Entanglement-assisted quantum communication employs pre-shared entanglement
between sender and receiver as a resource. We apply the same framework to
quantum metrology, introducing shared entanglement between the preparation and
the measurement stage, namely using some entangled ancillary system that does
not interact with the system to be sampled. This is known to be useless in the
noiseless case, but was recently shown to be useful in the presence of noise.
Here we detail how and when it can be of use. For example, surprisingly it is
useful when randomly time sharing two channels where ancillas do not help
(depolarizing). We show that it is useful for all levels of noise for many
noise models and propose a simple experiment to test these results.Comment: 5 pages, 5 figure
