Quantum Metrology with Atoms, NV Centres, and Mechanical Oscillators

M. S. J. Barson

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.

Karol Gietka

Seyed Mohammad Hosseiny, Jamileh Seyed‐Yazdi, Milad Norouzi +1 more

Lachlan J. Rogers

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.

Marcus W. Doherty, B Neil B. Manson, B Paul Delaney +3 more

The nitrogen-vacancy (NV) colour centre in diamond is an important physical system for emergent quantum technologies, including quantum metrology, information processing and communications, as well as for various nanotechnologies, such as biological and sub-diffraction limit imaging, and for tests of entanglement in quantum mechanics. Given this array of existing and potential applications and the almost 50 years of NV research, one would expect that the physics of the centre is well understood, however, the study of the NV centre has proved challenging, with many early assertions now believed false and many remaining issues yet to be resolved. This review represents the first time that the key empirical and ab initio results have been extracted from the extensive NV literature and assembled into one consistent picture of the current understanding of the centre. As a result, the key unresolved issues concerning the NV centre are identified and the possible avenues for their resolution are examined.

Satvika Bandarupally