Solid-state single-photon sources are the central blocks for many scalable quantum applications including quantum computing, quantum key distributions, and quantum simulations. Over the years, diverse quantum emitters have been studied. This thesis endeavored investigations into single-photon sources in diamond, silicon carbide and gallium nitride in three directions: Firstly, identification of the single-photon sources that have never been reported before and fabrication technique for scalable single germanium vacancy centers in diamond. Telecom wavelength single photon sources have been found in both gallium nitride and silicon carbide. More importantly, those emitters work even at room temperature and are promising for quantum key distribution applications in the future. Secondly, the spin properties study of the single silicon vacancy in diamond: the silicon vacancy optical transition has been coherently controlled in the work. The optical Rabi oscillations in the time domain have been observed. The ultrafast spin control of the silicon vacancy has been also achieved. Thirdly, quantum applications of the divacancy spins in silicon carbide: nano-scale thermometry based on the quantum control of the divacancy spin in silicon carbide has been introduced. Especially, we found a self-protected effect that the transverse strain can protect the spin from the local magnetic field and enables longer coherence time. Finally, some possible directions have been discussed.
Solid-state single-photon sources are the central blocks for many scalable quantum applications including quantum computing, quantum key distributions, and quantum simulations. Over the years, diverse quantum emitters have been studied. This thesis endeavored investigations into single-photon sources in diamond, silicon carbide and gallium nitride in three directions: Firstly, identification of the single-photon sources that have never been reported before and fabrication technique for scalable single germanium vacancy centers in diamond. Telecom wavelength single photon sources have been found in both gallium nitride and silicon carbide. More importantly, those emitters work even at room temperature and are promising for quantum key distribution applications in the future. Secondly, the spin properties study of the single silicon vacancy in diamond: the silicon vacancy optical transition has been coherently controlled in the work. The optical Rabi oscillations in the time domain have been observed. The ultrafast spin control of the silicon vacancy has been also achieved. Thirdly, quantum applications of the divacancy spins in silicon carbide: nano-scale thermometry based on the quantum control of the divacancy spin in silicon carbide has been introduced. Especially, we found a self-protected effect that the transverse strain can protect the spin from the local magnetic field and enables longer coherence time. Finally, some possible directions have been discussed.
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.
Single photon sources based on single emitters such as 2D materials and impurities in diamond are of great interest for many application fields including quantum communication, quantum metrology and quantum sensing. In photometry and radiometry, a single photon source, having the ability to emit only one photon in a well-defined time and frequency domain, represents a promising candidate to realize a photon number-based primary standard for quantum radiometry. At the Korea Research Institute of Standards and Science (KRISS), we have realized three kinds of single photon sources based on various single emitters at room temperature: silicon vacancy in diamond, defects in gallium nitride (GaN), and vacancy in hexagonal boron nitride (hBN). We found common factors related with the relaxation times of the internal states that indirectly affect the photon number stability. We observed a high photon number stability in the GaN emitter due to faster relaxation times compared with the hBN emitter, which on the other hand produced high rates of photons per second (> 106). Moreover, we demonstrate repeatable radiant flux measurements of a bright hBN single photon emitter for a wide radiant flux range from a few tens of femtowatts to one picowatt.
Over the past decade, quantum computers have emerged as a promising technology to enable transformational advances in information processing and communication and solve problems that are intractable to classical computers. While there is great promise in linking quantum computers together over long distances via quantum channels, these technologies are still under development. Solid-state emitters with coherent spin-photon interfaces, long spin lifetimes, and narrow optical transitions are a leading platform for use as quantum memories in networked quantum repeaters. However, while such emitters have already enabled advanced quantum networking demonstrations in laboratory settings, applying these devices as useful memory devices at scale is a key outstanding challenge. In this thesis, we experimentally investigate solid-state quantum memories for quantum information applications. First, we develop experimental techniques to characterize solid-state emitters with high throughput, enabling both better understanding of the distribution of emitter properties and improved feedback on material preparation and device fabrication. Next, we implement quantum frequency conversion to create a coherent spin-photon interface between silicon-vacancy centers in diamond and optical photons in the low-loss telecom band. Finally, we investigate color centers in other engineering materials, including silicon and silicon carbide, to better understand the fundamental trade space of requirements for solid-state hosts. Together, these efforts represent a significant advance in creating, controlling, and deploying telecom-compatible spin interfaces, paving the way for memory-enabled quantum repeaters.
Solid-state color centers with manipulatable spin qubits and telecom-ranged fluorescence are ideal platforms for quantum communications and distributed quantum computations. In this work, we coherently control the nitrogen-vacancy (NV) center spins in silicon carbide at room temperature, in which telecom-wavelength emission is detected. We increase the NV concentration sixfold through optimization of implantation conditions. Hence, coherent control of NV center spins is achieved at room temperature, and the coherence time T_{2} can be reached to around 17.1 μs. Furthermore, an investigation of fluorescence properties of single NV centers shows that they are room-temperature photostable single-photon sources at telecom range. Taking advantage of technologically mature materials, the experiment demonstrates that the NV centers in silicon carbide are promising platforms for large-scale integrated quantum photonics and long-distance quantum networks.
This chapter is organized as follows: The first half of the chapter is about quantum light sources based on diamond color centers. After the introduction in Section 7.1, we discuss the optical properties of the NV center and other color centers in diamond, indistinguishable photon sources in diamond, entanglement between photon and spin in diamond, electroluminescence (EL) of color centers in diamond, and diamond nanophotonic devices. The second half of the chapter is about quantum light sources based on color centers in SiC. Mainly, fundamental characteristics of silicon vacancies and divacancies are presented in each section.
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.