Experimental Realization of Improved Magnetic Sensing and Imaging in Ensembles of Nitrogen Vacancy Centers in Diamond

Abstract

Nitrogen-vacancy color centers in diamond constitute a leading quantum sensing platform, with particularly diverse applications in magnetometry. The negatively-charged NV center (NV) exhibits a long-lived electronic spin-triplet ground state with magnetically sensitive sublevels under ambient conditions. The NV spin state can be prepared optically and readout via spin-state-dependent fluorescence. Additionally, it is possible to engineer ensembles of NV at suitably high densities in favorable geometries. Together, these properties make NV- ensembles particularly advantageous for magnetic sensing and imaging applications spanning condensed matter physics, the life sciences, nuclear magnetic resonance, Earth and planetary science, and magnetic navigation and mapping. Despite the broad range of demonstrated applications, the magnetic sensitivities achieved using NV ensemble magnetometers remain orders of magnitudes from fundamental limits. Optical readout fidelities much less than unity, typical ensemble dephasing times over two orders of magnitude shorter than the spin lifetime, and ionization under strong optical illumination all contribute to limiting the NV ensemble magnetic sensitivity. Furthermore, the strength of NV centers as multi-modal sensors of magnetic and electric fields, temperature, crystal stress, and external pressure can become a vulnerability, resulting in poor magnetic specificity (the ability to isolate magnetic signals from non-magnetic sources) and inhibiting the scalable production of diamond material with reproducible properties.

This work pursues two complementary avenues towards improved NV ensemble magnetic sensitivity and specificity. First, the development of a characterization toolbox using NV centers as probes of their local environment provides feedback and informed metrics for the production of synthetic diamond material tailored to NV ensemble applications. This work is conducted in close collaboration with dia- mond manufacturers. Sources of heterogeneity are of particular concern; for example, the introduction of non-uniform crystal stress during synthesis results in pernicious NV ensemble dephasing. This crystal stress heterogeneity varies dramatically within and between samples, preventing the scalable production of material with consistent properties. Iterative efforts to characterize NV-diamond material and feedback on the synthesis methods provides a path toward improved NV ensemble magnetometer performance via material engineering. Second, this thesis reports the experimental realization of measurement protocols designed to simultaneously ameliorate the deficits of existing diamond material and relax requirements on diamond material under development. Double quantum coherence magnetometry is employed to mitigate the consequences of crystal stress inhomogeneity as well as temperature drift. Meanwhile, NV ensemble dephasing induced by dipolar interactions between the paramagnetic diamond spin bath and NV sensor spins is suppressed using resonant control of the constituent bath spins. These techniques provide improved magnetic sensitivity and specificity, including the demonstration of record volume-normalized magnetic sensitivity for a wide-field quantum diamond microscope (QDM).