Abstract: The variation of magnetic fields across the superconducting samples is quite small in the fragile genuine Meissner state, which is easily disturbed by Abrikosov vortex pining and field inhomogeneities due to large demagnetization effects. We developed minimally invasive optical magneto-sensing based on the ensembles of NV centers in diamond crystal to study Meissner state structure and to determine the lower critical field. I will show how different the structure of the Meissner state is in different superconducting materials. I will also discuss the application of the NV sensing to identify quantum phase transition in charge-doped iron-based superconductors.