Dependence of high density nitrogen-vacancy center ensemble coherence on electron irradiation doses and annealing time

Negatively charged nitrogen-vacancy (NV^-) center ensembles in diamond have proved to have great potential for use in highly sensitive, small-package solid-state quantum sensors. One way to improve sensitivity is to produce a high-density NV^- center ensemble on a large scale with a long coherence lifetime. In this work, the NV^- center ensemble is prepared in type-Ib diamond using high energy electron irradiation and annealing, and the transverse relaxation time of the ensemble-T ₂-was systematically investigated as a function of the irradiation electron dose and annealing time. Dynamical decoupling sequences were used to characterize T ₂. To overcome the problem of low signal-to-noise ratio in T ₂ measurement, a coupled strip lines waveguide was used to synchronously manipulate NV^- centers along three directions to improve fluorescence signal contrast. Finally, NV^- center ensembles with a high concentration of roughly 10¹⁵ mm^-3 were manipulated within a ∼10 μs coherence time. By applying a multi-coupled strip-lines waveguide to improve the effective volume of the diamond, a sub-femtotesla sensitivity for AC field magnetometry can be achieved. The long-coherence high-density large-scale NV^- center ensemble in diamond means that types of roomerature micro-sized solid-state quantum sensors with ultra-high sensitivity can be further developed in the near future.