A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item INTEGRATION OF ATOMIC EMITTERS IN PHOTONIC PLATFORMS FOR CLASSICAL AND QUANTUM INFORMATION APPLICATIONS(2024) Zhao, Yuqi; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Integrated photonics provide a powerful toolbox for a wide range of classical and nonclassical applications. In addition to their scalability and significantly lower power consumption, integrated photonic structures enable new design knobs and functionalities that are inaccessible in their bulk counterparts.Solid-state atomic emitters, such as rare-earth ions (REIs) and quantum dots, serve as excellent sources for scalable quantum memories and exhibit strong nonlinear resonant absorption. Integrating atomic emitters with photonic devices enhances light-matter interactions, unlocking new opportunities for advanced optoelectronic systems in both classical and quantum regimes. This thesis tackles two main challenges utilizing the integration of photonic devices and atomic emitters: (1) developing scalable quantum network components, and (2) creating low-power nonlinear components for classical on-chip optical signal processing. Specifically, we focus on a platform of rare-earth ion doped thin-film lithium niobate (TFLN), leveraging the ions’ stable optical transitions with thin-film lithium niobate’s rich toolbox of high-performance photonics. We first demonstrate an integrated atomic frequency comb (AFC) memory in this platform, an essential component for quantum networks. This memory exhibits a broad storage bandwidth exceeding 100 MHz and optical storage time as long as 250 ns. As the first demonstrated integrated AFC memory, it features a significantly enhanced optical confinement compared to the previously demonstrated REI memories based on ion-diffused waveguides, leading to a three orders of magnitude reduction in optical power required for a coherent control. Next, we develop reconfigurable narrowband spectral filters using ring resonators in the REI:TFLN platform. These on-chip optical filters, with linewidths in the MHz and kHz range and extinction ratios of 13 dB – 20 dB, are crucial for reducing background noise in quantum frequency conversion. By spectral hole burning at 100 mK temperature in a critical-coupled resonance mode, we achieve bandpass filters with a linewidth of as narrow as 681 kHz. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental component for optical signal processing and optical memories on-a-chip. We also demonstrate picowatt-threshold power nonlinearity in TFLN, utilizing the strong resonant nonlinear absorption induced by three-level REIs and enhanced by TFLN ring resonators. This work presents three distinct nonlinear transmission functions by adjusting the ring’s coupling strength. The lifetime of the nonlinear transmission is measured to be ~3 ms, determined by the ion’s third-level lifetime. Finally, we propose a novel nonlinear device design based on a different material system and mechanism - an ultrathin optical limiter with low threshold intensity (0.45 kW/cm2), utilizing a 500 nm-thick GaAs zone plate embedded with InAs quantum dots. The optical limiting performance, enabled by the zone plate’s nonlinear focusing behavior, is investigated using FDTD simulations. We also explore the effects of the zone plate’s thickness and radius on its optical limiting performance.Item Engineering a Control System for a Logical Qubit-Scale Trapped Ion Quantum Computer(2023) Risinger, Andrew Russ; Monroe, Christopher R; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum computing is a promising field for continuing to develop new computing capabilities, both in its own right and for continued gains as Moore's Law growth ends.Trapped ion quantum computing is a leading technology in the field of quantum computing, as it combines the important characteristics of high fidelity operations, individual addressing, and long coherence times. However, quantum computers are still in their infancy; the first quantum computers to have more than a handful of quantum bits (qubits) are less than a decade old. As research groups push the boundaries of the number of qubits in a system, they are consistently running into engineering obstacles preventing them from achieving their goals. There is effectively a knowledge gap between the physicists who have the capability to push the field of quantum computing forward, and the engineers who can design the large-scale & reliable systems that enable pushing those envelopes. This thesis is an attempt to bridge that gap by framing trapped ion quantum computing in a manner accessible to engineers, as well as improving on the state-of-the-art in quantum computer digital and RF control systems. We also consider some of the practical and theoretical engineering challenges that arise when developing a leading-edge trapped ion quantum computer capable of demonstrating error-corrected logical qubits, using trapped Ytterbium-171 qubits.There are many fundamental quantum operations that quantum information theory assumes, yet which are quite complicated to implement in reality. First, we address the time cost of rearranging a chain of ions after a scrambling collision with background gases. Then we consider a gate waveform generator that reduces programming time while supporting conditional quantum gates. Next, we discuss the development of a digital control system custom-designed for quantum computing and quantum networking applications. Finally, we demonstrate experimental results of the waveform generator executing novel gate schemes on a chain of trapped ions. These building blocks together will unlock new capabilities in the field of trapped ion quantum computers.Item A study of Quantum ALgorithms with Ion-trap Quantum Computers(2021) Zhu, Daiwei; Monroe, Christopher R; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum computing will be one of the most incredible breakthroughs in science and technology of our generation. Although the ultimate goal of building quantum computers that hold thousands of error-corrected qubits is still beyond our reach, we have made substantial progress. Compared with the first-generation prototypes, holding a few qubits with gate errors of several percent, the latest generation systems can apply more than a hundred gates (with fidelities above $99\%$) to tens of fully connected qubits. This thesis focuses on the applications of such state-of-the-art ion-trap quantum computers. The latest generation ion-trap quantum computers have become complex enough that automation is necessary for optimal operations. We present a full-stack automation scheme implemented on a system at the University of Maryland. With the automation scheme, the system can operate without human interference for a few days. With automation, such systems can efficiently demonstrate different categories of applications. We present the experimental study of several hybrid algorithms aiming for generation modeling and efficient quantum state preparation. We also present a gate-based digital quantum simulation with the trotterization method. Our result accurately reproduced all the features expected from running the algorithms. Verifying quantum computations with classical simulation is getting increasingly challenging as quantum computers evolve. We present two approaches to validate quantum computations. First, we demonstrate a method based on random measurement for comparing the results from different quantum computers. Our comparison captures the similarities between quantum computers made with the same technology. We then present experimental works in verifying quantum advantage classically with interactive protocols. We show that our results, at scale with real-time interaction, can demonstrate quantum advantages.Item CONSTRUCTION, OPTIMIZATION, AND APPLICATIONS OF A SMALL TRAPPED-ION QUANTUM COMPUTER(2019) Landsman, Kevin Antony; Monroe, Christopher; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A large-scale quantum computer will have the ability to solve many computational problems beyond the capabilities of today's most powerful computers. Significant efforts to build such a computer are underway, many of which are small prototypes that are believed to be extensible to larger systems. Such systems, like the one in this thesis built off of 171Yb+ ions, are enticing scientific endeavors for their potential to inform the production of large-scale systems, as well as the interesting experiments they can perform. In this work, experimental research is presented on both topics: scalability as well as compelling computations. The first half of this thesis discusses building and optimizing a quantum computer to have high-fidelity qubit operations. An experimental architecture that allows for individual addressing and individual detection of qubits is presented alongside a discussion of errors and error-reduction. We derive the coherent manipulation of qubits using Raman lasers for rotational gates and the criteria necessary for multi-qubit entangling gates. Methods for efficiently fulfilling these criteria are presented with experimental data. Lastly, we consider coherence-related properties of the system necessary to perform these operations and how they can be experimentally improved. The second half of the thesis features three experimental applications of the quantum computer: quantifying quantum scrambling, applying a quantum error correction code, and measuring Renyi entropy. Quantum scrambling is the coherent delocalization of information through a quantum system and is notably difficult to quantify experimentally. We present an efficient scheme to measure it using quantum teleportation. Quantum error correction is a set of techniques for mitigating the effect of imperfect operations performed on a quantum computer, and we demonstrate some of these techniques in order to fault-tolerantly prepare a logical qubit. Lastly, \renyi entropy is an information theoretic quantity that can be used to directly quantify the amount of entanglement in a system. We present a method for measuring it efficiently using a quantum gate known as a Fredkin gate.Item On Mapping Electron Clouds with Force Microscopy(2012) Wright, Charles Alan; Solares, Santiago D.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)At its core, this is a story about electrons. Electrons drive the interactions of matter at the nanoscale, so an understanding of electron behavior offers significant insight into the behavior of nanoscale materials. Atomic force microscopy (AFM) has demonstrated great success as a tool for probing matter at the nanoscale, and recent reports suggest that it may even be capable of mapping electron clouds on atomic surfaces. The most recent of these claims came in 2004, when Hembacher et al. [Science 305] observed subatomic features while imaging a graphite surface with a tungsten tip using higher-harmonics frequency modulation AFM (FM-AFM). The authors' interpretation of these features as the footprint of the electron density at the tungsten tip's apex atom has been met with much skepticism. But despite the potential significance of the results, a detailed theoretical study has not been performed. In this work, a computational method based in density functional theory (DFT) is developed in order to simulate the imaging process and draw fundamental conclusions regarding the feasibility of subatomic imaging with higher harmonics FM-AFM. The application of this method to the tungsten/graphite system reveals that the bonding lobes of increased charge density are in fact present at the tungsten tip's apex atom and that the corresponding higher harmonics images can exhibit subatomic features similar to those observed experimentally. We further show that the filtering process used to experimentally measure the harmonics does not introduce imaging artifacts but that harmonics averaging is not an appropriate method for enhancing contrast. We then suggest an alternate approach: the individual mapping of the first two harmonics, which are expected to dominate the contrast under the experimental conditions studied. Finally, we demonstrate the important role played by the surface atom used to probe the AFM tip. We find that a small, non-reactive atom is necessary for resolving subatomic features. Most importantly, we show that the observed features are not a direct reflection of the electron density at the AFM tip's front atom. Instead, they represent a measure of the bonding stiffness between the tip's front atom and the atoms in the layer above.