Spin relaxometry of single nitrogen-vacancy defects in diamond nanocrystals for magnetic noise sensing

We report an experimental study of the longitudinal relaxation time (T ₁) of the electron spin associated with single nitrogen-vacancy (NV) defects hosted in nanodiamonds (NDs). We first show that T₁ decreases over three orders of magnitude when the ND size is reduced from 100 to 10 nm owing to the interaction of the NV electron spin with a bath of paramagnetic centers lying on the ND surface. We next tune the magnetic environment by decorating the ND surface with Gd3⁺ ions and observe an efficient T₁ quenching, which demonstrates magnetic noise sensing with a single electron spin. We estimate a sensitivity down to ≈14 electron spins detected within 10 s, using a single NV defect hosted in a 10-nm-size ND. These results pave the way towards T₁-based nanoscale imaging of the spin density in biological samples.