Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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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 Quantum Dots in Photonic Crystals for Hybrid Integrated Silicon Photonics(2024) Rahaman, Mohammad Habibur; Waks, Edo Prof.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum dots are excellent sources of on-demand single photons and can function as stable quantum memories. Additionally, advanced fabrication techniques of III-V materials and various hybrid integration methods make quantum dots an ideal candidate for integration into fiber- and silicon-based photonic circuits. However, efficiently extracting and integrating quantum dot emissions into fiber- and silicon-based photonic circuits, particularly with high efficiency and low power consumption, presents a continued challenge. This dissertation addresses this challenge by utilizing photonic crystals to couple quantum dot emissions into fiber- and silicon-based photonic circuits. In this dissertation, we first demonstrate an efficient fiber-coupled single photon source at the telecom C-band using InAs/InP quantum dots coupled to a nanobeam photonic crystal. The tapered nanobeam structure facilitates directional emission that is mode-matched to a lensed fiber, resulting in a collection efficiency of up to 65% from the nanobeam to a single-mode fiber. Using this approach, we demonstrate a bright single photon source with a 575 ± 5 Kcps count rate. Additionally, we observe a single photon purity of 0.015 ± 0.03 and Hong-Ou Mandel interference from emitted photons with a visibility of 0.84 ± 0.06. A high-quality factor photonic crystal cavity is needed to further improve the brightness of the single-photon source through Purcell enhancement. However, photonic crystal cavities often suffer from low-quality factors due to fabrication imperfections that create surface states and optical absorption. To address this challenge, we employed atomic layer deposition-based surface passivation of the InP photonic crystal nanobeam cavities to improve the quality factor. We demonstrated 140% higher quality factors by applying a coating of Al2O3 via atomic layer deposition to terminate dangling bonds and reduce surface absorption. Additionally, changing the deposition thickness enabled precise tuning of the cavity mode wavelength without compromising the quality factor. This Al2O3 atomic layer deposition approach holds great promise for optimizing nanobeam cavities, which are well-suited for integration with a wide range of photonic applications. Finally, we propose a hybrid Si-GaAs photonic crystal cavity design that operates at telecom wavelengths and can be fabricated without the need for careful alignment. The hybrid cavity consists of a patterned silicon waveguide that is coupled to a wider GaAs slab featuring InAs quantum dots. We show that by changing the width of the silicon cavity waveguide, we can engineer hybrid modes and control the degree of coupling to the active material in the GaAs slab. This provides the ability to tune the cavity quality factor while balancing the device’s optical gain and nonlinearity. With this design, we demonstrate cavity mode confinement in the GaAs slab without directly patterning it, enabling strong interaction with the embedded quantum dots for applications such as low-power-threshold lasing and optical bistability (156 nW and 18.1 µW, respectively). In addition to classical applications, this cavity is promising for alignment-free, large-scale integration of single photon sources in a silicon chip.Item Designing Optical Quantum Computing with Minimal Hardware(2023) Shi, Yu; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Photons, while indispensable for quantum communication and metrology, fall short due to limited photon-photon interactions, thus suboptimal for quantum computing. This thesis explores the use of an atom-photon interface to foster entanglement between photons, thereby facilitating more scalable optical quantum computing with reduced resource demands. I initially discuss the deterministic generation of multi-dimensional cluster states via an atom-photon interface and time-delay feedback. These cluster states are essential resources for fault-tolerant measurement-based quantum computing. A diagrammatic method is introduced to derive tensor networks of highly entangled states, thereby aiding in the simulation of states produced from sequential photons. Subsequently, I investigate the implementation of the optical quantum Fourier transform through the interface, which facilitates photon-photon interactions and significantly reduces the dependence on linear optical devices. In addition to devising techniques, I introduce an error metric for non-trace-preserving quantum operations that aligns with fault-tolerant quantum computing theory. This metric is beneficial for assessing errors across various quantum platforms and post-selected protocols. Overall, this research advances the field of optical quantum information processing, proposing scalable, practical solutions for quantum computing. Concurrently, it pioneers novel error metrics, providing a promising benchmarking and optimization strategy for robust quantum information processing.Item INTEGRATION OF CLASSICAL/NONCLASSICAL OPTICAL NONLINEARITIES WITH PHOTONIC CIRCUITS(2023) Buyukkaya, Mustafa A; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent developments in nanofabrication have opened opportunities for strong light-matter interactions that can enhance optical nonlinearities, both classical and non-classical, for applications such as optical computing, quantum communication, and quantum computing. However, the challenge lies in integrating these optical nonlinearities efficiently and practically with fiber-based and silicon-based photonic circuits on a large scale and at low power. In this thesis, we aimed to achieve this integration of classical and quantum optical nonlinearities with fiber-based and silicon-based photonic circuits.For classical optical applications, optical bistability is a well-researched nonlinear optical phenomenon that has hysteresis in the output light intensity, resulting from two stable electromagnetic states. This can be utilized in various applications such as optical switches, memories, and differential amplifiers. However, integrating these applications on a large scale requires low-power optical nonlinearity, fast modulation speeds, and photonic designs with small footprints that are compatible with fiber optics or silicon photonic circuits. Thermo-optic devices are an effective means of producing optical bistability through thermally induced refractive index changes caused by optical absorption. The materials used must have high absorption coefficients and strong thermo-optic effects to realize low-power optical bistability. For this purpose, we choose high-density semiconductor quantum dots as the material platform and engineer nanobeam photonic crystal structures that can efficiently be coupled to an optical fiber while achieving low-power thermo-optical bistability. For applications that require non-classical nonlinearities such as quantum communication and quantum computing, single photons are promising carriers of quantum information due to their ability to propagate over long distances in optical fibers with extremely low loss. However, the efficient coupling of single photons to optical fibers is crucial for the successful transmission of quantum information. Semiconductor quantum dots that emit around telecom wavelengths have emerged as a popular choice for single photon sources due to their ability to produce bright and indistinguishable single photons, and travel long distances in fiber optics. Here, we present our advances in integrating telecom wavelength single photons from semiconductor quantum dots to optical fibers to realize efficient fiber-integrated on-demand single photon sources at telecom wavelengths. Finally, using the same methodology, we demonstrate the integration of these quantum dots with CMOS foundry-made silicon photonic circuits. The foundry chip is designed to individually tune quantum dots using the quantum confined stark shift with localized electric fields at different sections of the chip. This feature could potentially enable the tuning of multiple quantum emitters for large-scale integration of single photon sources for on-chip quantum information processing.Item STRONG OPTICAL NONLINEARITY AND SPIN CONTROL WITH PHOTONIC STRUCTURES(2023) singh, harjot; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Photons are excellent carriers of quantum information. Since they hardly interact with each other, they can maintain their quantum state over long distances. However, this poses a challenge if one wishes to create entanglement between the degrees of freedom of photons. Creating entangled states of photons is essential for quantum information processing with photons. One way to create interactions between photons is to create strong interactions between two level atoms and modes of electromagnetic radiation. This can be achieved by coupling optical transitions of two-level atoms with modes of optical cavities or waveguides. The nonlinearity of a two-level atom then effectively mediates interactions between two (or more) photons. However, due to a fundamental time-bandwidth limit, a two-level atom cannot enable arbitrary quantum operations on the states of two photons. In this thesis, we study theoretically the problem of splitting two indistinguishable photons to distinguishable output modes with a two-level atom. Due to the time-bandwidth limit, the achieved splitting efficiency is fundamentally limited to 82% using just a two-level atom. We show that a linear optical unitary transformation on the output modes of the two-level atom can exceed this limit. Via optimization of the input two photon wavefunction and the parameters of the linear optical unitary, we calculated a splitting efficiency of 92%. For experimental realization of strong atom-light interactions, we used InGaAs quantum dots coupled to a bullseye cavity. Bullseye cavities are promising towards realization of efficient collection of light due to their near Gaussian far field emission. We demonstrated a strong interaction between the quantum dot exciton and the Bullseye cavity mode, quantified by a Cooperativity of ~8. This high cooperativity with a low-quality factor cavity can be attributed to the charge stabilization enabled by the diode heterostructure of the quantum dot samples we used. Finally, we focus on the electron spin ground states of a negatively charged InGaAs quantum dot. The electron spin interacts with the nuclear spins of the In, Ga and As. We measure the spectrum of this interaction using all optical dynamical decoupling pulse sequencies. This work lays a path forward to realizing efficient and coherent spin-photon interfaces with InGaAs quantum dots.Item QUANTUM MODEM AND ROUTER FOR THE QUANTUM INTERNET(2022) Saha, Uday; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Like the internet, the quantum internet can change the world by connecting quantum computers over long distances. This connectivity can revolutionize different industries like banking, healthcare, and data analytics that utilize quantum computing and simulations. Additionally, it would enable us to connect multiple small quantum computers into powerful distributed quantum computers that can solve problems of significant societal impact. Despite the rise of excellent quantum computers, we don't yet have the core technologies to connect them. This is because modems and routers we use to connect classical computers don't work for quantum information. They destroy the coherence and entanglement of quantum information, which is vital for connecting quantum computers. In my thesis, I developed a quantum modem and router that can connect quantum computers and create a scalable quantum network. I have conceived the modem and router for the trapped ion quantum computers, the most promising quantum computing platform. However, we can easily use my developed concepts to connect different quantum computing platforms. I accomplished a quantum modem that provides an interface between a quantum computer and a fiber-optic network by generating telecommunication photons from the computer. I used a two-stage quantum frequency conversion scheme to realize the quantum modem. By calculating the second-order correlation function, I experimentally verified single-photon characteristics retained after the frequency conversion process. Telecommunication photons generated by the quantum modem can carry quantum information from ions over long distances. This will allow a long-distance quantum network to realize the quantum internet. On the other hand, I implemented a quantum router with photonic integrated circuits. Utilizing the thermo-optic property, I route photons from a trapped barium ion into different output ports of the quantum router in a programmable manner. This router can connect multiple quantum computers on-demand and in a scalable way. We are the first group to demonstrate a quantum modem and router working together with a quantum computer. This demonstration could lead to a scalable quantum network where photons from different quantum computers can be interfered with a programmable photonic chip to herald entanglement. Additionally, I developed visible photonic circuits for quantum data centers. In a quantum data center, there can be multiple trapped ion quantum computers that need to be connected. For this purpose, I designed a photonic circuit on a thin-film lithium niobate platform that can entangle two trapped ion quantum computers with >99% fidelity. Apart from achieving high fidelity entanglement, the circuit can achieve any polarization-independent power splitting ratio, which can have extensive use in integrated photonic technology. Finally, I invented a multiplexing scheme by which we can send quantum information from multiple quantum computers using a single fiber-optic cable. That will increase the channel capacity, where multiple quantum computers can communicate through the same channel. By encoding quantum information into the different wavelengths of photons, I devised my idea of multiplexing quantum information. These results will enable us to achieve a programmable and scalable network of quantum computers to increase the capability of quantum computing and quantum simulations and lead us to the future quantum internet.Item Quantum Light Generation from Bound Excitons in ZnSe(2022) Karasahin, Aziz; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum light sources and spin-based qubits are essential building blocks for on-chip scalable quantum computation and information processing. To achieve scalability, information-storing qubits should exhibit long coherence times. These qubits should also be efficiently interfaced with information-carrying single photons. Semiconductors are not only able to host such qubits and single photon sources but also, they offer a platform to interface them with the help of photonic structures. Hence, optically active solid-state qubits such as quantum dots, crystal defects and color centers have been extensively studied to date in various semiconductors. However, we still lack a suitable platform to satisfy all the requirements needed to realize a scalable quantum technology. Impurities in epitaxially grown ZnSe are particularly promising single photon sources and qubit candidates due to the direct bandgap of the material and potential for isotopic purification to achieve nuclear spin-zero background. These impurities possess impurity bound electrons that can serve as spin-qubit. They also form impurity-bound excitons that can generate single photons. Various impurities have been studied in ZnSe, but only F impurities have been isolated as single emitters to date. Despite the great potential suggested by previous results, there are many impurities waiting to be explored for their quantum capabilities. In this thesis, we study isolated Cl impurities in ZnSe for their photon emission and spin properties. We utilize a ZnMgSe/ZnSe/ZnMgSe quantum well to increase the binding energies bound excitons and to better separate donor bound exciton emission from the free excitons. In the PL spectrum, we observe narrow emission lines around 440 nm, which are originated from the single bound excitons. We calculate the average binding energy as 15 meV (at least 2 times higher than bulk values) and inhomogeneous broadening as 6 meV. We confirm the single photon emission by observing clear photon antibunching in the second order autocorrelation measurements. The time-resolved photoluminescence measurements show short radiative lifetimes of 192 ps. Our results demonstrate first time isolation of donor impurities in an unstructured ZnSe and provide complete characterization of radiative properties single Cl bound excitons. The bound electron of a donor impurity atom can serve as a spin qubit. We verify that the presence of ground state electron of the Cl donor complex by observing two electron satellite emission. We also characterize the Zeeman splitting of the exciton transitions by performing polarization-resolved magnetic spectroscopy on the single emitters. We also discover the presence of single biexcitons bound to Cl impurities. We demonstrate a radiative cascade from the decay of bound biexcitons. The emission exhibits both single photon statistics and clear temporal correlations revealing the time–ordering of the cascade. Finally, we discuss the design of nanophotonic cavities in the ZnSe platform. We develop a nanofabrication recipe to create suspended photonic crystal cavities. Then, we optically characterize the fabricated cavities. The results presented in this thesis provide the first complete study of single Cl impurities in ZnSe. Based on the results discussed, single Cl impurities in ZnSe manifest themselves as promising quantum light sources and appealing solid-state qubit candidates.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 SPECTROSCOPY OF TWO LEVEL DEFECTS & QUASIPARTICLES IN SUPERCONDUCTING RESONATORS(2021) Kohler, Timothy; Osborn, Kevin D; Anlage, Steven; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducting films are inherently limited by losses due to two-level system (TLS) defects within the amorphous oxide layers surrounding them and from quasiparticles in the film. In this thesis I will discuss novel theoretical and experimental methods toward understanding superconducting resonator loss from deleterious surface TLS defects as well as a loss transition from non-equilibrium quasiparticles in granular TiN. I will show using finite element solver software that a resonator with submicron linewidth and linespacing can be used to better characterize and simulate surface TLS as part of a circuit QED system. I have observed individual surface TLS and found coupling values in the range of g/2π =50 kHz -280 kHz with a maximum dipole moment pz-max = 4.5 Debye (.93 eÅ). I have found in in simulation of experiment that over 80% of the strongly coupled TLS reside within 50 nm of the corner between the Metal-Substrate (MS) and Substrate-Air (SA) interface. Additionally I have studied a loss transition from non-equilibrium quasiparticles in TiN films. These films exhibit an anomalous loss dependence on substrate treatment and film thickness. The films of interest are ones grown thin on oxidized substrates, which exhibit an order of magnitude decrease in internal quality factor (Qi) relative to either thicker ˝films or films grown without the oxidized substrate. These films additionally exhibit a grain size on average of 7.5 nm, a higher inhomogeneous gap, a transition to lower stress and a preference for the [111] crystal growth. The temperature dependence of the conductivity is fit and a factor of two difference in quasiparticle lifetime is found between the two films where the thinner film has a shorter lifetime. A two gap quasiparticle trapping model is fit to the temperature dependent loss data. The data is consistent with a model where non-equilibrium quasi-particles are trapped in low gapped grains on the inside of the films. From these works and others presented in my thesis the understanding of TLSs on surfaces and non-equilibrium quasiparticles in TiN has improved. This will help illuminate some of the most important absorption mechanisms plaguing superconducting qubits and resonators.Item An Integrated Photonic Platform For Quantum Information Processing(2021) Dutta, Subhojit; Waks, Edo EW; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum photonics provides a powerful toolbox with vast applications ranging from quantum simulation, photonic information processing, all optical universal quantum computation, secure quantum internet as well as quantum enhanced sensing. Many of these applications require the integration of several complex optical elements and material systems which pose a challenge to scalability. It is essential to integrate linear and non-linear photonics on a chip to tackle this issue leading to more compact, high bandwidth devices. In this thesis we demonstrate a pathway to achieving several components in the quantum photonic toolbox on the same integrated photonic platform. We focus particularly on two of the more nontrivial components, a single photon source and an integrated quantum light-matter interface. We address the problem of a scalable, chip integrated, fast single photon source, by using atomically thin layers of 2D materials interfaced with plasmonic waveguides. We further embark on the challenge of creating a new material system by integrating rare earth ions with the emerging commercial platform of thin film lithium niobate on insulator. Rare earth ions have found widespread use in classical and quantum information processing. However, these are traditionally doped in bulk crystals which hinder their scalability. We demonstrate an integrated photonic interface for rare earth ions in thin film lithium niobate that preserves the optical and coherence properties of the ions. This combination of rare earth ions with the chip-scale active interface of thin film lithium niobate opens a plethora of opportunities for compact optoelectronic devices. As an immediate application we demonstrate an integrated optical quantum memory with a rare earth atomic ensemble in the thin film. The new light matter interface in thin film lithium niobate acts as a key enabler in an already rich optical platform representing a significant advancement in the field of integrated quantum photonics.