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.
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.
When the word diamond comes up, the majority of people thinks about engagement rings or gemstones. In fact, it is additionally, one of the most promising host materials for the implementation of advanced technology, exploiting the nature of quantum physics. Such quantum technologies have disruptive potential, ones the quantum effects can be applied in macroscopic, commercial products. The nitrogen-vacancy (NV) centre in diamond is a promising candidate to perform such tasks in the fields of quantum computation, quantum cryptography or metrology. Especially in the field of quantum sensing, the NV sensor impresses with nano-scale spatial resolution paired with high sensitivity. The electronic spin of the atomic-sized point defect can be initialised, manipulated and read-out at ambient conditions and coherence lifetimes are comparably long. Diamond magnetometers based on negatively charged NV centres have shown to be very attractive, and the sensitivity goes down to the single-atom level. In order to exploit such a high sensitivity level for sensing prospects, in this thesis, the NV centres are fabricated well confined in a certain distance to the diamond surface, since the detectable field usually decays with the third power of the distance between sensor and target. The surface depicts a fracture of the spin wise protecting diamond lattice and charge fluctuations affect the stability of NV centres, which are overcome by diamond surface termination. In order to improve the sensitivity further, the sensor density, measurement contrast and interaction time are increased. The nitrogen doping technique, which is applied during a plasma-enhanced chemical vapour deposition (PECVD) diamond growth process is used to fabricate NV centre ensembles aligned along one diamond crystal axis, thereby improving the measurement contrast by a factor up to four. The interaction time limiting spin bath is analysed using the sensor capabilities of the NVs, which leads to enhanced growth strategies and the fabrication of sensitivity optimised NV centre ensembles for magnetometry applications.
The negative Nitrogen-Vacancy (NV) colour centre in diamond is an atomic-level impurity that is remarkably well suited for quantum information processing and quantum metrology applications. Underlying these properties is the fact that optical illumination causes the NV centre to preferentially populate one of its three spin levels. The mechanism that gives rise to this spin polarisation has not been well understood, despite intense research interest in the NV centre and its applications. This thesis arrives at an account of the spin polarisation mechanism, although a detailed theoretical treatment remains outstanding. The energy level structure of the NV centre is investigated using techniques of solid state spectroscopy. These include absorption and photoluminescence measurements made on diamonds at cryogenic and room temperatures in the presence of uniaxial stress, magnetic fields, and microwaves. Pulsed light experiments allow the dynamic processes within the NV centre to be measured. It is shown that at room temperature the orbital doublet excited state behaves like an orbital singlet, and this description is elegantly confirmed by single-site observations. This picture of the excited state is inconsistent with cryogenic measurements, and its behaviour is examined across a range of temperatures in order to resolve this tension. It is shown that electron-vibration interaction accounts for the quenching of orbital properties in the excited state with temperature. A new optical transition within the NV centre is reported at a wavelength of 1042 nm. It is established that this transition occurs between the intermediate singlet levels which are thought to provide the decay pathway that produces spin polarisation. The results are consistent with this understanding, and the order and properties of these singlet levels are established. Electron-vibration interaction is observed to be important within the singlet system. It is shown that electron-vibration interaction is an important detail in the NV optical pumping cycle. Taking it into consideration leads to the first plausible physical description of the spin polarisation mechanism. -- provided by Candidate.
The nitrogen-vacancy (NV) center in a nanodiamond (ND) crystal is a promising material for quantum information processing, sensing, and computing applications. It is one of the best candidate materials for quantum sensing and metrology expected to work at elevated temperatures and pressures conditions. We computationally show the effect of strain on the defect band edges and band gaps in the NV center diamond. A low energy Hamiltonian is developed for the ±1 spin manifold at the ground state. We show the quantum sensing device is a few orders of magnitude superior in sensing than the traditional optical sensing devices. We also discuss experimental results from the optically detected magnetic resonance (ODMR) and the spin relaxometry for the field sensing applications. The presentation concludes by providing a model for free spins detection of the rare earth ions.
This dissertation investigates applications of the Nitrogen vacancy in diamond (NV). The NV is a unique quantum system allowing for optical polarization and read out even at ambient conditions. Its electron spin (S=1) has coherence times in the order of milliseconds and is therefore an ideal candidate to investigate the foundations of quantum mechanics as well as exploiting quantum effects in metrology and information processing applications. To utilize the NV for quantum technologies, first the interaction of the NV with its surroundings has to be investigated and controlled. Here dynamical decoupling sequences (CPMG) could be utilized to extend the NV coherence time to a few milliseconds by decoupling the NV from its surrounding spin bath (given by the 13C isotope, natural abundance 1.1%). But not only decoupling sequences can be used to suppress the spin bath interaction, also eigen state tailoring can suppress the bath interaction. As long as strain is the dominant interaction, the NV eigen energies are not susceptible to small magnetic field changes. Therefore the magnetic dipole interaction with the spin bath is suppressed. But the spin bath is not only a nuisance limiting NV coherence times, but also a potential resource for quantum technologies. In order to harness the individual 13C nuclear spins, their interaction with the NV and among themselves has to be investigated. To not be limited by the NV coherence times, a new spectroscopy method, only limited by the NV lifetime, was developed. This can in principle be extended to the second range by using a nuclear spin memory. First advances in quantum technology were demonstrated in the field of metrology, where the Zeeman effect of the NV center allowed for precise nanoscale measurement of magnetic fields. Here this concept was extended to us the Stark effect to detect electric fields. A sensitivity of η=142.6±3.6 V/(cmHz1/2) was demonstrated. This is equivalent to the detection of a single elemental charge at a distance of 150 nm in one second. By using two NVs with a distance of less then 20 nm, a demonstration of nanoscale single fundamental charge detection was feasible. Here one NV was used as a controllable fundamental charge while the other NV was used as sensor NV, detecting the electric field of the fundamental charge. This allowed for the detection of a single electronic charge at ambient conditions. Another keystone of quantum technology is the reliable on demand or heralded creation of entanglement. In this dissertation, the successful entanglement of two electron spins in solid state at ambient conditions is demonstrated. The fundamental interaction used to create an entangled state was dipolar coupling between the two electron spins. In order to be able to harness weaker coupling strengths, an entanglement protocol based on dynamical decoupling was developed. A fidelity of F=0.67±0.04 was demonstrated. By using the intrinsic nuclear spins of the nitrogen it was possible to store the entangled state on the millisecond time scale. However, the theoretical limit for the fidelity given by the polarization and the coherence times is F=0.849. The discrepancy can be explained by pulse errors. These can be avoided by using optimal control yielding a fidelity of F=0.824±0.015. Also optimal control allowed for nuclear spin entanglement by storing the electron spin entanglement on the nitrogen nuclear spins. These experiments are first steps towards a room temperature quantum register, where the electron spins are used as bus to transport information and the nuclear spins act as a memory.
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.
In modern-day quantum metrology, quantum sensors are widely employed to detect weak magnetic fields or nanoscale signals. Quantum devices, exploiting quantum coherence, are inevitably connected to physical constants and can achieve accuracy, repeatability, and precision approaching fundamental limits. As a result, these sensors have shown utility in a wide range of research domains spanning both science and technology. A rapidly emerging quantum sensing platform employs atomic-scale defects in crystals. In particular, magnetometry using nitrogen-vacancy (NV) color centers in diamond has garnered increasing interest. NV systems possess a combination of remarkable properties, optical addressability, long coherence times, and biocompatibility. Sensors based on NV centers excel in spatial resolution and magnetic sensitivity. These diamond-based sensors promise comparable combination of high spatial resolution and magnetic sensitivity without cryogenic operation. The above properties of NV magnetometers promise increasingly integrated quantum measurement technology, as a result, they have been extensively developed with various protocols and find use in numerous applications spanning materials characterization, nuclear magnetic resonance (NMR), condensed matter physics, paleomagnetism, neuroscience and living systems biology, and industrial vector magnetometry. In this chapter, NV centers are explored for magnetic sensing in a number of contexts. In general, we introduce novel regimes for magnetic-field probes with NV ensembles. Specifically, NV centers are developed for sensitive magnetometers for applications where microwaves (MWs) are prohibitively invasive and operations need to be carried out under zero ambient magnetic field. The primary goal of our discussion is to improve the utility of these NV center-based magnetometers.