  We investigate the connection between the shot-noise limit in linear
interferometers and particle entanglement. In particular, we ask whether or not
sub shot-noise sensitivity can be reached with all pure entangled input states
of $N$ particles if they can be optimized with local operations. Results on the
optimal local transformations allow us to show that for $N=2$ all pure
entangled states can be made useful for sub shot-noise interferometry while for
$N>2$ this is not the case. We completely classify the useful entangled states
available in a bosonic two-mode interferometer. We apply our results to several
states, in particular to multi-particle singlet states and to cluster states.
The latter turn out to be practically useless for sub shot-noise
interferometry. Our results are based on the Cramer-Rao bound and the Fisher
information.
