  We present a novel technique for measuring the characteristics of a
magneto-optical trap for cold atoms by monitoring the spontaneous emission from
trapped atoms coupled into the guided mode of a tapered optical nanofiber. We
show that the nanofiber is highly sensitive to very small numbers of atoms
close to its surface. The size and shape of the MOT, determined by translating
the cold atom cloud across the tapered fiber, is in excellent agreement with
measurements obtained using the conventional method of fluorescence imaging
using a CCD camera. The coupling of atomic fluorescence into the tapered fiber
also allows us to monitor the loading and lifetime of the trap. The results are
compared to those achieved by focusing the MOT fluorescence onto a photodiode
and it was seen that the tapered fiber gives slightly longer loading and
lifetime measurements due to the sensitivity of the fiber, even when very few
atoms are present.
