This thesis decay rate enhancement and photophysics of nitrogen vacancy (NV) centers in diamond and quantum emitters in hexagonal boron nitride (hBN). The NV center in diamond is a unique physical system that finds use in many quantum technologies such as quantum sensing, quantum information processing and single photon generation. Consisting of a substitutional nitrogen atom next to a vacant lattice site, the NV center introduces discrete electronic energy levels within the wide bandgap of diamond. The NV center can be optically excited and will emit single photons upon relaxation that hold information of the spin state of the NV center. In this thesis, we use metallic structures to modify and enhance the decay rate of NV centers in bulk and nano diamonds in three different experiments. Such an enhancement is a way to improve the usability of NV as a single photon source. Using TiN films with metamaterial properties we observe a 2-fold decrease of the total excited state lifetime of NV centers in nano diamonds, when comparing to similar NV centers not influenced by the TiN films. For shallow implanted NV centers in bulk diamond we experimentally investigate how a decay rate enhancement influences the signal-to-noise ratio (SNR) in spin readout measurements. In this experiment we perform spin state readout before and after introducing a rate enhancement and find a 30 % drop in spin contrast. We model the spin readout measurements using a five-level model and use that to predict the SNR under higher rate enhancement. In the last NV related experiment we use a curved metallic mirror on top of shallow implanted NV centers, to measure the continuous lifetime modification as a function of distances to the mirror. This allows us to determine the internal quantum efficiency (IQE) by fitting the experimental data with a model that includes calculating the expected Purcell factor. We find IQE values of 0.70±0.07 and 0.82±0.08 for implantation depths of 4.5±1 and 8±2 nm, respectively. Last but not least, newly discovered bright single photon emitters in two dimensional hBN were investigated. Similar to the NV center they are believed to be crystal defects, but the defect configuration is not fully known. In this thesis, the photophysics of quantum emitters in hBN nano flakes are investigated. Intensity autocorrelation measurements indicate the presence of at least two metastable states, seen by distinct photon bunching levels on long time scales in the g(2)(τ)-functions. We also observe blinking behaviour, which we show can be explained by changes in the IQE. From rate equation fitting we find that transition rates scale linearly with excitation power. Based on the transition rates we estimate the IQE to be close to unity for excitation powers below saturation.
This thesis decay rate enhancement and photophysics of nitrogen vacancy (NV) centers in diamond and quantum emitters in hexagonal boron nitride (hBN). The NV center in diamond is a unique physical system that finds use in many quantum technologies such as quantum sensing, quantum information processing and single photon generation. Consisting of a substitutional nitrogen atom next to a vacant lattice site, the NV center introduces discrete electronic energy levels within the wide bandgap of diamond. The NV center can be optically excited and will emit single photons upon relaxation that hold information of the spin state of the NV center. In this thesis, we use metallic structures to modify and enhance the decay rate of NV centers in bulk and nano diamonds in three different experiments. Such an enhancement is a way to improve the usability of NV as a single photon source. Using TiN films with metamaterial properties we observe a 2-fold decrease of the total excited state lifetime of NV centers in nano diamonds, when comparing to similar NV centers not influenced by the TiN films. For shallow implanted NV centers in bulk diamond we experimentally investigate how a decay rate enhancement influences the signal-to-noise ratio (SNR) in spin readout measurements. In this experiment we perform spin state readout before and after introducing a rate enhancement and find a 30 % drop in spin contrast. We model the spin readout measurements using a five-level model and use that to predict the SNR under higher rate enhancement. In the last NV related experiment we use a curved metallic mirror on top of shallow implanted NV centers, to measure the continuous lifetime modification as a function of distances to the mirror. This allows us to determine the internal quantum efficiency (IQE) by fitting the experimental data with a model that includes calculating the expected Purcell factor. We find IQE values of 0.70±0.07 and 0.82±0.08 for implantation depths of 4.5±1 and 8±2 nm, respectively. Last but not least, newly discovered bright single photon emitters in two dimensional hBN were investigated. Similar to the NV center they are believed to be crystal defects, but the defect configuration is not fully known. In this thesis, the photophysics of quantum emitters in hBN nano flakes are investigated. Intensity autocorrelation measurements indicate the presence of at least two metastable states, seen by distinct photon bunching levels on long time scales in the g(2)(τ)-functions. We also observe blinking behaviour, which we show can be explained by changes in the IQE. From rate equation fitting we find that transition rates scale linearly with excitation power. Based on the transition rates we estimate the IQE to be close to unity for excitation powers below saturation.
The nitrogen vacancy center in diamond in its negative charge state is a promising candidate for quantum optic experiments that require single photon emitters. Important benefits of the NV center are its high brightness and photo-stability, even at room temperature. Engineering the emission properties of NV centers with optical resonators is a widely followed approach to meet the requirements for quantum technological applications, but the effect on non-radiative decay paths is yet to be understood. Here we report on modifying the internal quantum efficiency of a single NV center in a nanodiamond coupled to a 1D photonic crystal cavity. We assess the Purcell enhancement via three independent measurement techniques and perform autocorrelation measurements at elevated excitation powers in confocal microscopy. Employing a three-level model allows us to extract the setup efficiency, individual transition rates and thus the internal quantum efficiency of our system. Combining our results, we find that the enhancement of the radiative decay rate via the Purcell effect results in an internal quantum efficiency of 90 % for cavity-coupled NV centers. Our findings will facilitate the realization of nano-scale single photon sources with near-unity internal quantum efficiencies operating at high repetition rates.
Room temperature solid-state emitters provide a promising source of single photons for use in quantum technologies. The emitters studied in this thesis are the nitrogen-vacancy (NV) centre in diamond, and defects in two-dimensional hexagonal boron nitride (hBN). In this thesis, I present investigations into the coupling of these defects to open-access microcavities at room temperature. The optical microcavity serves to confine the electric field down to mode volumes on the order of λ3, increasing the spontaneous emission rate of these emitters and channelling this emission into well defined cavity modes. The general properties of both emitters will be presented, including examples of single photon emission, emission spectra, saturation behaviour and photo-stability. A few examples highlighting the long-term photo-stability of the NV centre under pulsed excitation will be displayed. This will highlight the suitability of the NV centre for a practical device. Following the preparation and characterisation of each emitter, single photon emission into the cavity modes is verified, reaching single photon purities greater than 97%. Enhancements in the spectral density of photons were observed for both the NV centre (~25×) and defect in hBN (~3×), along with decreases in the excited state lifetimes. While the NV centre provides a stable source of single photons, the cavity enhancement is limited by its broad emission spectrum. Defects in hBN present near-ideal optical properties in the absence of the cavity, and were expected to deliver superior results within the cavity. However, the poor photo-stability of these defects did not allow complete characterisation of the cavity coupled emission. FDTD simulations were combined with changes in the excited state lifetime to infer the quantum efficiency of these defects. The results of this thesis form the foundation for the realisation of a robust room temperature single photon source.
A quantum system with discrete, resolvable energy levels is an ideal system for sensing applications, taking into account that it can strongly interact with its environment. Although the strong response with the target properties such as magnetic or electric field is ideal, it can be a disadvantage when the system is interacting with other physical changes (noise) at the same time. However, the benefits of the strong coupling as well as small size of the sensor has motivated many researchers to explore quantum systems for measuring very small physical quantities. For instance, Nitrogen vacancy center in diamond is a unique quantum system enabling sensing of magnetic field from single molecules or electrons at room temperature.
This thesis has focused on two different quantum emitters: NV centers in nanodiamonds and quantum emitters in hexagonal Boron Nitride (hBN). In the first part a new assembly technique based on electron beam induced deposition and crosslinking chemistry is developed. It is demonstrated that fluorescent nanodiamonds containing NV centers can be assembled into arrays of various size and shapes on any substrate. The produced array is ideal for device fabrication due to its outstanding robustness. A potential application of such arrays as magnetic field sensor with high spatial resolution is shown.
The superior properties of quantum emitters in hBN has been studied in the second part of this thesis, where is has been shown that these emitters are not only stable but also maintain their single photon purity at elevated temperature. The highest measured quantum emission at 800 K in this study is the highest reported temperature for a quantum emitter so far, makes them a suitable candidate for temperature sensing. In addition, the level structure of emitters in hBN has been investigated in detail which reveals the unique photophysical properties of a class of these emitters which result in a nonlinear increase in the emission upon co-excitation with two lasers of different wavelength. Finally the photophysic property of these emitters has been employed to introduce a new modality of super-resolution microscopy with resolution down to about 70 nm. These findings will extend our understanding of quantum emitters in hBN and introduce a new functionality for them which paves the way toward their application in biology and sensing.
This thesis describes a series of experiments on the control of the optical properties of the nitrogen-vacancy (NV) center in diamond, and on control of the electron and nuclear spin states associated with the NV center. The NV center is a fluorescing atomic defect center in diamond, consisting of a substitutional nitrogen atom adjacent to a vacancy in the diamond carbon lattice. The electron spin of the NV center can be initialized and read out optically, and coherently manipulated with high fidelity even at room temperature. This unique combination of properties has attracted much attention to the NV center in recent years for application in quantum information technologies. The experiments in this work focus on exploiting the intrinsic hybrid nature of the NV center. The NV center forms a hybrid spin register, as its electron spin is always coupled to the nuclear spin of its own nitrogen nucleus, and can furthermore be coupled to additional nuclear carbon-13 spins in the diamond. In addition, the electronic spin state can be mapped onto the state of a photon, in principle allowing long range transmission of quantum information and measurement-based entanglement of distant NV center registers. The long-term goal is to achieve a large scale quantum network where the different constituents each perform the task they are most well suited for: the stable nuclear spins store the quantum information, photons transfer quantum information over long distances, and the fast electron spin interfaces between spin and photon states. For measurement-based entanglement of distant NV centers it is of primary importance to maximize the emission and detection of coherent photons emitted by the NV center. Spontaneous emission can be controlled and enhanced by coupling an emitter to a photonic cavity. Chapters 3, 4, and 5 of this thesis describe experiments aimed at maximizing the coherent photon emission rate of an NV center by coupling the NV center to a photonic crystal cavity. One of the main challenges is to position a single NV center into the nanometer-sized cavity mode. Chapter 3 describes the development of a nanopositioning technique, which allows diamond nanocrystals ~50 nm in size containing single NV centers to be placed at arbitrary locations on a chip, with nanometer precision. By using a sharp etched tungsten tip, individual nanocrystals can be picked up, moved and placed under real-time imaging with an electron microscope. We explicitly demonstrate that the unique optical and spin properties of the NV center are conserved by the nanopositioning process. In chapter 4, we apply the nanopositioning technique to couple single NV centers contained in a diamond nanocrystals to gallium phosphide photonic crystal cavities. These cavities resonate in the visible with high quality factors, and are designed to have the lowest energy mode coincide with the zero-phonon line of the NV center. Efficient coupling is evidenced by a strong enhancement of NV center emission at the cavity wavelength. The high quality factor of a photonic crystal cavity is a result of careful design and fabrication. The introduction of a nanocrystal may therefore be expected to have a detrimental effect on the optical properties of a cavity. In chapter 5 we investigate the effect of a nanocrystal on the optical properties of photonic crystal cavities. Our simulations and measurements show that the effect of a nanocrystal is in fact only minor, a promising result for future work on cavity-QED systems in the solid state and with diamond defect centers in particular. Understanding and mitigating decoherence is a key challenge for quantum science and technology. The main source of decoherence for solid-state spin systems is the uncontrolled spin bath environment. Chapter 6 describes experiments that exploit quantum control of the NV center and recently developed dynamical decoupling techniques to study the properties of the surrounding bath of spins belonging to single substitutional nitrogen atoms. The coherence properties of a single NV center are affected by changes in the state of the bath spins, allowing a single NV center to be used as a probe to detect bath spin resonances and study the quantum dynamics of the spin bath. We use quantum control of the spin bath to extend the dephasing time of the NV center, important for the application of the NV center in DC magnetometry. In chapter 7 we describe the development and implementation of decoherence protected quantum gates for the hybrid spin register formed by the NV center electron spin that is coupled to the nuclear spin of its own nitrogen atom. Electron and nuclear spins evolve and decohere at vastly different rates, making it challenging to use such a hybrid system as a fully functional quantum register. We create a universal set of two-qubit quantum gates by integrating dynamical decoupling techniques into the gate operation. We demonstrate the power of our gate design by implementing for the first time Grover's quantum search algorithm on a solid-state spin register.
Sources of single photons are integral to a number of applications in the emerging field of quantum technologies. For any practical implementation of single-photon sources, a variety of different properties and an overall comprehensive characterization of the sources are required. Color centers in hBN and group IV color centers in diamond are promising candidate sources due to their favorable properties and rich internal level structure. Both of these candidates, however, also present some serious challenges. Although bright and spectrally narrow, defects in hBN suffer from emission instabilities. Furthermore, limited knowledge about their atomic composition leads to behaviour, which is often unpredictable. Meanwhile, Group IV centers generally exhibit low extraction efficiencies due to total internal reflection inside the high-index diamond host material. In this thesis, several large-scale series of experiments are conducted to investigate both types of emitters in a confocal fluorescence microscope at room temperature. The excitation characteristics of a large number of single-photon emitters in hBN from a narrow spectral window are investigated using a fully automated procedure. The results are compared to theoretical predictions for several carbon defects from the literature to gain further knowledge of the atomic nature of the underlying defects. Based on this approach, the CB, CN, CBVN, and CNVB centers are confidently ruled out as possible origins of the observed emission. Some uncertainty remains about the assignment of the carbon trimer C2CN as well as carbon-based donor-acceptor pairs. In a separate series of experiments, emission instabilities of hBN emitters from the same spectral region are studied. The detected fluctuations in brightness were conclusively attributed to spectral diffusion of the emission. Furthermore, optical stabilization of these effects via short-wavelength irradiation favors an explanation of the observed behavior via nearby charge carriers and the resulting Stark effect. Tin-vacancies, as representatives of group IV centers, are studied in a novel type of diamond waveguide structure for light collection enhancement. A center with sharp zero-phonon emission around 619 nm, a g(2)(0) value of 0.32, and simultaneous stable count rate of 3.4 Mcps was found. By restricting the collection to the zero-phonon line, g(2)(0) = 0.1 could be achieved. In a widespread characterization of the sample, it was found that most structures contain more than one negatively charged SnV center, which leaves an opportunity to improve the implantation parameters. In additional experiments on single tin-vacancy centers, a possible transition to a higher excited state as well as the charge-state transitions of the defects were probed. The results of these measurements contribute to the growing body of research on tin-vacancies and other group IV centers.
Photon emission correlation spectroscopy (PECS) is an indispensable tool for the study of atoms, molecules, and, more recently, solid-state quantum defects. In solid-state systems, its most common use is as an indicator of single-photon emission, a key property for quantum technology. Beyond single-photon purity, photon correlation measurements can provide a wealth of information that can reveal details about an emitter's electronic structure and optical dynamics that are hidden by other spectroscopy techniques. This thesis explores the application of PECS to study and understand the optical dynamics of quantum emitters. The first part of this thesis presents a guide to a standardized framework for using PECS to facilitate materials exploration for qubit candidates. This includes discussion of theoretical background, considerations for data acquisition and statistical analysis, and interpretation of PECS. It also illustrates how this experimental technique can be paired with optical dynamics simulations to formulate an electronic model for unknown quantum emitters. The second part of this thesis implements the practices discussed in the first part to explore the optical dynamics of two systems: the nitrogen-vacancy (NV) center in diamond and a quantum emitter in hexagonal boron nitride (h-BN). The NV center is a promising platform for applications in quantum sensing, quantum communication, and quantum networks. In particular, its spin and charge dynamics constitute useful attributes that can be harnessed for quantum control protocols. This thesis models and quantifies the transition rates that govern spin and charge dynamics in the NV center, utilizing PECS measurements, analysis, and simulations as a function of magnetic field, and excitation power. These findings can further inform the design of quantum control protocols. H-BN hosts pure single-photon emitters that have shown evidence of optically detected electronic spin dynamics. However, the electrical and chemical structure of these optically addressable spins is unknown, and the nature of their spin-optical interactions remains mysterious. This thesis discusses time-domain optical and microwave experiments to characterize a single emitter in h-BN exhibiting room temperature optically detected magnetic resonance. It further discusses use of dynamical simulations to constrain and quantify transition rates in the model, and design of optical control protocols that optimize the signal-to-noise ratio for spin readout. This constitutes a necessary step towards quantum control of spin states in h-BN.