Sensitivity threshold defines the optimal spin subset for ensemble quantum sensing
Inhomogeneous broadening and spatial gradients in control fields inevitably produce large variations in characteristic sensitivity across spin ensembles. We derive an analytic expression for the sensitivity of an inhomogeneous ensemble and introduce a sensitivity threshold that identifies the optimal subset of spins. For both pulsed and continuous-wave magnetometry, the optimal ensembles deliver up to an eightfold improvement over conventional schemes relying on nominally uniform regions of the ensemble. We demonstrate phase-only digital holography to implement the optimal ensembles and show that the measured illumination non-uniformity limits the sensitivity by 14%. This approach enables optimal quantum sensing in optically scattering environments.