  Quantum diamond magnetometers using lock-in detection have successfully
detected weak bio-magnetic fields from neurons, a live mammalian muscle, and a
live mouse heart. This opens up the possibility of quantum diamond
magnetometers visualizing microscopic distributions of the bio-magnetic fields.
Here, we demonstrate a lock-in-based wide-field quantum diamond microscopy,
achieving a mean volume-normalized per pixel sensitivity of 43.9 $\mathrm{nT\mu
m^{1.5}/Hz^{0.5}}$. We optimize the sensitivity by implementing a double
resonance with hyperfine driving and magnetic field alignment along the
$<$001$>$ orientation of the diamond. Additionally, we show that sub-ms
temporal resolution ($\sim$ 0.4 ms) can be achieved while keeping the per-pixel
sensitivity at a few tens of nanotesla per second using quantum diamond
microscopy. This lock-in-based diamond quantum microscopy could be a step
forward in mapping functional activity in neuronal networks in micrometer
spatial resolution.
