Defect based qubit systems like the nitrogen vacancy center in diamond have recently emerged as promising candidates for quantum technologies due to their combination of long coherence times, room temperature operation, and robust optical interface. In order to realize many of their proposed applications, defect qubits must be incorporated into scalable devices architectures consisting of photonic, mechanical, or electrical degrees of freedom. Despite much recent progress, many challenges remain for diamond growth and device fabrication. As an alternate approach, we engaged in a search for nitrogen vacancy center analogues in alternative materials with the hope of obtaining a greater degree of control over defect and material properties. Ultimately, we discovered that divacancy-related point defects in all three of the most common forms of silicon carbide- termed 4H, 6H, and 3C- act as analogues to the nitrogen vacancy center in diamond. We chose to focus our research primarily on defects in 3C silicon carbide (termed 'Ky5' defects) because of its availability as a single crystal heteroepitaxial thin film grown on silicon, an advantage that greatly facilitates the fabrication of functional devices. We characterized the spin and optical properties of Ky5 defects in thin film geometries and observed many similarities to the nitrogen vacancy center. We performed the first measurements of spin dynamics in 3C silicon carbide and demonstrate coherent control of defect spins up to room temperature and observe coherence times of up to 22 microseconds. To demonstrate their use in real devices, we designed, fabricated, and characterized photonic crystal cavities in 3C silicon carbide thin films with mode volumes of less than (lambda/n)^3 and Q's as high as 1,500 with integrated Ky5 defects. Additionally, we performed simulations and analysis of the fabricated structures using observed structural imperfections to determine that the Q's are likely limited primarily by the non-vertical structure sidewall angle. Despite the modest Q's of these structures, they can be utilized to generate large local field intensities to enhance optical interactions with Ky5 qubit states within the cavities. We accomplish this by performing cavity enhanced photoluminescence excitation spectroscopy on cavity modes tuned to the zero phonon line of the defects and observe large (as high as 30 times) increases in the luminescence and optically detected magnetic resonance signals originating from the defect states and approximately 2x faster rates of ground state spin initialization. In addition, we use these techniques to probe the photoluminescence dynamics of the Ky5 defects' optical pumping cycle, perform excitation wavelength dependent studies of spin and spectral inhomogeneity, and use the small mode volume and narrowband photoluminescence enhancements of the cavities to observe spectrally distinct subensembles of defects with linewidths as narrow as 25 GHz within the inhomogeneously broadened zero phonon line. Although much is still unknown regarding the properties of these defects, they show great promise as a candidate system for defect qubit based quantum devices and technologies.
Defect based qubit systems like the nitrogen vacancy center in diamond have recently emerged as promising candidates for quantum technologies due to their combination of long coherence times, room temperature operation, and robust optical interface. In order to realize many of their proposed applications, defect qubits must be incorporated into scalable devices architectures consisting of photonic, mechanical, or electrical degrees of freedom. Despite much recent progress, many challenges remain for diamond growth and device fabrication. As an alternate approach, we engaged in a search for nitrogen vacancy center analogues in alternative materials with the hope of obtaining a greater degree of control over defect and material properties. Ultimately, we discovered that divacancy-related point defects in all three of the most common forms of silicon carbide- termed 4H, 6H, and 3C- act as analogues to the nitrogen vacancy center in diamond. We chose to focus our research primarily on defects in 3C silicon carbide (termed 'Ky5' defects) because of its availability as a single crystal heteroepitaxial thin film grown on silicon, an advantage that greatly facilitates the fabrication of functional devices. We characterized the spin and optical properties of Ky5 defects in thin film geometries and observed many similarities to the nitrogen vacancy center. We performed the first measurements of spin dynamics in 3C silicon carbide and demonstrate coherent control of defect spins up to room temperature and observe coherence times of up to 22 microseconds. To demonstrate their use in real devices, we designed, fabricated, and characterized photonic crystal cavities in 3C silicon carbide thin films with mode volumes of less than (lambda/n)^3 and Q's as high as 1,500 with integrated Ky5 defects. Additionally, we performed simulations and analysis of the fabricated structures using observed structural imperfections to determine that the Q's are likely limited primarily by the non-vertical structure sidewall angle. Despite the modest Q's of these structures, they can be utilized to generate large local field intensities to enhance optical interactions with Ky5 qubit states within the cavities. We accomplish this by performing cavity enhanced photoluminescence excitation spectroscopy on cavity modes tuned to the zero phonon line of the defects and observe large (as high as 30 times) increases in the luminescence and optically detected magnetic resonance signals originating from the defect states and approximately 2x faster rates of ground state spin initialization. In addition, we use these techniques to probe the photoluminescence dynamics of the Ky5 defects' optical pumping cycle, perform excitation wavelength dependent studies of spin and spectral inhomogeneity, and use the small mode volume and narrowband photoluminescence enhancements of the cavities to observe spectrally distinct subensembles of defects with linewidths as narrow as 25 GHz within the inhomogeneously broadened zero phonon line. Although much is still unknown regarding the properties of these defects, they show great promise as a candidate system for defect qubit based quantum devices and technologies.
This thesis describes studies on atomic point defects in semiconductors for applications in quantum information. In particular, its focus is on defects whose energy levels reside deep within the bandgap certain semiconductors because these systems can behave like artificial trapped atoms in the solid state, possessing optical addressability, long coherence times, and the ability to be manipulated at microwave frequencies. The most prominent defect in this field of study is the diamond nitrogen-vacancy center, whose functionality is well-known to persist to ambient temperature. The first part of this thesis investigates the persistence of this functionality to temperatures upwards of 600 K. Importantly, these investigations also demonstrate that the diamond nitrogen-vacancy center can be a sensitive thermometer (~10 mK/Hz^(1/2)) over a wide range of temperatures with nanoscale spatial resolution. The latter part of the thesis describes experimental measurements of divacancy defects in silicon carbide, a potential alternative materials host to diamond with established wafer-scale growth and microfabrication capabilities. Single divacancy spins are isolated and coherently controlled for the first time in the 4H polytype of silicon carbide, and the divacancy's coherence is shown to be a remarkably long T2 = 1.2 ms at T = 20 K. The last chapter extends these results by isolating single divacancies in the 3C polytype, experimentally demonstrating a qualitative and quantitative model of their spin and orbital dynamics, and revealing the divacancy's excited state level structure in both the 3C and 4H polytypes using resonant laser techniques. These excited state transitions are shown to posses a structure identical to the diamond nitrogen-vacancy center and a subset are shown to have favorable cycling properties. The location of a clear spin-photon interface has important implications for high-fidelity quantum state readout and photon-based entanglement schemes that use divacancy spins.
Silicon carbide (SiC) is a promising platform for scalable quantum technologies owing to its well-established, wafer-scale industrial processing. SiC also hosts a variety of optically active color centres including the nitrogen vacancy defect that has a spin-triplet ground state. However, strong phonon coupling in the infrared range limits photon extraction from these defects. Here, we use nanophotonic structures, specifically micro-pillar and micro-disk resonators, to enhance optical collection and spin-readout. The micro-pillar geometry yields a 4-fold increase in photon collection, accompanied by a 2.4-fold reduction in spectral noise in optically detected magnetic resonance measurements. Consequently, the magnetic field sensitivity is improved by 24%. The large mode volume of the micro-disk supports resonances spanning 1150-1250 nm, enabling broadband coupling to nitrogen vacancy emission lines. Our results demonstrate that fabrication of scalable photonic structures efficiently improves performance of silicon carbide color centers for integrated quantum light generation and sensing.
Color centers are optically active point defects in semiconductor materials, many of them carrying a spin degree of freedom that couples coherently to emitted photons. This makes them a leading candidate for building quantum computers, networks, and sensors, but realizing that promise depends on the interface between the emitter and light. Color centers must be integrated into nanophotonic structures, such as nanopillars that collect their emission efficiently and optical cavities that couple them to a single mode. This dissertation studies the physics of quantum emitters inside such structures along two complementary threads: the direct cryogenic spectroscopy of color centers integrated into silicon carbide nanostructures, and the study of the physics of many-body cavity quantum electrodynamics (CQED) through simulation on superconducting and trapped-ion quantum hardware. The first thread concerns the nitrogen-vacancy (NV) center in 4H silicon carbide integrated into nanopillars. We show that the sub-wavelength geometry of a nanopillar reshapes the local electromagnetic environment, granting optical access to transitions that selection rules forbid in bulk material: emission from the PL3 divacancy, nearly absent in planar devices, becomes pronounced in nanopillars through a polarization transformation of the excitation field, and the orientation-dependent collection of the pillar resolves the previously ambiguous NV4′ line, establishing nanophotonic integration as a symmetry-sensitive probe of defect structure. We then take up the complementary problem of quantifying how many emitters a device contains, adapting the number-and-brightness technique from fluorescence microscopy to solid-state emitters. This calibration-free method counts dense NV ensembles in silicon carbide nanopillars, a regime beyond the reach of existing optical techniques, and extracts their areal density and formation yield. The second thread turns to many-body cavity QED, in which placing N emitters in a common cavity enhances the light-matter coupling by √N and gives rise to collective effects that are central to strong-coupling applications yet difficult to achieve in solid-state devices based on defects. We realize one such effect on a superconducting circuit, demonstrating photon blockade, the conversion of a coherent drive into a stream of single photons, in a Tavis–Cummings system of N = 1, 2, 3 transmon qubits coupled to a common resonator. To study the open-system dynamics that govern these systems, we digitally simulate the dissipative Tavis–Cummings model on both a superconducting and a trapped-ion processor. Platform-specific compilation and error mitigation yield population dynamics in close agreement with exact solutions, providing a recipe for simulating cavity-emitter systems on contemporary quantum computers. Together, the two threads connect the fabrication and characterization of color-center devices to the many-body cavity-QED physics that will ultimately govern their performance.
Engineering the photonic interface with spins in color centers, such as the $N\phantom{\rule{0}{0ex}}V$ center in diamond or the neutral divacancy in silicon carbide, is crucial for applications in quantum communication and precision sensing. Exploiting SiC's compatibility with conventional thin-film fabrication, the authors use on-chip nanoscale photonic crystal cavities to dramatically enhance the measurement of the spin and optical properties of color-center ensembles. These results bring us a step closer to realizing scaled-up defect-based quantum technologies.
Efficient interfaces between optical photons and quantum bits are fundamental building blocks for quantum networks and large-scale quantum computers. We demonstrate an integrated platform for scalable quantum optics based on color centers coupled to diamond nanodevices. First, we incorporate nitrogen-vacancy (NV) centers in diamond into hybrid nanophotonic crystal cavities. Despite our progress towards coupling the NV to cavity photons, we find that the NV-cavity interface is limited by the imperfections of the color center's properties and its local environment. We approach this challenge first by characterizing and modifying the diamond surface to reduce the environmental noise affecting the NV optical transition. Subsequently, we develop a new approach that makes use of a quantum emitter with intrinsic protection from this noise: the silicon-vacancy (SiV) center in diamond. We demonstrate that SiV centers introduced through ion implantation feature highly coherent optical transitions, even inside nanostructures. By placing SiV centers inside all-diamond photonic crystal cavities, we achieve strong SiV-photon coupling with a cooperativity of greater than 20. Using this platform, we realize a quantum-optical switch controlled by the spin degree of freedom of a single SiV. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two probabilistically entangled SiV centers. Finally, we demonstrate controllable photon-mediated interactions between the spin degrees of freedom of two cavity-coupled SiV centers. When the optical transitions of the two SiV centers are tuned into resonance, the coupling of each SiV to the common cavity mode results in a coherent interaction between the two SiVs, leading to the observation of super- and sub-radiant two-SiV states.
Recently, developments in novel and high-purity materials allow for the presence of a single, solitary crystalline defect to define the electronic, magnetic, and optical functionality of a device. The discrete nature of the active dopant, whose properties are defined by a quantum mechanical description of its structure, enables radically new quantum investigations and applications in these arenas. Finally,there has been significant development in large-scale device engineering due to mature semiconductor manufacturing techniques. The diverse set of photonic device architectures offering light confinement, guiding, and extraction is a prime example. These three paradigms – solitary dopant photonics and optoelectronics (solotronics), quantum science\tand technology, and device engineering – merge in the development of novel quantum photonic devices for the next generation of information processing systems. We present in this thesis a series of investigations of optical nanostructures for single optically active spins in single crystal diamond. Chapter 1 introduces the Nitrogen-Vacancy (NV) color center, summarizes its applications, and motivates the need for their integration into photonic structures. Chapter 2 describes two prototype nanobeam photonic crystal cavities for generating strong light-matter interactions with NV centers. The first device consists of a silicon nitride photonic crystal nanobeam cavity with high quality factor \\(Q \\sim 10^5\\) and small mode volume \\(V \\sim 0.5*(\\lambda/n)^3\\). The second device consists of a monolithic diamond nanobeam cavity fabricated with the focused ion beam (FIB) directly in a single crystal diamond sample. Chapter 3 presents a high-efficiency source of single photons consisting of a single NV center in a photonic diamond nanowire. Early FIB prototypes are described, as is the first successful realization of the device achieved via reactive ion etching nanowires in a single crystal diamond containing NV centers, and finally a variation of this approach based on incorporation of NV centers in pure diamond via ion implantation. In chapter 4 we consider the optimal design of photonic devices offering both collection efficiency and cavity-enhancements and extend the model of the NV center to include photonic effects. In chapter 5 we briefly introduce a novel optically active spin discovered in a diamond nanowire. Finally, in chapter 6 we conclude with several proposals to extend this research program.