  We analyse photoionisation and ion detection as a means of accurately
counting ultra-cold atoms. We show that it is possible to count clouds
containing many thousands of atoms with accuracies better than $N^{-1/2}$ with
current technology. This allows the direct probing of sub-Poissonian number
statistics of atomic samples. The scheme can also be used for efficient single
atom detection with high spatio-temporal resolution. All aspects of a realistic
detection scheme are considered, and we discuss experimental situations in
which such a scheme could be implemented.
