  The evolution of a two-level system subjected to stimulated transitions which
is undergoing a sequence of measurements of the level occupation probability is
evaluated. Its time correlation function is compared to the one obtained
through the pure Schroedinger evolution. Systems of this kind have been
recently proposed for testing the quantum mechanical predictions against those
of macrorealistic theories, by means of temporal Bell inequalities. The
classical requirement of noninvasivity, needed to define correlation functions
in the realistic case, finds a quantum counterpart in the quantum nondemolition
condition. The consequences on the observability of quantum mechanically
predicted violations to temporal Bell inequalities are drawn and compared to
the already dealt case of the rf-SQUID dynamics.
