Proietti 2019 experiment: Doubt in measuring A1(B1)

[Joao]

TL;DR

Why measuring a and alfa with a bell state measurement means establishing Alice's own facts.

Hi everyone! Sorry for the bad english!

Paper: https://arxiv.org/abs/1902.05080

I guess I understood the experiment until the moment Alice and Bob chooses to measure A0(B0) or A1(B1).

I guess it's kind of straightforward that without a bell state measurement with photon alfa(beta) and photon a(b), Alice and Bob can know what her(his) friend measured, since alfa (beta) is carrying the message from the non destructive measurement made by the friend...

But why if Alice or Bob does a bell state measurement with photon alfa(beta) and photon a(b) the outcome will be that the friend is in an entangled state? It doesn't make much sense to me...

Thanks!

 

[RUTA]

See Section V in this paper https://arxiv.org/pdf/1901.10825.pdf