The nitrogen-vacancy center (NV) is a diamond spin defect consisting of a nitrogen atom and a carbon vacancy in two adjacent positions of the diamond lattice. Its remarkable spin-dependent optical properties make it a promising platform for quantum sensing and quantum information applications.In the present manuscript, two applications involving an ensemble of NV centers in diamond are investigated: a radio frequency (RF) spectrum analyser and a widefield imaging technique of the RF radiation emitted in near field by microwave devices.In the first chapter, the NV centers in diamond, their properties and the motivations behind the two NV centers based applications proposed in this manuscript are presented. The available solutions and the state-of-the-art techniques for both RF spectral analysis and near field imaging of RF radiation are reviewed and the potential of alternative solutions using NV centres are discussed.In the second chapter, a set-up for the widefield optical detection of the NV centers spin resonances is presented and used to investigate the main properties of this diamond defect. The NV center electron spin resonances are detected over a large frequency range (more than 10 GHz) and different phenomena are investigated: the behaviour of an ensemble of NV centers under a static magnetic field, the polarization properties of the NV center photoluminescence (PL) and how they can be exploited to recognise the NV centers orientations, the NV’s nitrogen nuclear spin polarization over a large frequency range (more than 2 GHz).The third chapter is devoted to the implementation of a real-time RF spectrum analyser based on an ensemble of NV centers. The main physical features of NV centers considered as MW detectors are investigated and an experimental architecture which allows a real time RF spectral analysis over a tuneable frequency range (10 MHz to 25 GHz), 4 GHz bandwidth, a MHz frequency resolution and a ms time resolution is demonstrated. The implementation of this device required an interesting analysis on the behaviour of an ensemble of NV centers under strong magnetic field gradient (tens of Tesla per meter) which is detailed in the chapter.The fourth and last chapter deals with the widefield imaging of RF field based on an ensemble of NV centers. Two techniques are proposed. The first is based on the RF power broadening of the NV center spin resonances in the continuous regime domain while the second is based on the widefield detection of NV centers Rabi oscillations in a pulsed regime. Both techniques allowed reconstructing the magnetic near field RF field emitted by a loop antenna. However, while the first technique requires a calibration to retrieve the amplitude of the RF field, the second allows a direct measurement of it. The widefield detection of the NV centers Rabi oscillations has thus been investigated both by means of experiments and simulations. Since the NV centers Rabi oscillations detected using our widefield experimental set-up showed an asymmetric shape, a model has been realized to figure out the NV centers behaviour during the pulse sequence employed to drive the Rabi oscillation. The results of the simulations, in agreement with the experimental results, enable to have a better insight of the NV center spin optical polarization and readout during the widefield detection of the Rabi oscillations.
The nitrogen-vacancy center (NV) is a diamond spin defect consisting of a nitrogen atom and a carbon vacancy in two adjacent positions of the diamond lattice. Its remarkable spin-dependent optical properties make it a promising platform for quantum sensing and quantum information applications.In the present manuscript, two applications involving an ensemble of NV centers in diamond are investigated: a radio frequency (RF) spectrum analyser and a widefield imaging technique of the RF radiation emitted in near field by microwave devices.In the first chapter, the NV centers in diamond, their properties and the motivations behind the two NV centers based applications proposed in this manuscript are presented. The available solutions and the state-of-the-art techniques for both RF spectral analysis and near field imaging of RF radiation are reviewed and the potential of alternative solutions using NV centres are discussed.In the second chapter, a set-up for the widefield optical detection of the NV centers spin resonances is presented and used to investigate the main properties of this diamond defect. The NV center electron spin resonances are detected over a large frequency range (more than 10 GHz) and different phenomena are investigated: the behaviour of an ensemble of NV centers under a static magnetic field, the polarization properties of the NV center photoluminescence (PL) and how they can be exploited to recognise the NV centers orientations, the NV’s nitrogen nuclear spin polarization over a large frequency range (more than 2 GHz).The third chapter is devoted to the implementation of a real-time RF spectrum analyser based on an ensemble of NV centers. The main physical features of NV centers considered as MW detectors are investigated and an experimental architecture which allows a real time RF spectral analysis over a tuneable frequency range (10 MHz to 25 GHz), 4 GHz bandwidth, a MHz frequency resolution and a ms time resolution is demonstrated. The implementation of this device required an interesting analysis on the behaviour of an ensemble of NV centers under strong magnetic field gradient (tens of Tesla per meter) which is detailed in the chapter.The fourth and last chapter deals with the widefield imaging of RF field based on an ensemble of NV centers. Two techniques are proposed. The first is based on the RF power broadening of the NV center spin resonances in the continuous regime domain while the second is based on the widefield detection of NV centers Rabi oscillations in a pulsed regime. Both techniques allowed reconstructing the magnetic near field RF field emitted by a loop antenna. However, while the first technique requires a calibration to retrieve the amplitude of the RF field, the second allows a direct measurement of it. The widefield detection of the NV centers Rabi oscillations has thus been investigated both by means of experiments and simulations. Since the NV centers Rabi oscillations detected using our widefield experimental set-up showed an asymmetric shape, a model has been realized to figure out the NV centers behaviour during the pulse sequence employed to drive the Rabi oscillation. The results of the simulations, in agreement with the experimental results, enable to have a better insight of the NV center spin optical polarization and readout during the widefield detection of the Rabi oscillations.
High-resolution wide-spectrum measurement techniques have important applications in fields such as astronomy, wireless communication, and medical imaging. Nitrogen-vacancy (NV) center in diamond is well known for its high stability, high sensitivity, real-time monitoring, single-point detection, and suitability for long-term measurement, and has an outstanding choice for spectrum analyzers. Currently, spectrum analyzers based on NV centers as detectors can perform real-time spectrum analysis in the range of several tens of gigahertz, but their frequency resolution is limited to a MHz level. In this study, we construct a quantum diamond microwave spectrum imaging system by combining continuous wave-mixing techniques. According to the spin-related properties of the NV center in diamond, we implement optical pumping by 532 nm green laser light illuminating the diamond NV center. A spherical magnet is used to produce a magnetic field gradient along the direction of the diamond crystal. By adjusting the size and direction of the magnetic field gradient, spatial encoding of the resonance frequency of the NV center is achieved. The magnetic field gradient induces the Zeeman effect on the diamond surface at different positions, generating corresponding ODMR signals. Through accurate programming, we coordinate the frequency scanning step size of the microwave source with the camera exposure and image storage time, and synchronize them circularly according to the order of image acquisition. Ultimately, after algorithmic processing, we successfully obtain comprehensive spectrum data in a range from 900 MHz to 6.0 GHz. Within the measurable spectrum range, the system employs continuous wave-mixing, simultaneously applying resonant microwaves and slightly detuning auxiliary microwaves to effectively excite the NV center. This method triggers off microwave interference effects, disrupting the balance between laser-induced polarization and microwave-induced spontaneous relaxation. Specifically, microwave interference causes the phase and amplitude of the fluorescence signal to change, leading to the generation of alternating current fluorescence signals. This further enhances the response of the NV magnetometer to weak microwave signals. The method enables the system to achieve a frequency resolution of 1 Hz in the measurable spectrum range, and it can separately measure the frequency resolution of multiple frequency points with a frequency step size of 1 MHz. The research results indicate that the wide-spectrum measurement based on NV centers can achieve sub-hertz frequency resolution, providing robust technical support for future spectrum analysis and applications.
We propose an original analog method to perform instantaneous and quantitative spectral analysis of microwave signals. An ensemble of nitrogen-vacancy (NV) centers held in a diamond plate is pumped by a 532 nm laser. Its photoluminescence is imaged through an optical microscope and monitored by a digital camera. An incoming microwave signal is converted into a microwave field in the area of the NV centers by a loop shaped antenna. The resonances induced by the magnetic component of that field are detected through a decrease of the NV centers photoluminescence. A magnetic field gradient induces a Zeeman shift of the resonances and transforms the frequency information into spatial information, which allows for the simultaneous analysis of the microwave signal in the entire frequency bandwidth of the device. The time dependent spectral analysis of an amplitude modulated microwave signal is demonstrated over a bandwidth of 600 MHz, associated to a frequency resolution of 7 MHz , and a refresh rate of 4 ms. With such integration time, a field of a few hundreds of μW can be detected. Since the optical properties of NV centers can be maintained even in high magnetic field, we estimate that an optimized device could allow frequency analysis in a range of 30 GHz, only limited by the amplitude of the magnetic field gradient. In addition, an increase of the NV centers quantity could lead both to an increase of the microwave sensitivity and to a decrease of the minimum refresh rate down to a few μs.
The fluorescence of the nitrogen-vacancy (NV) centre in artificial diamond has been widely studied for applications such as quantum communication protocols, quantum computation, fluorescence-based biolabelling, and optical sensing. Optical sensing by itself encompasses a wide range of possible applications: from magnetic and electric field sensing to temperature sensing and enabling, for example, the detection of paramagnetic ions and the identification of molecules by nuclear magnetic resonance. Compared to many other systems capable of optical sensing, the NV centre works at room temperature, and its spin-dependent fluorescence emission is the key property to achieve a high sensitivity to the surrounding environment. Additionally, the NV centre emission is highly photostable, has a relatively short lifetime (~ 12ns) and spans a broad wavelength range, from the red to the near-infrared region of the electromagnetic spectrum (600 - 800nm). While the common denominator of all mentioned applications is the NV centre, the properties of the artificial diamond surrounding the NV centre have a significant impact on its sensing performance. At present, the processing of artificial diamond has not been fully optimised, and research and development towards the realisation of the `best¿ diamond sample for optical sensing is far from over. Specifically, the role of other defects in diamond and their effects on the photo-physical properties of the NV centres are not entirely understood. In this thesis, I will discuss the potential benefits of a novel processing technique to create ensembles of NV centres, and the importance that surrounding nitrogen defects has on the achievable magnetic field sensitivity. Additionally, I will show both theoretical and experimental investigations of the NV centres photo-physical processes, with focus on magnetic field sensing. I will initially explore a protocol for microwave-free magnetometry. This protocol predicts a theoretical magnetic field sensitivity <10µT/√Hz in the range 0.3 - 30mT and I will demonstrate it experimentally with a maximum sensitivity of 3.6mT/Hz. Following, I will focus on exploring techniques to increase the magnetic field sensitivity by improving the material properties of the diamond samples. I will demonstrate that the simultaneous electron irradiation and annealing improves the creation efficiency of an ensemble of NV centres by up to 117%. Finally, I will investigate how substitutional nitrogen defects in the surrounding lattice affects the charge state of the NV centre, its emission intensity, its lifetime and its magnetic field sensitivity. I will study a sample characterised by multiple regions across its volume, with nitrogen concentration changing between 10ppm and 100ppm in each region. There si an optimal region with ~ 40ppm of substitutional nitrogen which demonstrates the highest fluorescence emission, mainly from the negatively-charged NV centre, and the highest magnetic field sensitivity of 1.6 ± 0.2µT/√Hz. However, both the regions with lower and higher substitutional nitrogen concentration than the optimal region show lower emission intensity and worse magnetic field sensitivity. Following is a summary of the content in each chapter: Chapter 1 will present an introduction to the diamond material and the classification of artificial diamond samples. It will introduce the NV centre, its fluorescence properties and the process to create NV centres in diamond. The chapter will also introduce the concepts around optical magnetometry and the working principle that allows the NV centre to sense magnetic fields as well as other external quantities, such as temperature or pressure. Finally, it will include a brief overview of the defects that are found in artificial diamond or are created during the sample processing. Chapter 2 will describe the working principle of the instruments utilised in this work. I will list the details of each instrument and describe the methods used for the analysis of the data. Chapter 3 will introduce the background of modelling the fluorescence emission from NV centres. I will discuss the implementation of a computational model based on the Lindblad master equation to simulate the photo-physical dynamics of the NV centre. I will demonstrate the successful simulation of the saturation emission, fluorescence lifetime and optically detected magnetic resonance using the theoretical model. Finally, I will explore the theoretically predicted sensitivity of microwave-free magnetometry, based on the NV centre emission intensity change under an external magnetic field. Chapter 4 will demonstrate experimentally the microwave-free magnetometry technique. In relation to the study of microwave-free magnetometry, this chapter will explore an unexpected increase in emission intensity at high magnetic fields > 100mT. Combining the experimental observations and the theoretical simulation developed in the previous chapter, I will identify and discuss a phenomenological decrease in the NV centre transition rate from the triplet state to the singlet state. The chapter will conclude comparing the experimental and theoretical sensitivities associated with the microwave-free magnetometry technique. Chapter 5 will show the characterisation of a diamond sample at each step of the process to create NV centres. I will study the electron irradiation followed by separate annealing on a high-pressure high-temperature diamond sample. I will then characterise the sample employing visible and infrared absorption spectroscopy, as well as measure the fluorescence spectra and lifetime of the defects identified at each step. Chapter 6 will examine an alternative method to create NV centres in diamond. I will compare diamond samples irradiated and annealed separately with samples simultaneously irradiated and annealed (referred to as 'annealing in situ'). To compare the two processes, I will estimate the concentration of nitrogen defects and NV centres and calculate their N-to-NV conversion efficiency. Chapter 7 will discuss the effects of substitutional nitrogen on the properties of the NV centres in diamond. For this discussion, I will use a single-crystal diamond that shows the presence of 4 growth sectors. Each growth sector is characterised by a different nitrogen concentration, as I will demonstrate at the beginning of the chapter. I will focus on 4 main properties affected by the concentration of nitrogen in the different sectors: the balance between the charge states of the NV centre, the overall fluorescence intensity from each sector, the fluorescence lifetime and its relation to non-radiative processes, and the magnetic field sensitivity obtained with the optically detected magnetic resonance technique. Chapter 8 will gather the results and conclusions of the entire body of work. It will also discuss future avenues of research in the topic of diamond processing and characterisations, to improve the existing optical sensing applications.
Abstract The rapid development of radio-frequency (RF) technologies requires tools which can efficiently monitor the electromagnetic landscape. Broadband real-time RF spectral analyzers need to operate at room temperature, with low power consumption and have a compact design for on-board device integration. Here we describe a Quantum Diamond Signal Analyzer (Q-DiSA) which detects RF signals over a tunable frequency range of 25 GHz with frequency resolution down to 1 MHz, a millisecond temporal resolution and a large dynamic range (40 dB). This approach exploits the room temperature spin properties of an ensemble of nitrogen-vacancy (NV) centers in diamond. Performance is enabled via our analyzer architecture which combines a specific diamond crystallographic cut with a simplified magnetic arrangement. This allows us to maintain the alignment of the magnetic field along the nitrogen-vacancy center axis whilst frequency tuning. These results demonstrate the potential of the Q-DiSA method for real-time broadband spectral analysis.
This thesis investigates the theoretical foundations, material properties, and experimental realization of quantum sensors based on the Nitrogen-Vacancy (NV) centers in diamond, a solid-state system that combines quantum coherence with room-temperature operability. The first chapter introduces the principles of quantum sensing, outlining the fundamental definitions, operating protocols, and physical processes underlying the quantum measurement paradigm. Emphasis is placed on the quantum sensing Hamiltonian and the distinction between various classes of quantum sensors. The second chapter focuses on diamond as a host material and on the NV center as a quantum defect. The electronic structure, optical transitions, and spin properties of the NV center are analyzed in detail. Several techniques of NV-based magnetometry are presented, including continuouswave and pulsed optically detected magnetic resonance (ODMR). The discussion also addresses coherence times (T1, T2, and T∗2 ) and methods of diamond synthesis together with NV-center generation through irradiation and annealing. Chapter three develops the physical theory describing the NV center. The Hamiltonian formalism includes spin-spin, hyperfine, and quadrupolar interactions, as well as couplings to external magnetic fields. The chapter further introduces the Bloch sphere representation, magnetic resonance phenomena, and models of lightmatter interaction, ranging from the semiclassical to the fully quantum Rabi framework. A seven-level rate-equation model is proposed to capture the spin dynamics and optical cycling processes of the NV center. The fourth chapter reports the experimental results. After describing materials and methods, several ODMR-based vector magnetometry experiments are presented, highlighting different spectral regimes and the dependence of resonance features on the external magnetic field geometry. Additional studies demonstrate temperature sensing and microwave-free magnetometry techniques for mapping magnetic field distributions. Sensitivity analysis and calibration procedures are also discussed.
We have realized a compact radiofrequency spectrum analyzer based on nitrogen-vacancy (NV) color centers in diamond. The RF spectral components are spatially encoded by means of a Zeeman shift of the NV resonances induced by a magnetic field gradient. The fluorescence of the NV centers is imaged on a CMOS detector array using a gradient index lens. The spectral components of the signal are revealed by the appearance of dark lines in the image. We observed frequencies up to 10 GHz, with a resolution of a few MHz and a RF sensitivity of the order of 100 µW.