Comment on “Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping”

In a recent letter, Jennewein et al. [1] reported an experiment demonstrating entanglement swapping among two pairs (0,1 and 2,3) of down-converted photons. They accomplished this by performing a Bell measurement on photons 1 and 2, and they verified the effect with regular polarization measurements on photons 0 and 3. By selecting data from runs in which 1,2 are measured to be in the Bell state |ψ−〉, they find that the measurement results for 0,3 violate a Bell (CHSH) inequality, and they conclude that 0,3 have exhibited quantum nonlocality. A peculiarity is noted for the case in which the Bell measurement on 1,2 is performed only after 0,3 have their polarizations measured. In this case, Jennewein et al. say that the measurements on 0 and 3 “indicate... that photons 0 and 3 were entangled.” But at the time 0 and 3 were measured, they were not entangled, because they started out unentangled and no entangling operation had been performed up to that point. They cannot be entangled retroactively. We are thus lead to conclude that photons 0 and 3 have violated a Bell inequality while not even being entangled with each other!