This paper reports on a study of the statistical properties of single photons emitted by a nitrogen vacancy centre using a confocal microscope and Hanbury-Brown and Twiss interferometer. High signal-to-noise photon correlation histograms from a single NV centre have been recorded for various pump laser powers. The form of these signals can be described by modelling the nitrogen vacancy as a three-level system. The parameters that characterize the model have been measured as a function of pump laser power.
This paper reports on a study of the statistical properties of single photons emitted by a nitrogen vacancy centre using a confocal microscope and Hanbury-Brown and Twiss interferometer. High signal-to-noise photon correlation histograms from a single NV centre have been recorded for various pump laser powers. The form of these signals can be described by modelling the nitrogen vacancy as a three-level system. The parameters that characterize the model have been measured as a function of pump laser power.
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.
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.
The present work focuses on laser excited nitrogen vacancies (NV-centres) in nanodiamonds. The emitted fluorescence of the investigated centres is observed after spectral filtering at (697 + 37) nm. To obtain the photon number distribution of different NV centres, the diagonal elements of the density matrix were experimentally determined using a transition-edge sensor (TES). This photon number resolving detector produces an output pulse proportional to the number of photons absorbed. Due to the absence of the two photon component, the g(2)(t = 0)-value calculated from the data is(2)function was measured by a Hanbury-Brown-Twiss (HBT) interferometer consisting of two single photon avalanche photodiodes (SPADs) and a beam splitter. By fitting the HBT-data, a g(2)(t = 0)-value of 0.03 ± 0.03 is determined, thus comparable with the TES value.
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.
We model and experimentally demonstrate the full time-dependent counting statistics of photons emitted by a single nitrogen-vacancy (NV) center in diamond under non-resonant laser excitation and resonant microwave control. A generalization of the quantum jump formalism for the seven electronic states involved in the fast intrinsic dynamics of an NV center provides a self-contained model that allows for the characterization of its emission and clarifies the relation between the quantum system internal states and the measurable detected photon counts. The model allows the elaboration of detection protocols to optimize the energy and time resources while maximizing the system sensitivity to magnetic-field measurements.
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.