  Multimode Gaussian quantum light, which includes multimode squeezed and
multipartite quadrature entangled light, is a very general and powerful quantum
resource with promising applications in quantum information processing and
metrology. In this paper, we determine the ultimate sensitivity in the
estimation of any parameter when the information about this parameter is
encoded in such light, irrespective of the information extraction protocol used
in the estimation and of the measured observable. In addition we show that an
appropriate homodyne detection scheme allows us to reach this ultimate
sensitivity. We 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-de ned 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 diff erent modes.
