Entanglement Swapping and FTL Communication

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Discussion Overview

The discussion revolves around the concept of entanglement swapping as proposed by Asher Peres, particularly focusing on its implications for faster-than-light (FTL) communication. Participants explore the experimental realization of this thought experiment, the nature of quantum correlations, and the conditions under which information might be transmitted between parties (Alice and Bob) through a third party (Victor).

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants describe the setup of the experiment involving four entangled photons and how Victor can influence the entanglement of Alice and Bob's photons after their measurements.
  • There is a proposal that Victor could send information to Alice and Bob faster than light by choosing to entangle his photons, leading to quantum correlations between Alice and Bob's particles.
  • Others challenge the feasibility of FTL communication, questioning how Alice and Bob could determine the entanglement of their particles without classical communication, which would be non-FTL.
  • Some participants express skepticism about the reliability of identifying entanglement based on observed correlations, suggesting that correlations might be misinterpreted without proper measurements.
  • A later reply emphasizes that the identification of quantum correlations is crucial and that Alice and Bob can recognize these correlations before Victor makes his choice.
  • Another participant argues that the results of the experiment can be interpreted in a way that does not imply any paradox, suggesting that the quantum state is merely a reflection of knowledge rather than a physical object.
  • There is a discussion about the measurement bases used by Alice, Bob, and Victor, and how these affect the interpretation of their results.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the implications of entanglement swapping for FTL communication. There is no consensus on whether the proposed method allows for FTL communication or whether it is fundamentally limited by classical information transfer requirements.

Contextual Notes

Participants note limitations in the understanding of how quantum correlations can be interpreted and the dependence on specific measurement choices. The discussion highlights unresolved questions about the nature of entanglement and the implications of delayed-choice experiments.

  • #61
Nugatory said:
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!
 
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  • #62
matrixrising said:
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.
 
  • #63
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.
 
  • #64
Thread closed pending Moderation...
 

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