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.

• Received 9 December 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.3031