  We investigate how multiphoton quantum states obtained through optical
parametric amplification can be manipulated by performing a measurement on a
small portion of the output light field. We study in detail how the macroqubit
features are modified by varying the amount of extracted information and the
strategy adopted at the final measurement stage. At last the obtained results
are employed to investigate the possibility of performing a
microscopic-macroscopic non-locality test free from auxiliary assumptions.
