A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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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.
HIGH-THROUGHPUT COMBINATORIAL EXPLORATION OF QUANTUM MATERIALS AND DEVICES FOR SPINTRONIC AND TOPOLOGICAL COMPUTING APPLICATIONS
(2024) Park, Jihun; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This doctoral dissertation aims to explore via high-throughput methodologies heavy-element-based quantum materials and devices for spintronic and topological computing applications. It is organized into three parts: (1) the development of spin wave devices based on magnetic insulators for magnon spintronics, (2) the search for spin-triplet superconductors based on Bi alloys (Bi–Ni and Bi–Pd) for superconducting spintronics, and (3) fabricating Josephson junctions based on topological insulators for topological quantum computing.The first part of this dissertation is to develop spin wave devices based on acoustically driven ferromagnetic resonance (ADFMR) using magnetic materials, including yttrium iron garnet (YIG). Spintronic devices based on ferromagnetic metals entail Joule heating and energy loss due to the moving of charge carriers. On the other hand, spin waves can be used without resistive losses. ADFMR is an efficient platform for generating and detecting spin waves via magneto-elastic coupling. While numerous ADFMR studies in ferromagnetic metals have been reported, there is no such report on magnetic insulators. This is due to (1) thermal degradation of piezoelectric substrates (e.g., LiNbO3) during the film crystallization (T > 800°C for YIG), (2) reaction between substrate and film materials, and (3) low ADFMR signals due to intrinsically low magnetostriction. The first part of this thesis attempts to address these issues to achieve YIG ADFMR devices by utilizing rapid thermal annealing to minimize thermal damage, a SiO2 buffer layer to avoid unwanted chemical reactions during crystallization, and a time-gating method for enhanced signal-to-noise ratio. YIG thin films deposited via pulsed laser deposition and crystallized by rapid thermal annealing show decent ferromagnetic behavior. YIG devices show exotic angle- and field-dependent absorption features, indicative of ADFMR. The observed ADFMR pattern is consistent with simulations. This result indicates the first demonstration of ADFMR in magnetic insulators. The second part of this work performs combinatorial synthesis of Bi–Ni and Bi–Pd alloys, which possibly show spin-triplet superconductivity. Such spin-triplet Cooper pairing would allow field-controllable spin polarization in superconductors, enabling superconducting spintronic applications. Furthermore, this type of device possibly provides evidence of superconducting pairing symmetries. In Bi–Ni spread study, Bi3Ni acts as a superconducting host material, where the superconductivity is identified to be varied according to two competing mechanisms: carrier doping and impurity scattering. These results can provide useful guidance in studying superconducting materials with stoichiometric defects. In the Bi–Pd spread films, two superconducting phases are identified with maximum Tc of 3.1 and 3.7 K, corresponding to BiPd and Bi2Pd phases, respectively. With Bi2Pd thin films, spin injection devices are fabricated and characterized. The Bi2Pd spin injection device showed unusual pair-breaking behavior where the superconductivity of Bi2Pd is destroyed significantly by unpolarized current injection. These superconducting spintronic studies demonstrate prompt device exploration via combinatorial methods, efficiently providing insight into spin-triplet superconductivity and its applications. Lastly, this dissertation aims to fabricate topological Josephson junctions based on Yb6/SmB6/Yb6 trilayers. SmB6 is a topological insulator characterized by a robust insulating bulk state and topological surface states. Superconducting proximity effects on the topological surface states can generate topological superconductivity, which can be utilized for fault-tolerant topological quantum computing. This dissertation addresses challenges in fabricating topological Josephson devices. With statistical analysis, device failure mechanisms are identified and addressed, allowing for improved design and fabrication. The improved devices showed Josephson junction-like behavior. The junction characterization revealed that 100% of measured samples showed Josephson features with prominent statistical reproducibility, possibly induced by the Klein effect. The dependence of SmB6 dimensions on the junction behavior is also investigated, along with possible proposed scenarios. These results demonstrate that the combinatorial approaches allow for efficient and prompt investigation of novel quantum materials and devices, facilitating phase diagram studies, materials screening, and stoichiometric controls.
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.
OPTIMIZATION OF PLASMA ASSISTED MOLECULAR BEAM EPITAXY GROWN NbxTi1-xN FOR EPITAXIAL JOSEPHSON JUNCTIONS
(2023) Thomas, Austin Michael; Richardson, Christopher; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis is an investigation into the growth and characterization of NbNx and TiN transition metal nitrides, along with the alloy NbxTi1-xN. These materials are commonly used in many applications ranging from superconducting quantum computing, superconducting conventional computing, high kinetic inductance devices such as single photon detectors, and hard coatings for industrial applications. This thesis will begin with an overview of superconducting quantum computing and superconducting materials, then review the fabrication of Josephson junctions and highlight the need for material improvement. The goal of this work is to grow a superconducting nitride material which can be engineered to lattice match with AlN, the barrier layer in a hypothetical all-nitride, epitaxially grown superconducting quantum computing structure. The alloy NbxTi1-xN is chosen as the superconducting alloy of choice due to the range of lattice constants available, the high critical temperature of these nitrides, and the high quality of material able to be grown using PAMBE. The first aim of this thesis studies the binary transition metal nitrides NbNx and TiN to generate endpoints for various properties of the alloy NbxTi1-xN. This thesis is one of the first investigations of multi-phase growth of ε-NbN and γ-Nb4N3, and demonstrates control over the phase, crystal orientation, superconducting properties, and surface morphology by changing PAMBE growth parameters. The second aim of this thesis demonstrates the growth of NbxTi1-xN and is the first investigation of tunable material properties for this alloy by adjusting the composition. The last aim of this work is the development of a novel annealing scheme used to prepare NbxTi1-xN thin films for Josephson junction integration. The novel annealing scheme ensures excellent surface roughness of NbxTi1-xN thin films, increases the superconducting critical temperature of this alloy from approximately 14 K to 16.8 K, and improves the crystal quality by way of nitrogen incorporation and improvement of the crystal quality. The results from this work will be crucial in developing NbxTi1-xN / AlN / NbxTi1-xN Josephson junctions with smooth, uniform interfaces and low-loss, defect free nitride materials. Additionally, this thesis represents an investigation into the relationship between phases of NbNx and TiN, the role of nitrogen incorporation caused by in-situ annealing, and a useful record of control over this material using PAMBE growth conditions and alloy composition.
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.
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.
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.
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.
MATERIALS CHARACTERIZATION FOR SUB-MICRON SUPERCONDUCTING INTERCONNECTS IN RECIPROCAL QUANTUM LOGIC CIRCUITS
(2022) Garcia, Cougar Alessandro Tomas; Anlage, Steven M; Talanov, Vladimir V; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Today's server based computing consumes a considerable amount of energy.Reciprocal Quantum Logic (RQL) is a classical logic family within superconducting electronics, and is a candidate for energy efficient computing technologies. Similar to the current complimentary metal-oxide semiconductor technologies, RQL interconnects are responsible for dissipating the majority of the energy. The energy dissipated in RQL interconnects comes from finite resistive losses in the superconducting wires and embedded dielectrics at radio frequencies. Therefore, material properties, processing, and performance are critical to understanding the mechanisms of loss and mitigation of power dissipation in RQL interconnects. This dissertation presents work on three aspects of materials characterization of RQL interconnects: implementing a method to deconvolve superconducting and dielectric losses, evaluating losses in three generations of RQL fabrication, and understanding the microscopic physics that determines the performance of RQL interconnects in a temperature and frequency range from 1.5-6 K and 3-12 GHz, respectively. A novel method to accurately deconvolve superconducting and dielectric losses by exploiting their frequency dependence is described. Furthermore, a finite element modeler is used to accurately extract the losses. This method is termed Dispersive Loss Deconvolution. The designed microstrip transmission line resonators are fabricated in a 0.25 $\mu m$ RQL fabrication process composed of Nb wires embedded in Tetraethyl orthosilicate (TEOS) dielectric. The Nb and TEOS losses as a function of microstrip width down to 0.25 $\mu m$ are modeled and measured. The electrical and physical material properties for 3 RQL processes over 5 wafers are evaluated.The electrical properties were evaluated by characterizing resonators in cryogenic dip probes and a dry system with $\pm 10 \: mK$ temperature control. The physical properties were evaluated using Transmission Electron Microscopy and Energy-Dispersive Spectroscopy. Two of the processes use chemical mechanical polishing (CMP) to planarize the Nb wires, and the other using reactive ion etching (RIE) to define Nb wires. At 4.2 K, the Nb loss in the 0.25 $\mu m$ resonators between the 3 processes were surprisingly distinct. The two CMP processes yield Nb losses up to 2 times higher relative to the RIE process, and have a discernible increase in loss by as much as 20\% going from 4 to 0.25 $\mu m$ microstrip widths. For the RIE process, there is no detectable upturn in Nb losses for microstrip widths down to 0.25 $\mu m$. Most notably, the RIE process produced 0.25 $\mu m$ Nb wires with loss reaching the theoretical lower limit of intrinsic surface resistance $R_s = 17 \: \mu \Omega$ at 4.2 K and 10 GHz. The superior RIE process may be linked to the incorporation of thin metal passivation layers protecting the Nb, which prevented Nb oxide from participating in additional loss mechanisms. %The CMP processes had detectable Ar concentrations in the Nb up to 1 $at\%$ most likely due to the trench filling process. For all 3 processes and microstrip widths from 0.25-4 $\mu m$, the TEOS losses had negligible width dependence and varied by as much as $\pm 20\%$. From the electrical characterization at 4.2 K, it was found that the Nb wires are the limiting loss mechanisms in RQL interconnects. As temperature is decreased below 4.2 K, it is well known that Nb loss will exponentially decrease and amorphous dielectrics like TEOS can have loss with a non-monotonic temperature dependence depending on the input power. This offered the opportunity to explore a possible optimum operating temperature to minimize power dissipation by the RQL interconnects. At relatively low input powers, TEOS became the limiting loss mechanism for temperatures below 3 K, and I conclude this can be attributed to losses coming from two-level system tunneling relaxation and resonant absorption processes. %Estimates of peak currents and voltages are used to %A loss spectroscopy method is presented as a tool to The work in this dissertation describes the development of methods to aid in characterization, design, and fabrication of RQL interconnects, and can be extended to potentially other Single Flux Quantum and Quantum Computing technologies.
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.

A. James Clark School of Engineering

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