Super-resolution covariance magnetometry with NV centers: detecting field correlators at nanometer distances

Nitrogen vacancy (NV) centers in diamond are robust quantum sensors with long spin coherence times that can be used for high-sensitivity local magnetometry with nanoscale resolution. Recently, covariance magnetometry has been developed in which two or more NV centers are simultaneously measured to detect the presence of correlations in fluctuating magnetic fields across space or time. So far, covariance magnetometry has been limited to the detection of NV centers that are spatially resolved, with separations beyond the optical diffraction limit. I will describe methods to detect the presence of correlated noise using two NV centers that are unresolved, enabling covariance magnetometry at nanometer length scales. These methods include a new phase cycling protocol for low magnetic fields, the use of local 13C nuclei as spin handles for phase cycling at high magnetic fields, and entangled Bell-pair states as a sensing resource for closely-spaced NV centers.