  Different approaches have improved the sensitivity of either electron or
nuclear magnetic resonance to the single spin level. For optical detection it
has essentially become routine to observe a single electron spin or nuclear
spin. Typically, the systems in use are carefully designed to allow for single
spin detection and manipulation, and of those systems, diamond spin defects
rank very high, being so robust that they can be addressed, read out and
coherently controlled even under ambient conditions and in a versatile set of
nanostructures. This renders them as a new type of sensor, which has been shown
to detect single electron and nuclear spins among other quantities like force,
pressure and temperature. Adapting pulse sequences from classic NMR and EPR,
and combined with high resolution optical microscopy, proximity to the target
sample and nanoscale size, the diamond sensors have the potential to constitute
a new class of magnetic resonance detectors with single spin sensitivity. As
diamond sensors can be operated under ambient conditions, they offer potential
application across a multitude of disciplines. Here we review the different
existing techniques for magnetic resonance, with a focus on diamond defect spin
sensors, showing their potential as versatile sensors for ultra-sensitive
magnetic resonance with nanoscale spatial resolution.
