  Multimode Gaussian quantum light, including multimode squeezed and/or
multipartite quadrature entangled light, is a very general and powerful quantum
resource with promising applications to quantum information processing and
metrology involving continuous variables. In this paper, we determine the
ultimate sensitivity in the estimation of any parameter when the information
about this parameter is encoded in such Gaussian light, irrespective of the
exact information extraction protocol used in the estimation. We then show
that, for a given set of available quantum resources, the most economical way
to maximize the sensitivity is to put the most squeezed state available in a
well-defined light mode. This implies that it is not possible to take advantage
of the existence of squeezed fluctuations in other modes, nor of quantum
correlations and entanglement between different modes. We show that an
appropriate homodyne detection scheme allows us to reach this Cramr-Rao bound.
We apply finally these considerations to the problem of optimal phase
estimation using interferometric techniques.
