  The ability to measure the passage of electrical current with high spatial
and temporal resolution is vital for applications ranging from inspection of
microscopic electronic circuits to biosensing. Being able to image such signals
passively and remotely at the same time is of high importance, to measure
without invasive disruption of the system under study or the signal itself. A
new approach to achieve this utilises point defects in solid state materials,
in particular nitrogen vacancy (NV) centres in diamond. Acting as a high
density array of independent sensors, addressable opto-electronically and
highly sensitive to factors including temperature and magnetic field, these are
ideally suited to microscopic widefield imaging. In this work we demonstrate
such imaging of signals from a microscopic lithographically patterned circuit
at the micrometer scale. Using a new type of lock-in amplifier camera, we
demonstrate sub-millisecond (up to 3500 frames-per-second) spatially resolved
recovery of AC and pulsed electrical current signals, without aliasing or
undersampling. Finally, we demonstrate as a proof of principle the recovery of
synthetic signals replicating the exact form of signals in a biological neural
network: the hippocampus of a mouse.
