What particles that never met "know" of one another?

Abstract

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.