  The nitrogen-vacancy (NV) center in diamond allows room-temperature
wide-field quantum magnetometry and metrology for a small volume, which is an
important technology for applications in biology. Although coherence of the NV
center has a limited frequency resolution of diamond magnetometry to 10-100
kHz, recent studies have shown that a phase sensitive protocol can beat the
coherence limit on a confocal setup. Here, we report a new measurement
protocol, "iQdyne," for improving the frequency resolution of wide-field
imaging beyond the coherence limit of the NV center. We demonstrate wide-field
magnetometry with a frequency resolution of 238 mHz and a magnetic sensitivity
of 65 nT/sqrtHz, which are superior to the conventional XY8-based technique,
which paves the way to in vivo microscale nuclear magnetic resonance imaging.
We find that the experimental performance of iQdyne agrees well with that of an
analytical model.
