  Quantum sensing based on nitrogen-vacancy (NV) centers in diamond has been
developed as a powerful tool for microscopic magnetic resonance. However, the
reported sensor-to-sample distance is limited within tens of nanometers because
the signal of spin fluctuation decreases cubically with the increasing
distance. Here we extend the sensing distance to tens of micrometers by
detecting spin polarization rather than spin fluctuation. We detected the
mesoscopic magnetic resonance spectra of polarized electrons of a
pentacene-doped crystal, measured its two typical decay times and observed the
optically enhanced spin polarization. This work paves the way for the NV-based
mesoscopic magnetic resonance spectroscopy and imaging at ambient conditions.
