  We present high-resolution optically detected magnetic resonance (ODMR)
spectroscopy on single nitrogen-vacancy (NV) center spins in diamond at and
around zero magnetic field. The experimentally observed transitions depend
sensitively on the interplay between the microwave (MW) probing field and the
local intrinsic effective field comprising strain and electric fields, which
act on the NV spin. Based on a theoretical model of the magnetic dipole
transitions and the MW driving field, we extract both the strength and the
direction of the transverse component of the effective field. Our results
reveal that for the diamond crystal under study, strain is the dominant
contribution to the effective field. Our experiments further yield a method for
MW polarization analysis in a tunable, linear basis, which we demonstrate on a
single NV spin. Our results are of importance to low-field quantum sensing
applications using NV spins and form a relevant addition to the ever-growing
toolset of spin-based quantum sensing.
