Quantum Sensing with NV Centers in Diamond

Abstract

This thesis is concerned with studies involving quantum sensing based on Nitrogen-Vacancy (NV) center in diamond. Essentially, it attempts to address two important problems. First, enhancing the sensitivity of a single NV center to external static magnetic fields, and second, probing the phase transitions in a soft condensed matter system using a near surface NV center. NV center's sensitivity to time-varying or AC external magnetic fields is in the range of a few nT and is limited by coherence time of NV. For various applications, including the bio-magnetic measurements, it is necessary to sense nearly static or DC fields rather than the AC fields. However, in diamond samples with natural abundance 13 C concentration (1.1%), the best reported sensitivity to DC fields is in µT range and is limited by short lived spin dephasing time of NV. In the present work, a novel hybrid magnetometer consisting of a ferromagnetic material and a single NV center is utilized to achieve DC field sensitivity down to tens of nT. The ferromagnetic material exhibits a property known Giant magneto-impedance (GMI) and is sensitive to DC fields under certain conditions. The achievable sensitivity of GMI sensors is in the range of pT. By positioning a GMI microwire in close vicinity of a NV center, magnetic interaction between the microwire and NV center can be realized. In the presence of minute changes in the DC fields in the surroundings, the GMI wire responds and encodes information to the NV center. By employing a standard magnetometry sequence on NV this information can be readout. As the sequence relies on long lived NV coherence time, its overall sensitivity could be enhanced by over two orders of magnitude. Second study deals with detecting the temperature driven phase transitions in a soft matter system, namely a liquid crystal (LC) material. The chosen LC material shows distinct ordered phases close to room temperature. By varying the temperature, transitions from solid-like phases to liquid-like phases in a thin layer of LC can be induced. The NV sensors located at a few nm depths detect these transitions in terms of changes in the spin noise signal emanating from nanoscopic volumes containing LC molecules above the diamond surface. Temperature plays a key role in determining the soft matter properties. Since NV centers are also known as nanoscale temperature sensors, it is possible to tune the temperature precisely to the transition points. This way, NV based method is demonstrated as a dual mode sensing for studying soft matter systems at nanoscale, with a control over temperature. The work aims at extending NV centers as novel probes for exploring soft matter systems and address some important questions in that area.