For applications in Quantum Information and Quantum Key Distribution an on-demand source of single photons is desirable because absolute security is of utmost importance. Photons are quantum systems; hence encoding information onto them offers a secure alternative to classical cryptography as a measurement cannot be performed on photons without altering their properties. The Nitrogen Vacancy (NV) centre in diamond is a good source of such photons. It is photo-stable and its location in diamond offers robustness. It has zero phonon line at 637 nm and its relative short luminescence life-time of about 12 ns makes it suitable for generating single photons. This thesis covers two aspects: Firstly the characterization of defects in diamonds and subsequent selection of diamonds suitable for use in the single photon setup and secondly, the development of the experimental setup for single photon generation. This thesis sets out to describe the development of a laboratory based single photon source using the NV centre in diamond. For this purpose a suite of diamond samples were selected and subjected to various spectroscopic tests in order to characterize and classify the samples, especially the presence of the NV centres and their concentrations. The characterization of the defects was done through the use of the following spectroscopic techniques: UltravioletVisible-Near infrared spectroscopy, Infrared spectroscopy, Electron Spin Resonance and Photoluminescence. These techniques enabled us to understand the types and origins of crystal defects that were present in the diamond samples used in this study and to use this to select diamonds that are most suitable for use in generation of single photons. The experimental setup for single photon generation using the NV centre is based on a confocal microscope arrangement. Single NV centres were identified by measuring the second order autocorrelation function of the fluorescence light emitted by the sample when illuminated with a laser. This measurement was done using a Hanbury-Brown Twiss (HBT) interferometer.
For applications in Quantum Information and Quantum Key Distribution an on-demand source of single photons is desirable because absolute security is of utmost importance. Photons are quantum systems; hence encoding information onto them offers a secure alternative to classical cryptography as a measurement cannot be performed on photons without altering their properties. The Nitrogen Vacancy (NV) centre in diamond is a good source of such photons. It is photo-stable and its location in diamond offers robustness. It has zero phonon line at 637 nm and its relative short luminescence life-time of about 12 ns makes it suitable for generating single photons. This thesis covers two aspects: Firstly the characterization of defects in diamonds and subsequent selection of diamonds suitable for use in the single photon setup and secondly, the development of the experimental setup for single photon generation. This thesis sets out to describe the development of a laboratory based single photon source using the NV centre in diamond. For this purpose a suite of diamond samples were selected and subjected to various spectroscopic tests in order to characterize and classify the samples, especially the presence of the NV centres and their concentrations. The characterization of the defects was done through the use of the following spectroscopic techniques: UltravioletVisible-Near infrared spectroscopy, Infrared spectroscopy, Electron Spin Resonance and Photoluminescence. These techniques enabled us to understand the types and origins of crystal defects that were present in the diamond samples used in this study and to use this to select diamonds that are most suitable for use in generation of single photons. The experimental setup for single photon generation using the NV centre is based on a confocal microscope arrangement. Single NV centres were identified by measuring the second order autocorrelation function of the fluorescence light emitted by the sample when illuminated with a laser. This measurement was done using a Hanbury-Brown Twiss (HBT) interferometer.
Defects in diamond have evolved as unique object for their potential application for Optical imaging and Nano-photonics. High pressure high temperature (HPHT) grown type-Ib diamond particles were milled down to the size of ~ 80 nm and purified to remove iron & graphitic carbon impurities and irradiated with 35 KeV He+ ions to create vacancies in the diamond lattice. Vacancies diffuse close to substitutional nitrogen impurities of nano-diamond particles upon thermal treatment. Characteristic sharp emission of NV centers were observed at 575.5 nm and 637.9 nm corresponding to NV° and NV− charged states in addition to a broad emission in the range 550-750 nm. Structural and photo-physical behavior of these fluorescent diamonds was found to depend on the method of purification adopted. Samples purified through acid reflux were found suitable for bio-imaging applications while, sample purified through air oxidation shows properties desirable for Nano-photonic applications. Photon-correlation measurements of emission from individual diamond nanoparticles using Hanbury Brown and Twiss interferometer (HBT) shows anti-bunching curve with g2(t)= 0 confirming their suitability as single photon source. Such sources of indistinguishable photons are highly desirable for quantum computation and quantum optics applications.
The development of reliable devices to generate single photons is crucial for applications in quantum cryptography, as well as for fundamental quantum optics experiments. Due to their extremely high photostability at room temperature, optically active defects in solids, the so called color centers, are among the most promising candidates. Single NV (nitrogen-vacancy) centers in diamond have been demonstrated to be able to generate single photons and have already shown advantages compared to attenuated laser pulses in a quantum cryptography experiment. The nitrogen-vacancy center (NV center) in diamond consists of a substitutional nitrogen atom and an adjacent vacancy. This complex exhibits an optical transition with a fluorescence lifetime of 11.6 ns. The room temperature spectrum of the NV center consists of the zero-phonon line at 637 nm and broad phononic sidebands with a FWHM of more than 100 nm. Due to the existence of a shelving level the efficiency of the NV center as a single photon source is limited. Additionally, the high diffraction index of bulk diamond restricts the efficiency to collect the fluorescence light.
The nitrogen-vacancy (NV) center is a defect in the structure of diamond, roughly the size of a single atom, that possesses properties that make it applicable to quantum information processing, high-resolution magnetic resonance imaging, and probing biological systems. When an NV center is excited by green light of wavelength 523 nm, it emits red light of in the range from 600-800 nm tens of nanoseconds later. A confocal microscope can be used to excite and collect emitted photons from NV centers. By detecting the intensity of the emitted red photons, we can determine the quantum state of a single NV center, which is needed to build various quantum computing, sensing and imaging devices. The autocorrelation method is used to detect antibunching in the emission of red photons and ascertain that the observed photons are being emitted from a single NV center. We have built an autocorrelation device that measures, with sub-nanosecond accuracy, the statistical distribution of the arrival times of the red photons emitted from a diamond sample at a photon counter. The device can measure fixed time values with a time resolution of 120 ps FWHM. For uncorrelated sources measured for 120 seconds at the TAC full range of 100 ns, the average counts in each histogram bin were about 0.01% of the predicted value. The autocorrelation device that we have built costs about 10 times less than commercial units used by other NV researchers.
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.
The nitrogen-vacancy (N-V⁻) colour centre in diamond has potential applications in quantum information processing and single photon generation. It is currently the only known defect in a solid detected at a single site level that has a non-zero spin in the electronic ground state. For the proposed applications it is desirable to have a good understanding of the electronic structure and photodynamics of the centre, however this is currently not the case. Various models have been proposed to explain the fluorescence characteristics observed in single site experiments. The challenge has been to also account for the properties of the centre well-known from observations of the fluorescence from large ensembles of N-V⁻ in diamond. Firstly, that a non-Boltzmann population distribution between the. ground state spin levels is induced by optical excitation. Secondly, that the fluorescence intensity exhibits a strong dependence on the spin orientation. The models proposed to date either cannot account for these properties, or do so only by invoking optical processes that are arbitrary or, in some cases, not physical. In this thesis an alternative model is presented. The derivation of the model, from group theoretical considerations, does not form part of this thesis. This thesis is concerned primarily with a series of independent measurements to determine the transition rates which govern the photodynamics of the centre. When these transition rates are known, there will be no free parameters in the model and the N-V⁻ emission can be simulated for an arbitrary optical field by solving the classical rate equations. To conclude the first part of this thesis, a two-pulse optical excitation is considered and the results of experiment are compared to the predictions of the model. The latter part of t his thesis is concerned with optically detecting single N-V⁻ centres. A confocal microscope system, to enable single site detection, was developed as part of this work. The photon statistics from a single N-V⁻ centre is compared to the statistics predicted by the model. The implications for the modelling of individual N-V⁻ photon statistics are discussed.
Single photon source can be produced by using spontaneous parametric down conversion or quantum emitter such as ions, molecules, atoms, quantum dots and colour centres. Main objective of current research is to automate single photon generation module and detection module based on nitrogen vacancy colour centre in diamond into one system. In single photon generation, diamond sample is held at a holder mounted on a 3D piezo translation stage. Laser source with wavelength 527nm is focused using a standard microscope objective to a spot size at the nitrogen vacancy centre to produce fluorescence. Since a single photon is generated by exciting an isolated nitrogen vacancy in a diamond crystal, it is critical that position of nitrogen vacancies in the crystal to be known. For this purpose, a scanning system was designed and constructed to determine the 3D position of nitrogen vacancy and identified their coordinates for later use. The system consists of a high precision 3D piezo translation stage and was controlled by a scanning programme built using LabVIEW. This programme will map the location of the vacancies in an intensity graph where axis X and Y show the scanning position while the bright colour spots determine the position of the vacancies. In single photon detection which is based on the Hanbury-Brown-Twiss setup, the fluorescence emitted from the nitrogen vacancy is split by a beamsplitter and directed to single photon detectors. A digital pulse is produced for each photocount detected. At the same time, output from the detectors is fed into a time to amplitude converter/single channel analyzer to produce coincidence counts. In order to read and record the number of photon counts and number of coincidences, a detection system was designed and built. This detection system interfaces a series of high performance single photon detectors to the same computer that controls the scanning system via a detection programme. Besides reading and recording data, the detection programme can also calculate the second order correlation function, g2(τ) from a subVI written in LabVIEW 8.2.