We report the detection of the $\sqrt{N}$ statistical polarization in a small ensemble of electron spin centers in ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ by magnetic resonance force microscopy. A novel detection technique was employed that captures the statistical polarization and cycles it between states that are either locked or antilocked to the effective field in the rotating frame. Using field gradients as high as $5\mathrm{G}/\mathrm{n}\mathrm{m}$, we achieved a detection sensitivity equivalent to roughly two electron spins, and observed ultralong spin-lock lifetimes, as long as 20 s. Given a sufficient signal-to-noise ratio, this scheme should be extendable to single electron spin detection.

• Received 14 July 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.207604