  One of the critical milestones in the intensive pursuit of quantitative
nanoscale magnetic imaging tools is achieving the level of sensitivity required
for detecting the field generated by the spin magnetic moment {\mu}B of a
single electron. Superconducting quantum interference devices (SQUIDs), which
were traditionally the most sensitive magnetometers, could not hitherto reach
this goal because of their relatively large effective size (of the order of 1
{\mu}m). Here we report self-aligned fabrication of nano-SQUIDs with diameters
as small as 46 nm and with an extremely low flux noise of 50 n{\Phi}0/Hz^1/2,
representing almost two orders of magnitude improvement in spin sensitivity,
down to 0.38 {\mu}B/Hz^1/2. In addition, the devices operate over a wide range
of magnetic fields with 0.6 {\mu}B/Hz^1/2 sensitivity even at 1 T. We
demonstrate magnetic imaging of vortices in type II superconductor that are 120
nm apart and scanning measurements of AC magnetic fields down to 50 nT. The
unique geometry of these nano-SQUIDs that reside on the apex of a sharp tip
allows approaching the sample to within a few nm, which paves the way to a new
class of single-spin resolved scanning probe microscopy.
