  In the context of quantum state engineering we analyze the effect of
observation on nonlinear optical $n$-photon Fock state generation. We show that
it is possible to truncate the vacuum component from an arbitrary photon number
superposition without modifying its remaining parts. In the course of the full
dynamical analysis of the effect of observation, it is also found that the Zeno
and the anti-Zeno effects repeat periodically. We discuss the close
relationship between vacuum state truncation and so-called "interaction-free"
measurement.
