Entanglement Swapping and FTL Communication

[matrixrising]

I'm sorry, I may not have been completely clear about what my question is. I'll try again.

What exactly is this "distinct signature"? How do Alice and Bob recognize it when they see it?

Or, to be even more specific: Alice measures photon one and gets a result, either up or down, on some measurement axis. Bob measures photon four and gets a result, either up or down, on some measurement axis. How do they use the two measurement results to determine whether Victor has chosen to do an entanglement swap? Do they need any other information than those two measurements, and if so, what is it?

BINGO!

You asked the right question so let me repeat the answer.

What's the other information Alice and Bob have??

THE DATA FROM THE ENTANGLEMENT THAT'S BEING SWAPPED!

Victor, Bob and Alice know the data on the entanglement of photons 1&2 and 3&4. Bob and Alice use this data to know whether entanglement swapping has occurred. If all 3 bases show strong quantum correlation between photons 1&4 then they know entanglement swapping has occurred if not then they know it hasn't occurred.

BINGO!

 

[Nugatory]

THE DATA FROM THE ENTANGLEMENT THAT'S BEING SWAPPED!

Victor, Bob and Alice know the data on the entanglement of photons 1&2 and 3&4. Bob and Alice use this data to know whether entanglement swapping has occurred. If all 3 bases show strong quantum correlation between photons 1&4 then they know entanglement swapping has occurred if not then they know it hasn't occurred.

What exactly do you mean by "the data from the entanglement that's being swapped"? Again, be precise - how is it measured, by whom, when?

If you're suggesting that Alice knows the initial state of photon two and Bob knows the initial state of photon three, then that's the source of the disagreement right there - it's impossible for them to acquire that information without breaking the entanglement of the one-two and three-four pairs.

If you're suggesting that they also use a result that Victor has obtained, then there's no possibility of FTL communication, because Victor's results have to be conveyed to them through some other means before they can make the determination that Victor did or did not entangle two and three. We're just looking at the same quantum non-locality that entanglement always demonstrates.

 

[matrixrising]

Here's more from the paper:

When Victor performs a Bell state measurement this swaps entanglement which is CONFIRMED by strong correlation between 3 bases for photons 1&4. When this occurs the state fidelity is 0.681+/-0.034 and the entanglement witness value is -0.181+/-0.034 which shows entanglement between photons 1&4.

Again, Victor isn't choosing which Bell state will occur. He's choosing whether a bell state or a separable state will occur for photons 1&4.

If Victor chooses a separable state measurement, the state fidelity is 0.421+/-0.029 and the entanglement witness value is 0.078+/-0.029 between photons 1&4.

It doesn't get much clearer than this.

 

[berkeman]

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