further optimization of the pulse using predistortion or leakage reduction strategies may yield even greater performance.Item TOPOLOGICAL PHOTONICS: NESTED FREQUENCY COMBS AND EDGE MODE TAPERING(2024) Flower, Christopher James; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Topological photonics has emerged in recent years as a powerful paradigm for the designof photonic devices with novel functionalities. These systems exhibit chiral or helical edge states that are confined to the boundary and are remarkably robust against certain defects and imperfections. While several applications of topological photonics have been demonstrated, such as robust optical delay lines, quantum optical interfaces, lasers, waveguides, and routers, these have largely been proof-of-principle demonstrations. In this dissertation, we present the design and generation of the first topological frequency comb. While on-chip generation of optical frequency combs using nonlinear ring resonators has led to numerous applications of combs in recent years, they have predominantly relied on the use of single-ring resonators. Here, we combine the fields of linear topological photonics and frequency microcombs and experimentally demonstrate the first frequency comb of a new class in an array of hundreds of ring resonators. Through high-resolution spectrum analysis and out-of- plane imaging we confirm the unique nested spectral structure of the comb, as well as the confinement of the parametrically generated light. Additionally, we present a theoretical study of a new kind of valley-Hall topological photonic crystal that utilizes a position dependent perturbation (or “mass-term”) to manipulate the width of the topological edge modes. We show that this approach, due to the inherent topological robustness of the system, can result in dramatic changes in mode width over short distances with minimal losses. Additionally, by using a topological edge mode as a waveguide mode, we decouple the number of supported modes from the waveguide width, circumventing challenges faced by more conventional waveguide tapers.Item Radiative Plasmas in Pulsar Magnetospheres(2024) Chernoglazov, Alexander; Philippov, Alexander; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pulsars are highly magnetized rotating neutron stars known for their periodic bursts of radio emission. Decades of astronomical observations revealed that pulsars produce non-thermal radiation in all energy bands, from radio to gamma rays, covering more than 20 decades in photon energy. Modern theories consider strongly magnetized relativistic electron-positron plasmas to be the source of the observed emission. In my Thesis, I investigate physical processes that can be responsible for plasma production and the observed high-energy emission in the wide range of photon energies, from eV to TeV. In the first Chapter of my Thesis, I investigate relativistic magnetic reconnection with strong synchrotron cooling using three-dimensional particle-in-cell kinetic plasma simulations. I characterize the spectrum of accelerated particles and emitted synchrotron photons for varying strengths of synchrotron cooling. I show that the cutoff energy of the synchrotron spectrum can significantly exceed the theoretical limit of 16 MeV if the plasma magnetization parameter exceeds the radiation reaction limit. Additionally, I demonstrate that a small fraction of ions present in the current sheet can be accelerated to the highest energies, making relativistic radiative reconnection a promising mechanism for the acceleration of high-energy cosmic rays. In the second Chapter, I present the first multi-dimensional simulations of the QED pair production discharge that occurs in the polar region of the neutron star. This process is believed to be the primary source of the pair plasma in pulsar magnetospheres and also the source of the radio emission. In this work, I focus on the self-consistently emerging synchronization of the discharges in different parts of the polar region. I find that pair discharges on neighboring magnetic field lines synchronize on a scale comparable to the height of the pair production region. I also demonstrate that the popular “spark” model of pair discharges is incompatible with the universally adopted force-free magnetospheric model: intermittent discharges fill the entire polar region that allows pair production, leaving no space for discharge-free regions. My findings disprove the key assumption of the spark model about the existence of distinct discharge columns. In the third Chapter, I demonstrate how the key findings of two previous chapters can provide a self-consistent explanation of the recently discovered very-high-energy, reaching 20TeV, pulsed emission in Vela pulsar. Motivated by the results of recent global simulations of pulsar magnetospheres, I propose that this radiation is produced in the magnetospheric current sheet undergoing radiative relativistic reconnection. I show that high-energy synchrotron photons emitted by reconnection-accelerated particles efficiently produce electron-positron pairs. The density of secondary pairs exceeds the supply from the polar cap and results in a self-regulated plasma magnetization parameter of $\sim 10^7$. Electrons and positrons accelerate to Lorentz factors comparable to $\sim 10^7$ and emit the observed GeV radiation via the synchrotron process and ~10 TeV photons by Compton scattering of the soft synchrotron photons emitted by secondary pairs. My model self-consistently accounts for the ratio of the gamma-ray and TeV luminosities.Item Unifying Searches for New Physics with Precision Measurements of the W Boson Mass(2024) Sathyan, Deepak; Agashe, Kaustubh; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Standard Model (SM) of particle physics has been extremely successful in describing the interactions of electromagnetic, weak nuclear, and strong nuclear forces. Yet, there are both unexplained phenomena and experimentally observed tensions with the SM, motivating searches for new physics (NP). Collider experiments typically perform two kinds of analyses: direct searches for new physics and precision measurements of SM observables. For example, experimental collaborations use collider data to search for NP particles like the heavy superpartners of the SM particles, whose observation would be clear evidence of supersymmetry (SUSY). These direct searches often consider kinematic regions where the SM background is small. This strategy is unable to probe regions of the NP parameter space where the SM background is dominant. The same collaborations also measure the masses of SM particles, which not only serve as consistency tests of the SM, but can also probe effects of NP. In 2022, the Collider Detector at Fermilab (CDF) collaboration published the most precise measurement of the $W$ boson mass: $m_W$ = 80433.5 $\pm$ 9.4 MeV. This measurement is in $7\sigma$ significance tension with the SM prediction via the electroweak (EW) fit, $m_W^{\rm pred.}$ = 80354 $\pm$ 7 MeV. Many extensions to the SM can affect the prediction of $m_W$ with indirect effects of heavy NP. However, in 2023, the ATLAS re-measurement of the $W$ boson mass, $m_W$ = 80360 $\pm$ 16 MeV, was found to be consistent with the SM prediction. Both collaborations found a high-precision agreement between the measured kinematic distributions and the SM prediction of the kinematic distributions for their corresponding extracted $m_W$. We propose using the precision measurements of $m_W$ to directly probe NP contributing to the same final state used to measure $m_W$: a single charged lepton $\ell$ and missing transverse energy $\met$. This strategy is independent of modifying the EW fit, which tests indirect effects of NP on the predicted value of $m_W$. Any NP producing $\ell+\met$ which modifies the kinematic distributions used to extract $m_W$ can be probed with this method. With this strategy, since these distributions are used to search for NP while measuring $m_W$, a simultaneous fit of NP and SM parameters is required, thus unifying searches and measurements. This simultaneous fitting can induce a bias in the measured $m_W$, but only to a limited extent for our considered models. We consider three categories of NP which can be probed: ($i$) modified decay of $W$ bosons; ($ii$) modified production of $W$ bosons; and ($iii$) $\ell+\met$ scenarios without an on-shell $W$ boson. We also show that models whose signals extend beyond the kinematic region used to measure $m_W$ can be probed in an intermediate kinematic region. Our results highlight that new physics can still be discovered at the LHC, including light new physics, via SM precision measurements. Additionally, anticipated improvements in precision SM measurements at the High Luminosity LHC further enables new searches for physics Beyond the Standard Model (BSM).Item EXCURSION IN THE QUANTUM LOSS LANDSCAPE: LEARNING, GENERATING AND SIMULATING IN THE QUANTUM WORLD(2024) Rad, Ali; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Statistical learning is emerging as a new paradigm in science. This has ignited interestwithin our inherently quantum world in exploring quantum machines for their advantages in learning, generating, and predicting various aspects of our universe by processing both quantum and classical data. In parallel, the pursuit of scalable science through physical simulations using both digital and analog quantum computers is rising on the horizon. In the first part, we investigate how physics can help classical Artificial Intelligence (AI) by studying hybrid classical-quantum algorithms. We focus on quantum generative models and address challenges like barren plateaus during the training of quantum machines. We further examine the generalization capabilities of quantum machine learning models, phase transitions in the over-parameterized regime using random matrix theory, and their effective behavior approximated by Gaussian processes. In the second part, we explore how AI can benefit physics. We demonstrate how classical Machine Learning (ML) models can assist in state recognition in qubit systems within solid-state devices. Additionally, we show how ML-inspired optimization methods can enhance the efficiency of digital quantum simulations with ion-trap setups Finally, in the third part, we focus on how physics can help physics by using quantum systems to simulate other quantum systems. We propose native fermionic analog quantum systems with fermion-spin systems in silicon to explore non-perturbative phenomena in quantum field theory, offering early applications for lattice gauge theory models.Item Constraining Higgs Boson Self-coupling with VHH Production and Combination, and Searching for Wgamma Resonance using the CMS Detector at the LHC(2024) Lai, Yihui; Palmer, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since the discovery of the Higgs boson (H) with a mass of 125 GeV by the ATLAS and CMS collaborations at the CERN LHC in 2012, the focus of the particle physics community has expanded to include precise measurements of its properties, and so far the measurements align with the Standard Model (SM) predictions. Of particular interest among these properties is the Higgs boson self-coupling, which can be directly probed by measuring the cross section for the production of Higgs boson pair (HH). This thesis presents three analyses using proton- proton collision data at \sqrt{s} = 13TeV with an integrated luminosity of 138 fb−1: a search for SM Higgs boson pair production with one associated vector boson (VHH), a combination of H measurements and HH searches, and a search for a new particle decaying to a W boson and a photon (\gamma). The VHH search focuses on Higgs bosons decaying to bottom quarks, and vector boson decaying to electrons, muons, neutrinos, or hadrons, with a novel background estimation approach. An observed (expected) upper limit on the VHH production cross section is set at 294 (124) times the SM predicted value. The combination of H measurements and HH searches aims to constrain the Higgs self-coupling with the best possible precision. The search for Wgamma resonance focuses on leptonic W boson decays, achieving the world’s best sensitivity for this resonance in the mass ranges considered.