  Precise readout of spin states is crucial for any approach towards physical
realization of a spin-based quantum computer and for magnetometry with single
spins. Here, we report a new method to strongly improve the optical readout
fidelity of electron spin states associated with single nitrogen-vacancy (NV)
centers in diamond. The signal-to-noise ratio is enhanced significantly by
performing conditional flip-flop processes between the electron spin and the
nuclear spin of the NV center's nitrogen atom. The enhanced readout procedure
is triggered by a short preparatory pulse sequence. As the nitrogen nuclear
spin is intrinsically present in the system, this method is universally
applicable to any nitrogen-vacancy center.
