Highly sensitive quantum magnetometry using Nitrogen-Vacancy centers in diamond

The work presented in this thesis deals with magnetic field sensing using the nitrogen-vacancy (NV) defect center in diamond. The NV center is a solid-state defect in diamond with a level structure and properties that render its electron spin state sensitive to many environmental factors including magnetic fields.

The NV center can be used for sensing even under ambient conditions, and the diamond substrate is both mechanically stable and chemically inert. These properties would represent significant advantages compared to existing quantum magnetometers if the sensitivity of NV magnetometers were higher. The ultimate goal of the presented work was thus to explore and investigate various different ways to improve the sensitivity of NV magnetometry.

Ensembles of NV centers yield higher sensitivity than single NV centers, but controlling an ensemble is complicated by inhomogeneous broadening of the transition frequencies and drive field inhomogeneities. Smooth optimal control theory was used to design shaped control pulses that are robust against inhomogeneous broadening and drive amplitude variations.

Furthermore, the theory was expanded to explicitly include the hyperfine splitting of the NV center electron spin states. The resulting optimal control pulses were found theoretically and shown experimentally to yield significant improvements in the sensitivity compared to the best equivalent standard control pulses.

The typical sensing approach based on measuring changes in the red fluorescence is limited by the need to measure a low contrast on a bright background. These limitations can be avoided by utilizing the green absorption to perform laser threshold magnetometry. A setup where the drive-dependent change in green absorption is used to push an external laser cavity across the lasing threshold was proposed and theoretically investigated.

The predicted sensitivity was found to reach the pT/pHz range for realistic optimal parameters. However, it was also shown that the effect of amplified spontaneous emission near lasing threshold can significantly reduce the achievable sensitivity. Different sensing schemes are affected differently by inhomogeneous broadening (IHB) and drive amplitude variations (DAV), which makes it challenging to select the optimal scheme for a given situation.

The maximum achievable sensitivity of the three most commonly used low-frequency sensing schemes was simulated and compared for different levels of IHB and DAV. The three schemes were continuouswave (CW) optically detected magnetic resonance (ODMR), π-pulse ODMR and Ramsey interferometry. It was found that Ramsey interferometry only yields the best sensitivity for low inhomogeneous broadening, while CW ODMR yields the best sensitivity for large drive amplitude variations. π-pulse ODMR was found to yield the best sensitivity when the inhomogeneous broadening is not low and the drive amplitude variations are not large.