Key points are not available for this paper at this time.
When a single particle from two independent entangled pairs is detected in a manner such that it is impossible to determine from which pair the single particle came, the remaining three particles become entangled in a GHZ state. This procedure can be realized with existing sources of entangled photons and with future sources of entangled atoms.
Zeilinger et al. (Mon,) studied this question.