Fig. 2: Nano-NMR based on NV centers.
From: Achieving the ultimate precision limit with a weakly interacting quantum probe
The NV center (red) is situated at a depth d below the diamond surface. The nuclear spin ensemble (blue), which is located on top of the surface, is partially polarized due to the external magnetic field B. The local magnetic field at the NV’s position is only affected by the nuclear spins within a hemisphere of radius d centered above the NV’s position (dashed red line), since the dipolar interaction creates an effective cutoff. The sensor is initialized and then measured after a given time, in order to reproduce the probability distribution, which is analogous to the classic NMR signal (top right). However, as the sensor is brought into close proximity with the sample, strong back-action causes the signal to change (bottom right). This change dramatically affects the precision of the frequency estimation (the full expression of the probability distribution plotted on the right hand side is taken from Eq. (13)).
