  One of the most surprising consequences of quantum mechanics is the
entanglement of two or more distant particles. In an entangled EPR two-particle
system, the value of the momentum (position) for neither single subsystem is
determined. However, if one of the subsystems is measured to have a certain
momentum (position), the other subsystem is determined to have a unique
corresponding value, despite the distance between them. This peculiar behavior
of an entangled quantum system has surprisingly been observed experimentally in
two-photon temporal and spatial correlation measurements, such as ghost
interference and ghost imaging. This article addresses the fundamental concerns
behind these experimental observations and to explore the nonclassical nature
of two-photon superposition by emphasizing the physics of 2 is not 1 + 1.
