Probing condensed matter systems with nanoscale covariance magnetometers based on diamond quantum sensors*

Correlated phenomena play a central role in condensed matter physics, but in many cases, there are no tools available for measurements of correlations at the relevant length scales (nanometers - microns). We recently demonstrated that nitrogen vacancy (NV) centers in diamond can be used as point sensors for measuring two-point magnetic field correlators, in contrast to standard techniques where the magnetic field is measured by averaging sequential measurements of single NV centers, or by spatial averaging over ensembles of many centers. This novel quantum sensing platform allows us to measure new physical quantities that are otherwise inaccessible with current tools, particularly in condensed matter systems where two-point correlators can be used to characterize charge transport, magnetism, and non-equilibrium dynamics. Being able to measure correlations between two arbitrary centers that are close to a system of interest also opens the possibility to probe anisotropic phenomena such as variation in correlation parallel or perpendicular to the current flow that could not be probed with a single center otherwise. I will describe an NV center platform for measuring correlations in condensed matter targets, using excess quasiparticle noise near a superconducting phase transition as a model system. The platform features multiplexed sensing with low readout noise and high Rabi frequency microwave manipulation in a cryogenic environment, giving us access to a wide range of condensed matter phenomena.