  Magnetic resonance detection is one of the most important tools used in
life-sciences today. However, as the technique detects the magnetization of
large ensembles of spins it is fundamentally limited in spatial resolution to
mesoscopic scales. Here we detect the natural fluctuations of nanoscale spin
ensembles at ambient temperatures by measuring the decoherence rate of a single
quantum spin in response to introduced extrinsic target spins. In our
experiments 45 nm nanodiamonds with single nitrogen-vacancy (NV) spins were
immersed in solution containing spin 5/2 Mn^2+ ions and the NV decoherence rate
measured though optically detected magnetic resonance. The presence of both
freely moving and accreted Mn spins in solution were detected via significant
changes in measured NV decoherence rates. Analysis of the data using a quantum
cluster expansion treatment of the NV-target system found the measurements to
be consistent with the detection of ~2,500 motionally diffusing Mn spins over
an effective volume of (16 nm)^3 in 4.2 s, representing a reduction in target
ensemble size and acquisition time of several orders of magnitude over
state-of-the-art electron spin resonance detection. These measurements provide
the basis for the detection of nanoscale magnetic field fluctuations with
unprecedented sensitivity and resolution in ambient conditions.
