  This thesis consists of four parts. In the first part it is shown that
optimal universal cloning of photons can be realized with the help of
stimulated emission. Possible schemes based on three-level systems and on
parametric down-conversion are analyzed in detail.
  The second part shows that the dynamical rules of quantum physics can be
derived from its static properties and the condition that superluminal
communication is forbidden.
  In the third part a very simple form of the Kochen-Specker theorem is
presented, which leads to a proposal for a simple experimental test of
non-contextual hidden variables.
  The fourth part shows how hidden-variable theorems can be derived in a
completely operational way. In particular a Kochen-Specker theorem is derived
that applies to experiments with finite precision.
