Entanglement shown between photons that never coexist

Science : The entanglement of two particles (or photons) is a quantum mechanical effect in which measuring one of the particles instantaneously determines the state of the other, regardless of the distance between them. And entanglement can be swapped between pairs of entangled particles by creating two sets of particles and then performing a “projective measurement” of one particle of each pair. The measurement simultaneously entangles and destroys the measured particles, and it entangles the two other particles even if they had previously been measured. Now, Eli Megidish and Hagai Eisenberg of the Hebrew University of Jerusalem and their colleagues have used this swapping technique to entangle two photons that never coexisted. The time-separated effect was predicted by the original quantum theory, but this recent work is the first demonstration of it. The technique could be useful in the development of quantum communications systems.