Can Entangling Separate Photon Pairs Affect Previously Measured Photons?

[StevieTNZ]

If we take two pairs of entangled photons (A+B and C+D), and take one from each pair (B and C) and perform a bell-state measurement on them (or another scheme that entangles photons A and D together):

(a) if the two other photons have not had their wave function collapsed, four-photon entanglement is created: http://arxiv.org/abs/quant-ph/0104047

(b) if the two photons had been measured, and then a bell state measurement occurred (or similar), a different result is evident (i.e. not the same result as (a)).

I've seen (b) realized in delayed-choice entanglement swapping, but is there another scheme that would generate different results if measurement had occurred before photons A and D became entangled?

 

[DrChinese]

I've seen (b) realized in delayed-choice entanglement swapping, but is there another scheme that would generate different results if measurement had occurred before photons A and D became entangled?

Keep in mind this rule: ordering of observation does not change the observed results in any discernible fashion.

 

[GTOM]

Maybe this is a right topic.

Some people had the idea, that we could use entangled photons to send an emergency signal once, by deliberately disrupting their entanglement.
Is is possible to notice somehow, that the link between them is broken, but not break it ourselves?

 

[GTOM]

Any opinions about this idea?

 

[IttyBittyBit]

In short, no.

If you take a pair of particles, entangle them, then separate them by a large distance and disrupt the entanglement on one side, it would not immediately affect the state of the particle on the other side in any physically measurable way.