  Wide-field magnetometry can be realized by imaging the optically-detected
magnetic resonance of diamond nitrogen vacancy (NV) center ensembles. However,
NV ensemble inhomogeneities significantly limit the magnetic-field sensitivity
of these measurements. We demonstrate a double-double quantum (DDQ) driving
technique to facilitate wide-field magnetic imaging of dynamic magnetic fields
at a micron scale. DDQ imaging employs four-tone radio frequency pulses to
suppress inhomogeneity-induced variations of the NV resonant response. As a
proof-of-principle, we use the DDQ technique to image the dc magnetic field
produced by individual magnetic-nanoparticles tethered by single DNA molecules
to a diamond sensor surface. This demonstrates the efficacy of the diamond NV
ensemble system in high-frame-rate magnetic microscopy, as well as
single-molecule biophysics applications.
