The mechanical and thermal properties of the nitrogen-vacancy centre in diamond

Abstract

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.