Entanglement Swapping and FTL Communication

[matrixrising]

Just close this thread.

It's funny how people can stand to have their existing paradigms challenged.

 

[matrixrising]

bhobba,

And for something that would rock physics to it core, dethroning very well verified stuff like SR.

Nothing I have said conflicts with SR. Information isn't traveling between point A and point B. There's no violation of causality.

 

[StevieTNZ]

It's funny how people can stand to have their existing paradigms challenged.

What are you even going on about? Don't make assumptions about me.

 

[Nugatory]

WHETHER ENTANGLEMENT SWAPPING OCCURS STRICTLY DEPENDS ON VICTORS MEASUREMENT.

Of course it does, and everyone here is agreeing with you about that. That's not the issue.

The question is, can Alice and Bob, just on the basis of the measurements they make and share, detect whether entanglement swapping has occurred. If they cannot, then there's no possibility of FTL communication from Victor to them, we're just looking at the routine and well-accepted non-locality of quantum mechanics.

So I have to repeat the question I asked a few posts back: can you tell me, in detail, precisely how Alice and Bob, given just their measurements of photons one and four, can determine whether they have been entanglement-swapped? I assert that that it is not possible but will look at any counterexample with an open mind.

 

[StevieTNZ]

So I have to repeat the question I asked a few posts back: can you tell me, in detail, precisely how Alice and Bob, given just their measurements of photons one and four, can determine whether they have been entanglement-swapped? I assert that that it is not possible ...

Exactly the example I gave earlier, where determining whether entangle swapping occurred or not was not possible without knowing from Victor what measurement he performed (which requires a classical channel).

 

[matrixrising]

Nugatory,

Asked and answered. The published paper has spelled it out and I have spelled it out in this thread. I think someone on here even went so far to say they just didn't agree with the observed experiment. They realize that FTL communication is possible based on entanglement swapping.

Again, it's right there.

Bob and Alice know when Victor chooses to swap entanglement or not swap entanglement.

How do they know this?

It's because photons 1&2 are entangled and so are photons 3&4.

Photons 1&4 have never interacted.

When Victor chooses a bell state measurement with photons 2&3, the entanglement is swapped and projected onto photons 1&4. Alice and Bob have the data about the entanglement of 1&2 and 3&4 that's being swapped.

When Victor chooses a separable measurement, the entanglement isn't swapped and this is projected onto photons 1&4.

So again, Alice&Bob know when Victor has chosen to swap entanglement and when Victor has chosen not to swap entanglement because they are armed with the data of entanglement between photons 1&2 and 3&4.

The mistake that I keep pointing out is, Victor isn't making a choice between bell states. Nobody is making that claim. Victor is making the choice to swap entanglement or not to swap entanglement which will allow for FTL communication.

Like I said, read the published paper. It's right there plain as day. Entanglement swapping leaves a distinct signature that can be used to encode information and causality isn't violated because information isn't traveling between points A and B.

 

[Nugatory]

Exactly the example I gave earlier, where determining whether entangle swapping occurred or not was not possible without knowing from Victor what measurement he performed (which requires a classical channel).

Yes - I think we also all agree that it's possible if we have access to Victor's records as well, and enough samples to detect the correlations with statistical significance. Indeed, that's the point of the paper that seems to have so misled matrixrising. The impossible challenge is to determine whether swapping has occurred just on the basis of Alice's and Bob's measurements.

(I'm not arguing with StevieTNZ here, I'm just summarizing and restating for the benefit of any brave soul who comes across this thread in the future. I agree with the earlier post - just close this thread already)

 

[DrChinese]

Edit: It would look like this:

HH23HH23VV23HH23VV23VV23 = 0 ( a separable state)

+23+23-23-23-23+23+23-23+23-23=1 (a Bell state)

I have no idea what the above represents. What's +23 or -23? Alice has 1 bit (H or V), and Bob has another bit. Those form pairs of bits. I asked what those pairs would look like, and made it clear how to answer. Please address this, and write it in the form per below so you can see the true situation:

Alice (photon 1): ? ? ? ? ? ? ? ?
Bob (photon 4): ? ? ? ? ? ? ? ?

Please note: They cannot see what Victor sees, which is any information about photons 2 & 3 which perhaps is somehow tied to your "23" above. Please note this crucial sentence from the referenced paper:

"In our experiment, the primary events are the polarization measurements of photons 1 and 4 by Alice and Bob. They keep their data sets for future evaluation. Each of these data sets by itself and their correlations are completely random and show no structure whatsoever."

Hopefully you will read that paragraph from Zeilinger et al and understand that this is what I have been saying: a completely random pattern can contain no useful information without a key to decode it. Victor has that key, but must transmit it by classical (normal light speed) channels to Alice and Bob. Which means no FTL signalling is possible here.

I am going to tell you - as a kind service (and you can thank me later) - that if you are serious about learning more about quantum physics, you are at the right place here at PF. There are many here in this thread that are attempting to assist you, and your words are unnecessarily harsh. Even were you correct, no one talks like that in the scientific world. I hope that once you crash on this particular idea, you will continue to come back and learn. All of us have been where you are at one time or another, so don't let embarrassment keep you from returning.

-DrC

 

[matrixrising]

Nurgatory,

Of course you and others want the thread closed because you're not debating against what was actually published. People have their minds made up that FTL communication is impossible but it's not as long as causality isn't violated.

Of course Bob and Alice can know if entanglement swapping has occurred because they have access to 2 crucial pieces of information.

The data on the entanglement between photons 1&2 and photons 3&4 along with the photons 1&4 where entanglement is being projected onto.

Like I said read the paper, especially page 6.

I would like someone to show me where it says Bob and Alice can't know whether entanglement swapping has occurred. Of course they can and this is clear from the published article.

 

[Nugatory]

Alice and Bob have the data about the entanglement of 1&2 and 3&4 that's being swapped.
...
Entanglement swapping leaves a distinct signature that can be used to encode information ...

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?

 

[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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