Entanglement Swapping and FTL Communication

[DrChinese]

Victors choice determines whether photons 1 and 4 will find strong correlations in all three bases. ... Again, which state Alice and Bob find photons 1 and 4 in is determined by Victor. If there's strong correlation between all three bases then Alice and Bob know it was a Bell state correlation (1). If they don't find this strong correlation between all 3 bases, they know it was a separable state correlation (0).

Yes, the correlations are strong when Victor sends 1 & 4 to a Bell state. But there are 2 very different Bell states (and you can see the correlations):

Bell state + (what I call symmetric correlations)
1THHTTTHH etc
4THHTTTHH

Bell state - (what I call anti-symmetric correlations)
1THHTTTHH etc
4:THTTHHHTT

Victor cannot choose which Bell state is selected! That occurs randomly, outside of Victor's control. So when Victor throws every 1 &4 into a Bell state (your 1), what actually shows up for Alice/Bob is a completely random set of H's and T's. This is completely indistinguishable from your 0 case.

A couple of comments:
1. In reality, Victor cannot throw every pair into a Bell state. Only some can be so created in actual experiments.
2. If your system worked - and this is the cool part - then Victor could send a message from the future to the past! This is because Victor's decision can be made before, during OR after detection of 1&4. Wild stuff, eh?

 

[DrChinese]

And just to be glaringly obvious:

What does Victor see? He sees his 2&3 cast into + or - Bell states that read something like the following (when he is choosing to allow the Bell state):

2&3: +-+--+--+++-+-++-+-- etc.

So naturally, he knows he is not transmitting anything more than random gibberish. Victor's results must be combined with Alice/Bob's to see the underlying pattern of correlations.

 

[matrixrising]

Thanks for the response Dr. Chinese and elo,

You guys have it wrong and I refer you to page 6 of the study.

First, entanglement between photons 1&2 and 3&4 occur. Photons 1 and 4 are sent to Bob and Alice and Victor is sent photon 2&3. In my scenario, Bob and Alice are in the same place. When Victor performed the separable state measurement, the entanglement remained between 1&2 and 3&4. When a bell state measurement occurred, this entanglement vanished.

So Victors choice effects photons 1&4 which are with Bob and Alice.

Victor isn't choosing which Bell State occurs. This point is obvious but it has nothing to do with my scenario. My scenario depends on Victors free choice to entangle or not to entangle.

This has nothing to do with Victor choosing which Bell State will occur.

It has everything to do with whether there's strong quantum correlations between photons 1&4 or separable state correlations between photons 1&4 based on Victors choice.

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.

Now, how do Alice and Bob know that entanglement has been swapped or it hasn't been swapped?

They know because they have the data for entanglement between photons 1&2 and 3&4. Armed with this data, Alice and Bob can know whether strong quantum correlations exist in all 3 bases and entanglement swapping has occurred or if entanglement swapping hasn't occurred.

Victor is sending information to Alice and Bob faster than the speed of light. Because swapping or no swapping shows up in Bell or separable correlations in photons 1&4.

I do think it's wild he can send a message into the past. I think causality would be preserved though because nothing could be changed on our worldline because of entropy.

Let's say you send the lottery numbers to yourself in the past. These lottery numbers are from a week ago on December 27th. Well December 27th has already passed for you and your history of that day can't change in your current worldine. This would mean, you would be making yourself rich in a parallel universe. So again, causality would be preserved.

 

[DrewD]

You guys have it wrong and I refer you to page 6 of the study.

Since this thread is about to be closed, I would like to sneak in and try to make some money. Since FTL travel of information is possible (completely overturning 100 years of experiment and strong theoretical reasoning), your exact experiment will soon be tried and published. I would like to pay pay you $1000 when this happens. Of course, if it doesn't happen, I would like some money from you in return. It really is amazing though that this experiment was completed a few years ago and nobody else noticed that it overturns a century of scientific thought.

But in all seriousness, how much QM have you studied? I am not terribly familiar with this experiment (I read the paper when it came out, but I don't remember much), but I think the examples that have been given illustrate the problem quite clearly. All of the information you are quoting comes from a set up where Victor's measurements are compared to Alice's and Bob's so it does not apply when they are space-like separated.

 

[matrixrising]

DrewD,

Again, you're wrong. The only reason people can't accept FTL communication is because they can't get their head around the fact that FTL communication via quantum entanglement doesn't violate causality. Information isn't passing through any intervening space between points A or B.

Alice & Bob already have all the information they need to know if entanglement swapping has occurred. They have entanglement data on photons 1&2 and 3&4. Victors choice to swap or not to swap shows up in photons 1&4. So yes, information is being sent faster than light but causality is still preserved.

 

[Nugatory]

Alice & Bob already have all the information they need to know if entanglement swapping has occurred. They have entanglement data on photons 1&2 and 3&4. Victors choice to swap or not to swap shows up in photons 1&4.

Can you be more precise about exactly how Victor's choice to swap or not to swap shows up in photons one and four?

Alice and Bob each get exactly one measurement on their photons, and that one measurement will return either H or T (along whatever axis they choose). How do Alice and Bob compare their measurements to determine whether Victor has chosen to swap or not?

 

[DrChinese]

Thanks for the response Dr. Chinese and elo,

You guys have it wrong and I refer you to page 6 of the study.

First, entanglement between photons 1&2 and 3&4 occur. Photons 1 and 4 are sent to Bob and Alice and Victor is sent photon 2&3. In my scenario, Bob and Alice are in the same place. When Victor performed the separable state measurement, the entanglement remained between 1&2 and 3&4. When a bell state measurement occurred, this entanglement vanished.

So Victors choice effects photons 1&4 which are with Bob and Alice.

Victor isn't choosing which Bell State occurs. This point is obvious but it has nothing to do with my scenario. My scenario depends on Victors free choice to entangle or not to entangle.

This has nothing to do with Victor choosing which Bell State will occur.

Yes, of course the idea is that Alice and Bob are in the same place comparing their outcomes (for 1 & 4). They measure polarization at the same angle setting, and see if they get matching results.

The thing is: there results MAY be correlated + or -. HH or TT is + correlated, HT or TH is - correlated. So any of the following outcomes are possible for Alice and Bob when Victor casts 2&3 to a Bell state:

HH, TT, HT or TH.

The problem for you is that these 4 permutations are the same as would be seen in ANY random set as well. They are completely indistinguishable. So Victor can do something or nothing - and the results are the same as far as Alice & Bob are concerned. They always see some set of the above. Now, if you could fix it so that Alice and Bob see only HHs or TTs when Victor did something, then a message could be sent from Victor to Alice/Bob. But as I keep telling you, Victor cannot select the particular output Bell state as being + or -. Just because photons 1 & 4 are entangled does not mean they have the same values. That only happens in the + case, which occurs 1/2 of the time. In the other 1/2, you have the - case and 1&4 - still entangled - have instead OPPOSITE values.

You might ask yourself whether your proposed scenario ever occurred to the top scientists who ran the experiment, or perhaps they were blind to this ground-breaking outcome. Or alternately, maybe my description is more apropos and there is no FTL mechanism here.

 

[DrChinese]

I do think it's wild he can send a message into the past. I think causality would be preserved though because nothing could be changed on our worldline because of entropy.

Let's say you send the lottery numbers to yourself in the past. These lottery numbers are from a week ago on December 27th. Well December 27th has already passed for you and your history of that day can't change in your current worldine. This would mean, you would be making yourself rich in a parallel universe. So again, causality would be preserved.

The experiments show that regardless of WHEN Victor decides to entangle 1&4, the results are the same. This test was actually performed. Photons detected now were entangled in the relative future. In fact, the 1&4 photons do not need to have ever existed at the same time or be in a common light cone to be entangled. Ordering makes no difference for the outcomes of any of the 4 photons in this setup.

As to your idea that entropy is what prevents causality from being violated: there really is no evidence of that either way. It's possible, I guess. Who knows? Ditto for your comments about a parallel universe. What parallel universe? Where is it? It too is possible, I guess. But it is pure speculation and meaningless words past that. All I can really say is that IF your idea worked (and it doesn't), then someone in the future could send you a message.

By the way: In the past, I have had similar FTL epiphanies as you. Only to wake up the next day realizing what a wrong idea it was. And I had been told: in vino veritas! I had certainly indulged the "in vino" part adequately. Oh well, time for you to take some medicine.

 

[bhobba]

I do think it's wild he can send a message into the past. I think causality would be preserved though because nothing could be changed on our worldline because of entropy.

If you can send information FTL you have destroyed SR.

To understand why get a good book on relativity like my go-to book - Rindler - Introduction To Special Relativity:
https://www.amazon.com/dp/0198539525/?tag=pfamazon01-20

Basically the POR implies the maximum speed you can send information is the same in all inertial frames. Call that speed C, and you can derive the Lorentz equations from it. They show a few things such that there can only be one such speed and that speed is the only one that does not depend on the speed of the source. From Maxwells equations we know the speed of light does not depend on the speed of the source so the speed of light must be the maximum speed information can be sent.

There are countless observations that show SR is correct eg the existence of magnetic fields (that is in fact a relativistic effect - and one can actually derive Maxwell's equations from it) and mass increase. All these have been confirmed to high accuracy. If SR is wrong you have destroyed a massive amount of physics.

This is why if you can do what you are claiming you will get a Nobel Prize immediately. Physics as we know it would be destroyed.

The fact you do not seem to understand this makes me believe you need to study quite a bit more physics before coming up with proposals that are contradictory to a massive amount of both theoretical knowledge and hard experimental fact.

Thanks
Bill

 

[matrixrising]

Dr. Chinese,

Again, this isn't the case.

Again, Alice and Bob don't see the same results and I spelled out why earlier. Alice and Bob know whether Victor has swapped entanglement from the data they have from the entanglement of photons 1&2 and 3&4.

Nobody is saying Victor can choose which Bell state will occur. The paper is titled "Experimental Delayed-Choice Entanglement Swapping"

This is because Bob and Alice know when Victor has swapped entanglement and when he hasn't. It's plain as day in the study.

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.

Again, when Victor chooses to swap entanglement this can be seen by Bob and Alice in photons 1&4. When Victor chooses not to swap entanglement this is seen in photons 1&4.

There's growing evidence for parallel universes. Look at the data from Planck's satellite and the work of Laura-Mersini Haughton. You can also look at others work in these areas. At the end of the day, just ask yourself this simple question:

When Victor chooses to swap entanglement or not to swap entanglement, does Alice and Bob know that Victor swapped entanglement?

The answer is clearly yes and this is why Alice and Bob have the data from the entanglement of photons 1&2 and 3&4.

 

[bhobba]

By the way: In the past, I have had similar FTL epiphanies as you

Not regarding FTL, but I also have had similar half backed ideas about the measurement problem in QM. Then I would think better of it and realize what an idiot I as.

Now when such things overtake me I say - hold on - guys like Bohr, Einstein, Heisenberg etc etc are really smart dudes - surely they would have thought of similar stuff. Then sure enough I would see what an idiot I was - it may take a while for my subconscious to work through it but eventually I go - dah - what a fool.

To the OP - physics is an interconnected whole. There are basic and fundamental reasons you can't do what you are proposing.

Thanks
Bill

 

[bhobba]

Again, this isn't the case.

Exactly what isn't the case?

You have not explained how to ensure 1 & 4 are always correlated.

As Dr Chinese explained 50% of the time it's correlated in which case its the same as what's being sent, and for the rest it will ant-correlated in which case its the opposite, and which occurs will be random. This means the information received will be random.

Please stop simply saying this isn't true. You need to address the issue - not simply avoid it.

Thanks
Bill

 

[DrChinese]

When Victor chooses to swap entanglement or not to swap entanglement, does Alice and Bob know that Victor swapped entanglement?

Of course not! How would they? They have pairs of entangled photons (photons 1&4 form the pair). For each pair, they can see an H or T (heads/tails) value and compare them (let's assume polarization is measured at 0 degrees for both). What do you think they see when Victor has chosen to place those into a Bell state? For this example, let's assume that everything is ideal.

Alice: ? ? ? ? ? ? ? ?
Bob: ? ? ? ? ? ? ? ?

Please write an example set of about 8 or so down and then I will explain your misunderstanding. This is the part that trips up most people, because it is too difficult.

(And again, I wonder how you imagine the experimenters performed this experiment and missed your conclusion. Doesn't that seem unusual in the least to you? These are some of the top physicists in the world.)

 

[bhobba]

And again, I wonder how you imagine the experimenters performed this experiment and missed your conclusion. Doesn't that seem unusual in the least to you? These are some of the top physicists in the world.

I think its being carried away with an idea that makes you go WOW. You loose your sanity a bit.

I remember one similar discussion I had with a guy who thought it was possible to derive QM from classical mechanics. At the end of the day you had to hit him with all sorts of stuff. He said - enough - he was just starting a course on QM - so please be gentle.

Thanks
Bill

 

[DrChinese]

I think its being carried away with an idea that makes you go WOW. You loose your sanity a bit.

... so please be gentle.

Thanks
Bill

I lost my sanity more than a few times... so I am always gentle.

 

[matrixrising]

Dr. Chinese,

Who is making the claim that Bob and Alice need to differentiate between Bell states? Who is saying that Victor can decide which Bell state to send?

You're debating against something that's not even in contention. You keep avoiding debating the actual paper and you keep making a strawman argument that has nothing to do with the paper. It says:

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. The other two photons (photons 2 and 3) are delayed
until after Alice and Bob’s measurements, and sent to Victor for measurement. His measurement then decides
the context and determines the interpretation of Alice and Bob’s data. In our setup, using two-photon
measurement which projects photons 2 and 3 either onto |Φ+〉23 or onto |Φ− 〉23 . This would swap entanglements onto photons 1 and 4. Instead of a Bell measurement, Victor may perform a Bell-state polarization of these photons individually and project photons 2 and 3 either onto |HH〉23 or onto |VV〉23 ,entanglement to photons 1 and 4. Instead of a Bell-state measurement, Victor could also decide to measure the polarization of these photons individually and project photons 2 and 3 either onto HH 23 or onto VV 23 which would result in a well-defined polarization for photons 1 and 4, i.e. a separable state.

According to Victor’s choice of measurement (i.e. entangled or separable state) and his results (i.e. |Φ+ 〉23 ,
|Φ− 〉23 , or |HH〉23 , |VV〉23 ), Alice and Bob can sort their already recorded data into 4 subsets. They can now
verify that when Victor projected his photons onto an entangled state (|Φ+ 〉23 or |Φ− 〉23), each of their joint subsets behaves as if it consisted of entangled pairs of distant photons. When Victor projected his photons on a separable state (|HH〉23 or |VV〉23 ) Alice and Bob's joint subsets behave as if they consisted of separable pairs of photons. Whether Alice and Bob's earlier measurement outcomes indicate entanglement of photons 1 and 4 strictly depends on which measurements Victor performs
at a later time on photons 2 and 3.

THE CHOICE TO SWAP ENTANGLEMENT DEPENDS ON VICTORS CHOICE.

This choice projects an entangled state or a separable state onto photons 1&4. Alice and Bob have the data needed to verify when entanglement swapping has occurred and when it hasn't occurred.

I repeat, it has nothing to do with Victor choosing between Bell states.

Anyone who wants to read what the paper actually says can go here:

http://arxiv.org/abs/1203.4834

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)

This will tell Alice and Bob if Victor swapped entanglement or if he didn't.

 

[DrewD]

My mom always told me not to respond to a troll, but I just can't help it.

According to Victor’s choice of measurement (i.e. entangled or separable state) and his results \left(\mbox{i.e. }\left|Φ^+\rangle\right._{23} ,\left|Φ^-\right\rangle_{23}, \mbox{or } \left|HH\right\rangle_{23} , \left|VV\right\rangle_{23}\right), Alice and Bob can sort their already recorded data into 4 subsets

Notice the first 6 words in this quote. I didn't even have to go back through and reread the paper! When you read through your entire quote it explicitly mentions what everyone has been saying... Victor's classical information is necessary to make sense of Alice's and Bob's information (that was previously recorded). You do know this, right? You're just messing around, right? You didn't accidentally choose a passage to quote that (without even using the formalism that has been used to disprove your claim) shows you are wrong, right?

My father often reminds me not to attribute to malice that which can be attributed to ignorance, so maybe you really did just misunderstand the paper. While there is always the possibility that somebody without a lot of training will make some sort of breakthrough, it is unlikely that a world changing result would be missed for almost two years by EVERY physicist and only noticed by laymen on the internet.

 

[matrixrising]

DrewD,

You should have went back and read the paper.

Alice and Bob know Victors results. If you read the paragraph I quoted above, it tells you when Victor chooses to swap entanglement, his choice is projected onto photons 1 & 4 as +23 or - 23. If Victor chooses not to swap entanglement it's, HH23 or VV23.

Again, +23 and -23 = 0

HH23 and VV23 = 1.

No matter which combination occurs, Victor can control the 1's and 0's sent to Alice and Bob.

This is the point I was making to Dr. Chinese. Victor isn't choosing between random Bell states. He's choosing between entanglement swapping and no entanglement swapping. Notice the last line.

WHETHER ENTANGLEMENT SWAPPING OCCURS STRICTLY DEPENDS ON VICTORS MEASUREMENT.

That says it all.

 

[StevieTNZ]

Just close this thread.

 

[bhobba]

While there is always the possibility that somebody without a lot of training will make some sort of breakthrough, it is unlikely that a world changing result would be missed for almost two years by EVERY physicist and only noticed by laymen on the internet.

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

Just as an example, from Coulombs law and SR one can derive Maxwell's Equations:
http://www.cse.secs.oakland.edu/haskell/SpecialRelativity.htm

So out goes one of the most accurately verified physical theories of all time - stuff engineers use all the time to design just about every piece of electrical gear you have. Physics and engineering would be in DEEP DEEP trouble.

Personally I think the guy is just a bit early on in his physics education and is a bit carried away with something he thinks he has spotted without understanding if it was true then all sorts of problems would be raised.

Thanks
Bill

 

[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?