Nitrogen-vacancy (NV) centers in diamonds play a large role in advanced quantum sensing with solid-state spins for potential miniaturized and portable application scenarios. With the temperature sensitivity of NV centers, the temperature fluctuations caused by the unknown environment and the system itself will mix with the magnetic field measurement. In this research, the temperature-sensitive characteristics of different diamonds, alongside the temperature noise generated by a measurement system, were tested and analyzed with a homemade NV magnetometer in a fiber-optic scheme. In this work, a multi-frequency synchronous manipulation method for resonating with the NV centers in all axial directions was proposed to compensate for the temperature fluctuations in a fibered NV magnetic field sensing scheme. The symmetrical features of the resonance lines of the NV centers, the common-mode fluctuations including temperature fluctuations, underwent effective compensation and elimination. The fluorescence change was reduced to 1.0% by multi-frequency synchronous manipulation from 5.5% of the single-frequency manipulation within a ±2 °C temperature range. Additionally, the multi-frequency synchronous manipulation improved the fluorescence contrast and the magnetic field measurement SNR through an omnidirectional manipulation scheme. It was very important to compensate for the temperature fluctuations, caused by both internal and external factors, to make use of the NV magnetometer in fiber-optic schemes' practicality. This work will promote the rapid development and widespread applications of quantum sensing based on various systems and principles.
Nitrogen-vacancy (NV) centers in diamonds play a large role in advanced quantum sensing with solid-state spins for potential miniaturized and portable application scenarios. With the temperature sensitivity of NV centers, the temperature fluctuations caused by the unknown environment and the system itself will mix with the magnetic field measurement. In this research, the temperature-sensitive characteristics of different diamonds, alongside the temperature noise generated by a measurement system, were tested and analyzed with a homemade NV magnetometer in a fiber-optic scheme. In this work, a multi-frequency synchronous manipulation method for resonating with the NV centers in all axial directions was proposed to compensate for the temperature fluctuations in a fibered NV magnetic field sensing scheme. The symmetrical features of the resonance lines of the NV centers, the common-mode fluctuations including temperature fluctuations, underwent effective compensation and elimination. The fluorescence change was reduced to 1.0% by multi-frequency synchronous manipulation from 5.5% of the single-frequency manipulation within a ±2 °C temperature range. Additionally, the multi-frequency synchronous manipulation improved the fluorescence contrast and the magnetic field measurement SNR through an omnidirectional manipulation scheme. It was very important to compensate for the temperature fluctuations, caused by both internal and external factors, to make use of the NV magnetometer in fiber-optic schemes' practicality. This work will promote the rapid development and widespread applications of quantum sensing based on various systems and principles.
Efficient collection of photoluminescence arising from spin dynamics of nitrogen vacancy (NV) centers in diamond is important for practical applications involving precise magnetic field or temperature mapping. These goals may be realized by the integration of nanodiamond particles with optical fibers and volumetric doping of the particles alongside the fiber core. That approach combines the advantages of robust axial fixation of NV diamonds with a direct spatial overlap of their fluorescence with the guided mode of the fiber. We developed a suspended core silicate glass fiber with 750 nm-diameter nanodiamonds located centrally in the 1.5 µm-core cross-section along its axis. The developed fiber probe was tested for its magnetic sensing performance in optically detected magnetic resonance measurements using a 24 cm-long fiber sample, with the NV excitation and fluorescence collection from the far ends of the sample and yielding optical readout contrast of 7% resulting in 0.5 µT·Hz-1/2 magnetic field sensitivity, two orders of magnitude better than in earlier designs. Thanks to its improved fluorescence confinement, the developed probe could find application in magnetic sensing over extended fiber length, magnetic field mapping or gradiometry.
Portable quantum sensors are crucial for developing practical quantum sensing and metrology applications. Fiberized nitrogen-vacancy (NV) centers in diamonds have emerged as one of the most promising candidates for compact quantum sensors. Nevertheless, due to the difficulty of coherently controlling the ensemble spin and noise suppression in a large volume, it often faces problems such as reduced sensitivity and narrowed bandwidth in integrated lensless applications. Here, we propose a fluorescence signal treatment method for NV spin ensemble manipulation by the exponential fitting of spin polarization processes, instead of integrating the photon emission. This enables spin state readout with a high signal-to-noise ratio and applies to the pulse sensing protocols for large-volume NV spins. Based on this, we further developed a fiberized diamond-based AC magnetometer. With an XY8-N dynamical decoupling pulse sequence, we demonstrated a T2-limited sensitivity of 8 pT/Hz and T1-limited frequency resolution of 90 Hz over a wide frequency band from 100 kHz to 3 MHz. This integrated diamond sensor leverages quantum coherence to achieve enhanced sensitivity in detecting AC magnetic fields, making it suitable for implementation in a compact and portable endoscopic sensor.
Among quantum sensors, the single nitrogen-vacancy (NV) defect in diamond has the highest sensitivity-to-size factor. For instance, a single-NV spin when used as a magnetometer could achieve sensitivities of the order of nT/Hz, while the dimensions of the sensor are merely atomic in size. The sensitivity is limited only by the photon shot noise. One method to boost the magnetometer sensitivity to pico-tesla scales is to use many NV defect centers as an ensemble sensor [Phys. Rev. X5, 041001 (2015)PRXHAE2160-330810.1103/PhysRevX.5.041001]. However, during the absorption-emission (fluorescence) optical cycles, the NV centers transfer a portion of the irradiation laser energy into phonons and heat the diamond matrix. This results in unintended fluorescence decrease, spin resonance lines shifts, and fluctuations. Hence, the advantages gained by packing a high density of NV centers are significantly reduced. Here we investigate the heat generation of ensemble NV centers in micrometer-sized diamond under 532 nm laser irradiation and its effects pertaining to sensing applications. These investigations help us to find strategies that mitigate the detrimental effects of heating and yet permits the use of ensemble NV defects for improved metrology applications.
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.
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.
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.