Electron paramagnetic resonance spectroscopy (EPR) is among the most important analytical tools in physics, chemistry, and biology. The emergence of nitrogen-vacancy (NV) centers in diamond, serving as an atomic-sized magnetometer, has promoted this technique to single-spin level, even under ambient conditions. Despite the enormous progress in spatial resolution, the current megahertz spectral resolution is still insufficient to resolve key heterogeneous molecular information. A major challenge is the short coherence times of the sample electron spins. Here, we address this challenge by employing a magnetic noise-insensitive transition between states of different symmetry. We demonstrate a 27-fold narrower spectrum of single substitutional nitrogen (P1) centers in diamond with linewidth of several kilohertz, and then some weak couplings can be resolved. Those results show both spatial and spectral advances of NV center-based EPR, and provide a route towards analytical (EPR) spectroscopy at single-molecule level.
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