  We give instructions for the construction and operation of a simple apparatus
for performing optically detected magnetic resonance measurements on diamond
samples containing high concentrations of nitrogen-vacancy (NV) centers. Each
NV center has a spin degree of freedom that can be manipulated and monitored by
a combination of visible and microwave radiation. We observe Zeeman shifts in
the presence of small external magnetic fields and describe a simple method to
optically measure magnetic field strengths with a spatial resolution of several
microns. The activities described are suitable for use in an advanced
undergraduate lab course, powerfully connecting core quantum concepts to
cutting edge applications. An even simpler setup, appropriate for use in more
introductory settings, is also presented.
