The integration of Nitrogen-Vacancy color centers into diamond anvil cells has opened the door to quantum sensing at megabar pressures. Despite a multitude of experimental demonstrations and applications ranging from quantum materials to geophysics, a detailed microscopic understanding of how stress affects the NV center remains lacking. In this work, using a combination of first principles calculations as well as high-pressure NV experiments, we develop a complete description of the NV's optical properties under general stress conditions. In particular, our ab initio calculations reveal the complex behavior of the NV's inter-system crossing rates under stresses that both preserve and break the defect's symmetry. Crucially, our proposed framework immediately resolves a number of open questions in the field, including: (i) the microscopic origin of the observed contrast-enhancement in (111)-oriented anvils, and (ii) the surprising observation of NV contrast-inversion in certain high-pressure regimes. Our work lays the foundation for optimizing the performance of NV high-pressure sensors by controlling the local stress environment, and more generally, suggests that symmetry-breaking stresses can be utilized as a novel tuning knob for generic solid-state spin defects.
The integration of Nitrogen-Vacancy color centers into diamond anvil cells has opened the door to quantum sensing at megabar pressures. Despite a multitude of experimental demonstrations and applications ranging from quantum materials to geophysics, a detailed microscopic understanding of how stress affects the NV center remains lacking. In this work, using a combination of first principles calculations as well as high-pressure NV experiments, we develop a complete description of the NV's optical properties under general stress conditions. In particular, our ab initio calculations reveal the complex behavior of the NV's inter-system crossing rates under stresses that both preserve and break the defect's symmetry. Crucially, our proposed framework immediately resolves a number of open questions in the field, including: (i) the microscopic origin of the observed contrast-enhancement in (111)-oriented anvils, and (ii) the surprising observation of NV contrast-inversion in certain high-pressure regimes. Our work lays the foundation for optimizing the performance of NV high-pressure sensors by controlling the local stress environment, and more generally, suggests that symmetry-breaking stresses can be utilized as a novel tuning knob for generic solid-state spin defects.
The diamond anvil cell-based high-pressure technique is a unique tool for creating new states of matter and for understanding the physics underlying some exotic phenomena. In situ sensing of spin and charge properties under high pressure is crucially important but remains technically challenging. While the nitrogen-vacancy (NV) center in diamond is a promising quantum sensor under extreme conditions, its spin dynamics and the quantum control of its spin states under high pressure remain elusive. In this study, we demonstrate coherent control, spin relaxation, and spin dephasing measurements for ensemble NV centers up to 32.8 GPa. With this in situ quantum sensor, we investigate the pressure-induced magnetic phase transition of a micron-size permanent magnet Nd 2 Fe 14B sample in a diamond anvil cell, with a spatial resolution of ∼2 μm, and sensitivity of ∼20 μT/Hz 1/2. This scheme could be generalized to measure other parameters such as temperature, pressure and their gradients under extreme conditions. This will be beneficial for frontier research of condensed matter physics and geophysics.
Nitrogen-Vacancy (NV) centers in diamond have been established as exceptionally versatile probes for quantum technologies from sensing to simulation.Owing to the versatility of diamond fabrication, NVs can be integrated into probes with nanoscale resolution and superb sensitivity.\nHowever, because the diamond is host to many other types of defects, the local environment of the NV is unique for each NV, which poses challenges for any devices instrumenting NVs.\nThis thesis contributes to the future of NV technologies along two essential directions.\nFirst, we work towards a more complete understanding of the local charge and strain environments of the NV.\nIn addition to permitting the calibration of these effects for NV probes, we suggest potential uses for the local charges and strain that had previously been considered hindrances.\nSecond, we develop new technologies for NV sensing and simulation.\nOur work integrating NVs into diamond anvil cells opens the door to detecting high pressure phenomena using the NVs as all-in-one sensors.\nFinally, our study of the coherence regimes of the local defect environment of the NV is a step towards versatile quantum simulation in dimensions lower than three.
The nitrogen vacancy (NV) center has emerged as a powerful quantum sensor in high-pressure research, with the observation of optically detected magnetic resonance at megabar pressures. However, some aspects of NV physics require further investigation to optimize the development of NV-based sensing under pressure. Here, we study both experimentally and theoretically the optical properties of the NV center under hydrostatic pressure. We investigate the evolution of the zero-phonon line (ZPL) position, radiative lifetimes, optical lineshapes, and photoionization thresholds of the NV center under pressures up to ~120 GPa. We also provide spectroscopic guidelines for performing high-pressure optical experiments. Our results confirm that the NV center remains a robust quantum sensor under extreme hydrostatic pressures, especially for magnetic characterization.
When high static pressure is applied to a material, the arrangement of its atoms and electrons can dramatically change to allow higher density, thus leading to the creation of new phases or modifying electronic and magnetic properties of solids. Pressures of tens of kilobars (gigapascals) or even megabars can be achieved in laboratories using Diamond Anvil Cells (DACs), at the expense of working with samples of minute size. Recent studies have claimed to have uncovered materials, super-hydrides, retaining superconductivity close to room temperature under extreme pressure conditions. However, performing reliable measurements of the hallmarks of superconductivity on these samples synthesized at high-pressure is remarkably challenging, sometimes leading to controversial results. In this thesis, we chose to pursue the development of a new high-pressure optical magnetic measurement method based on the adaptation of a mature quantum technology to the DAC: Nitrogen-Vacancy (NV) centers of diamond. We fabricate ensembles of these color centers inside the tip of a diamond anvil, using them as local quantum magnetic probes based on their spin-dependent photoluminescence properties. We show that NV centers can be used to spatially map the Meissner effect in the presence of a cuprate superconductor at high pressure and record the sample's critical temperature evolution with pressure up to 31 GPa. We then systematically study the NV centers' magnetic sensing capabilities at high pressure, identifying non-hydrostatic anvil stress as a limitation above 40 to 50 GPa. Finally, we propose an anvil micro-structuring strategy lifting this constraint and enabling efficient NV magnetometry at least up to 130 GPa, paving the way toward a future settlement of the super-hydride dispute by this new diagnosis method.
Hydrostatic deformation is an effective approach for tuning the quantum properties of color centers in diamond, with significant implications for quantum sensing, computing, and communication. Compared to the widely studied nitrogen-vacancy (NV) centers, silicon-vacancy (SiV) centers exhibit more than a tenfold increase in coherent photon emission. In this work, we investigate the effects of hydrostatic pressure and tension on the SiV center in diamond using first-principles calculations with the r2SCAN meta-GGA (Generalized Gradient Approximation) functional. We demonstrate that under hydrostatic tension corresponding to an isotropic expansion exceeding 4%, the SiV center undergoes spontaneous symmetry breaking from the inversion-symmetric D3d structure to the asymmetric C3v configuration, similar to that of the NV center. Within the hydrostatic compression and tension range corresponding to isotropic deformations of −8%–4%, the optical properties and hyperfine parameters of the SiV center change monotonically, indicating promising potential for pressure- or deformation-sensing applications. A microscopic explanation of these trends is provided from an electronic structure perspective. The r2SCAN meta-GGA functional shows high accuracy in calculating hyperfine parameters, in agreement with experimental results. This study enhances our understanding of the optical properties and hyperfine interactions of SiV defects in diamond, laying the groundwork for their potential use in hydrostatic pressure or strain sensing applications.
Quantum technologies offer revolutionary new ways to perform metrology and process information. However, successfully exploiting quantum devices for new practical technology is a challenging problem. Due to the fragile nature of quantum states, precision measurements and operations using quantum objects are often confined to systems that are well protected from their environment. This can limit the practical use of such quantum devices. A quantum tool that can successfully and simply operate in ambient conditions would provide major advances in quantum technology. In the past decade or so, the nitrogen-vacancy (NV) centre in diamond has proven itself to be a remarkably powerful tool for nanoscale quantum sensing and quantum information processing in ambient conditions. Despite these achievements, there are still several fundamental features of the NV centre which are not completely understood. This thesis addresses these areas in two parts, firstly the mechanical properties and secondly the thermal properties of the NV centre. The effect of crystal stress or strain on the spin resonances on the NV centre ground state is theoretically described and then experimentally characterised; correcting previously contradictory attempts to explain the observed behaviour. The utility of this knowledge is demonstrated by force sensing in a microscopic diamond cantilever using a single NV centre. New unique concepts of force sensing and metrology based on the NV spin-mechanical interaction are explored. The thermal properties of the NV centre’s optical and spin resonances are theoretically described and experimentally characterised; providing the first successful description that details the origin of the effect of temperature on the spin resonance. Furthermore, the atomscopic changes in the NV centre's electronic orbitals due to the effect of crystal distortion are directly probed. This complete understanding of the NV centre's mechanical and thermal behaviour enables metrology that spans the full magnetic-temperature-pressure range of the NV centre. For example, this is ideally suited to studying superconducting phase changes in high-pressure materials. A proof of principle measurement of phase changes in superconductors is demonstrated and new magnetic-temperature-pressure metrology devices are discussed. Magnetic circular dichroism (MCD) spectroscopy measurements are used to unpick some of the remaining mysteries of the NV centre. The magnetic structure of the singlet levels are directly measured for the first time and a large quenching of orbital angular momentum is observed. This provides further evidence of a Jahn-Teller interaction and its role in the lower inter-system crossing. These MCD observations greatly enhance the knowledge of the poorly understood, but critically important lower inter-system crossing. Using MCD the fine-structure of the NV0, ground and excited states are measured for the first time and the reasons of their absence from previous measurements are discussed. The observation of the fine structure of NV0, ground state has been a long-standing mystery of the NV centre, this information will enable the pursuit of new applications of the NV centre that also incorporate NV0.