  An experiment proposed by Yurke and Stoler, and similar to that realized
experimentally by Sciarrino et al., is analyzed. In Sciarrino's realization,
identical photons from a degenerated down-conversion pair are used, i.e. the
photons met in the past. In the experiment analyzed here the particles are also
identical, but from different sources. As long as one can tell from which
source came each particle, the joint wave function remains factorizable.
However, a configuration is created in which one cannot tell anymore which
particle came from which source. As a result, the wave function becomes
non-factorizable, symmetrical (for bosons) or antisymmetrical (for fermions).
In part of the cases the situation is even more surprising: the particles never
meet, s.t. the symmetry (antisymmetry) is produced at-a-distance without the
particles having had the possibility to interact in any way.
