  There has been much recent interest in extending the technique of magnetic
resonance imaging (MRI) down to the level of single spins with sub-optical
wavelength resolution. However, the signal to noise ratio for images of
individual spins is usually low and this necessitates long acquisition times
and low temperatures to achieve high resolution. An exception to this is the
nitrogen-vacancy (NV) color center in diamond whose spin state can be detected
optically at room temperature. Here we apply MRI to magnetically equivalent NV
spins in order to resolve them with resolution well below the optical
wavelength of the readout light. In addition, using a microwave version of MRI
we achieved a resolution that is 1/270 size of the coplanar striplines, which
define the effective wavelength of the microwaves that were used to excite the
transition. This technique can eventually be extended to imaging of large
numbers of NVs in a confocal spot and possibly to image nearby dark spins via
their mutual magnetic interaction with the NV spin.
