Could they possibly create 1-2 and 3-4 entangled pairs at the same time, measure 1 & 4 and then use this "projective measurement" trick to entangle 3 and 4? If so, could this affect the measurements of 1 and 4 in the past?
yes, i think, if you mean 2 and 3. you have 1-2 and 3-4, then you entangle 1-4 which I think should lead to 2-3 entanglement without the 2-3 couple ever interacting at all.
>For example, last year a team showed that entanglement swapping still works even if they make the projective measurement after they’ve already measured the polarizations of photons 1 and 4.
So yes, they can. Which leaves me wondering which option is right:
A. I'm missing something major here.
B. They can measure 1 and 4 and then decide whether to use their box on 2 and 3 and contradict causality.
C. This experiment isn't actually exciting at all and the 'entanglement' isn't the good kind.
This can't possibly work because it would mean that you cou
ld measure 1&4 and get result that proves that they were not entangled and then entangle 2&3.
Of course it's possible that 2&3 entanglement trick doesn't always work and it never works if preceeding 1&4 measurment proved that they were unentangled. But this is some serious cheating. :)
