Scanning Gradiometry of Magnetic and Electric Fields with a Single Nitrogen-Vacancy Sensor

We report on the development of quantum sensing protocols for the quantitative measurement of magnetic and electric field gradients with nanoscale spatial resolution. With a single nitrogen-vacancy (NV) center at the apex of a scanning diamond probe, we synchronize coherent spin manipulations and optical readout measurements with the mechanical oscillations of the scanning probe. This process converts the sample's static magnetic/electric field into an AC gradient field which can be measured with increased sensitivity while rejecting low frequency noise. We switch between measuring magnetic and electric field gradients by changing the alignment of a milli-Tesla strength bias magnet with respect to the NV symmetry axis. Notably, this technique provides roughly an order of magnitude improvement in magnetic field sensitivity and provides a method to overcome the significant electric field screening currently hindering static field NV electrometry. We demonstrate our scanning gradiometry technique by imaging magnetic and electric field gradients of several well-studied material systems.