  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.
