  In this work we investigate the relation between quantum measurements and
decoherence, in order to formally express the necessity of the latter for
obtaining an informative output from the former. To this aim, referring to the
Von Neumann scheme for ideal quantum measurements, we first look for the
minimal structure that the interaction between principal system and measurement
apparatus must have for properly describing the process, beyond the quantum
measurement limit, and then analyze the dynamical evolution induced by one such
interaction. The analysis is developed by means of a recently introduced method
for studying open quantum systems, namely the parametric representation with
environmental coherent states, that allows us to determine a necessary
condition that the quantum state of the apparatus must fullfil in order to give
information on the observable being measured. We find that this condition
strictly implies decoherence in the principal system, with respect to the
eigenstates of the hermitian operator that represents the measured observable,
thus establishing that there cannot be information flux from a quantum system
towards a readable analyser unless decoherence occurs. The relevance of
dynamical entanglement generation is highlighted, and consequences of the
possible macroscopic structure of the measurement apparatus are also commented
upon.
