All-optical low-field magnetometry of superconductors using NV nanodiamonds

We report a microwave-free, all-optical sensing approach for characterization of superconducting materials using nitrogen-vacancy (NV) centers in nanodiamonds. By implementing near-zero-field cross-relaxation magnetometry within a custom-built widefield fluorescence microscope integrated with a lock-in amplifier, we measured critical superconducting parameters including transition temperature and penetration fields by depositing nanodiamonds on a 200 nm YBa 2 Cu 3 O 7 − δ thin film. This minimally invasive, all-optical approach provides a general platform for measuring local magnetic responses in superconducting materials without the perturbative effects of traditional microwave-based sensing.