  Antiferromagnetic insulators (AFIs) are of significant interest due to their
potential to develop next-generation spintronic devices. One major effort in
this emerging field is to harness AFIs for long-range spin information
communication and storage. Here, we report a non-invasive method to optically
access the intrinsic spin transport properties of an archetypical AFI
{\alpha}-Fe2O3 via nitrogen-vacancy (NV) quantum spin sensors. By NV
relaxometry measurements, we successfully detect the time-dependent
fluctuations of the longitudinal spin density of {\alpha}-Fe2O3. The observed
frequency dependence of the NV relaxation rate is in agreement with a
theoretical model, from which an intrinsic spin diffusion constant of
{\alpha}-Fe2O3 is experimentally measured in the absence of external spin
biases. Our results highlight the significant opportunity offered by NV centers
in diagnosing the underlying spin transport properties in a broad range of
high-frequency magnetic materials, which are challenging to access by more
conventional measurement techniques.
