Entanglement Swapping and FTL Communication

Recent experiments realized the thought experiment of Asher Peres on entanglement swapping. Here's the abstract.

Motivated by the question, which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping. There, entanglement can be "produced a posteriori, after the entangled particles have been measured and may no longer exist". In this work we report the first realization of Peres' gedanken experiment. Using four photons, we can actively delay the choice of measurement-implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator-on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as "quantum steering into the past".
Couldn't Alice and Bob in the experiment receive information faster than light from Victor? Here's the set up:

Both pairs of photons are entangled, so that the two particles in the first set are entangled with each other, and the two particles in the second set are entangled with each other. Then, one photon from each pair is sent to a person named Victor. Of the two particles that are left behind, one goes to Bob, and the other goes to Alice.

But now, Victor has control over Alice and Bob's particles. If he decides to entangle the two photons he has, then Alice and Bob's photons, each entangled with one of Victor's, also become entangled with each other. And Victor can choose to take this action at any time, even after Bob and Alice may have measured, changed or destroyed their photons.
http://www.livescience.com/19975-spooky-quantum-entanglement.html

Victor could use this scheme to send information to Alice and Bob. Bob and Alice would be in the same place and Victor could be a mile or 5 light years away.

101100 = dog
100110 = cat

If Victor has 6 particle pairs with him, he can then send Bob and Alice the word dog or cat. Let's say he wants to send dog.

Victor would do this. He would entangle the first pair, he would choose not to entangle the second pair, he would then entangle the next two pairs and he would choose not to entangle the last two pairs.

When Bob and Alice check their particles pairs, they would see:

quantum correlations, uncorrelated, quantum correlations, quantum correlations, uncorrelated, uncorrelated or 101100 which = dog.

The beauty of this is causality will be preserved because nothing is actually moving through space-time faster than light.

This is an experiment that could be carried out today with random number generators and atomic clocks. Why wouldn't this be FTL communications?
 
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The problem is with assigning 0's and 1's to whether the pair Alice and Bob hold is entangled or not.

How would know that Alice and Bob's photons are entangled? They might exhibit correlations that may make it look like they're entangled, when in fact they are not. I can't go into the technical details right now, but may be able to answer more fully tomorrow.
 
To DennisN:

This wouldn't be the case because you can still show the information was transferred faster than light with atomic clocks and random number generators. The Author of the paper even talked about communication through computers and quantum computers.

To StevieTNZ:

Of course Alice and Bob would know their photons exhibit quantum correlations. The whole idea behind the experiment hinges on Alice and Bob knowing if their particles showed quantum correlation or classical correlations. They can even see the quantum correlations before Victor even makes us choice to entangle his particle pair.
 
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To StevieTNZ:

Of course Alice and Bob would know their photons exhibit quantum correlations. The whole idea behind the experiment hinges on Alice and Bob knowing if their particles showed quantum correlation or classical correlations. They can even see the quantum correlations before Victor even makes us choice to entangle his particle pair.
Not quite. I would encourage you to read the Nature article itself. Especially when Alice and Bob measure in the H/V and 45/135 bases, and no bell-state measurement occurs on Victor's side. You'll find Alice and Bob's photons sometimes exhibit quantum correlations despite not being entangled. Remember, you're limiting yourself to 6 pairs of photons.
 
To StevieTNZ:

That's not the case. When Victor chooses to entangle his particle pair, the quantum correlation between Alice and Bob's particle pair is clearly identifiable or you couldn't even carry out the test. How would you know when Victor's choice cause quantum correlations between Bob and Alice's particle pair? Here's how the ended the article:

With our ideal realization of the delayed-choice entanglement swapping gedanken experiment, we have
demonstrated a generalization of Wheeler’s “delayed-choice” tests, going from the wave-particle duality of a
41
single particle to the entanglement-separability duality of two particles . Whether these two particles are
entangled or separable has been decided after they have been measured. If one views the quantum state as a
real physical object, one could get the seemingly paradoxical situation that future actions appear as having an
influence on past and already irrevocably recorded events. However, there is never a paradox if the quantum
2
state is viewed as to be no more than a “catalogue of our knowledge” . Then the state is a probability list for all
possible measurement outcomes, the relative temporal order of the three observer’s events is irrelevant and no
physical interactions whatsoever between these events, especially into the past, are necessary to explain the
delayed-choice entanglement swapping. What, however, is important is to relate the lists of Alice, Bob and
Victor’s measurement results. On the basis of Victor’s measurement settings and results, Alice and Bob can
group their earlier and locally totally random results into subsets which each have a different meaning and
interpretation. This formation of subsets is independent of the temporal order of the measurements. According
to Wheeler, Bohr said: “No elementary phenomenon is a phenomenon until it is a registered phenomenon.” We would like to extend this by saying: “Some registered phenomena do not have a meaning unless they are put in relationship with other registered phenomena.”
On page 6 of the study this is clearly spelled out.

One more thing.

According to Victor’s choice and his results, Alice and Bob can sort their already recorded data into
subsets and can verify that each subset behaves as if it consisted of either entangled or separable pairs of distant photons, which have neither communicated nor interacted in the past.
This pretty much sums it up.
 
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Okay:

So we have a two pairs of entangled photons, as such that each is created in the state |H>|V> - |V>|H>.

We send one photon from each pair to Victor, who either performs a separable state, or a bell-state measurement.

Alice and Bob measure in the H/V basis. So does Victor.

If Alice and Bob's photons are measured and both are found in |H>|H>, we know either that Victor performed a separable state measurement and got |V>|V> for his two photons; or that Victor performed a bell-state measurement and Alice and Bob's photons are now entangled in the bell-state |H>|H> - |V>|V> or |H>|H> + |V>|V>.

How can we differentiate between the two without knowing what kind of measurement Victor made (either a separable or bell-state)? That requires a classical communication channel, and thus renders your FTL communication scheme invalid.
 
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To DennisN:

This wouldn't be the case because you can still show the information was transferred faster than light with atomic clocks and random number generators. (my bolding)
What kind of information? Do you have a scientific source to support the words I put in bold? (a peer-reviewed paper?)

Furthermore, you did not answer my question in post #2. Care to consider it? Exactly how would they "check" their particle pairs without using a classical information channel? (non-FTL). Determining correlations means comparing the results of two distant measurements. How do you compare the results without any classical communication?

The Author of the paper even talked about communication through computers and quantum computers.
Not any faster-than-light communication. I can't find it neither in the article nor paper.

Here's the paper:
Experimental delayed-choice entanglement swapping
http://arxiv.org/abs/1203.4834

Download it and search for e.g. the words "faster" and "communication".
 
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Both of you guys seem to be making the same point which doesn't apply. You say they would have to use a classical information channel but that doesn't mean we can't determined if Bob and Alice received the message from Victor faster than light by using atomic clocks.

It will not take Bob and Alice 5 light years to check for quantum correlations so the point is mute. Entanglement has already been measured using atomic clocks. We can know if Bob and Alice are receiving the information faster than light.
 
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Well, we can get into the measurement problem which has bearing on your scheme.
 
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Both of you guys seem to be making the same point which doesn't apply. You say they would have to use a classical information channel but that doesn't mean we can't determined if Bob and Alice received the message from Victor faster than light by using atomic clocks.

It will not take Bob and Alice 5 light years to check for quantum correlations so the point is mute. Entanglement has already been measured using atomic clocks. We can know if Bob and Alice are receiving the information faster than light.
Alice and Bob, if they're people, won't know if they received a message FTL due to the reason I stated in my earlier post.
 
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We can know if Bob and Alice are receiving the information faster than light.
Its impossible.

To send information the person sending would need to know the outcome of his/her measurement so the person receiving it knows what was sent. Since QM says you cant do that its not possible to send information this way.

Its utterly trivial once you understand that you can't predict the spin that will be observed.

Thanks
Bill
 
At the end of the day, this could easily be tested using random number generators and atomic clocks today.

You can set this up where Victor, Bob and Alice are 1 mile apart. Scientist have already clocked entanglement using atomic clocks. You determine the speed that light will carry a message from Victor to Alice and Bob. You then set up three words that could be sent.

101100 = dog
100110 = cat
110001 = rat

You then have a random number generator determine which word will be sent. You also have atomic clocks set up with Alice and Bob and one with Victor.

To determine which word is being sent, you just need to check for quantum correlation which = 1 and when there's no quantum correlation it = 0.

Quantum correlation occurs when Victor entangles a particle pair that's already entangled with the particle pairs Alice and Bob have. The thing that makes this possible is Alice and Bob's particle pair doesn't exhibit quantum correlation until Victor chooses to entangle his particle pair.

So it's simple. When Victor wants to send a 1, he entangles his particle pair and then the particle pair of Alice and Bob will show quantum correlation. If Victor wants to send an 0, he doesn't entangle his particle pair and Alice and Bob will not find quantum correlation.

I think people can't think past Relativity when it comes to FTL communication. This isn't violating anything because there's no information passing through space-time between Victor, Bob and Alice.
 
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So it's simple. When Victor wants to send a 1, he entangles his particle pair and then the particle pair of Alice and Bob will show quantum correlation. If Victor wants to send an 0, he doesn't entangle his particle pair and Alice and Bob will not find quantum correlation.
So we have a two pairs of entangled photons, as such that each is created in the state |H>|V> - |V>|H>.

We send one photon from each pair to Victor, who either performs a separable state, or a bell-state measurement.

Alice and Bob measure in the H/V basis.

If Alice and Bob's photons are measured and both are found in |H>|H> (a quantum AND classical correlation), we know either that Victor performed a separable state measurement and got |V>|V> for his two photons; or that Victor performed a bell-state measurement and Alice and Bob's photons are now entangled in the bell-state |H>|H> - |V>|V> or |H>|H> + |V>|V>.

How can we differentiate between the two without knowing what kind of measurement Victor made (either a separable or bell-state)? That requires a classical communication channel, and thus renders your FTL communication scheme invalid.
 
bhobba,

You don't have to predict anything. You're just seeing if Victor made the choice to entangle or not to entangle. You're not trying to predict which spin will be observed.

I direct you to the experiment posted in 2012:

http://arxiv.org/abs/1203.4834

Motivated by the question, which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping. There, entanglement can be "produced a posteriori, after the entangled particles have been measured and may no longer exist". In this work we report the first realization of Peres' gedanken experiment. Using four photons, we can actively delay the choice of measurement-implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator-on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as "quantum steering into the past".
 
1,456
365
So we have a two pairs of entangled photons, as such that each is created in the state |H>|V> - |V>|H>.

We send one photon from each pair to Victor, who either performs a separable state, or a bell-state measurement.

Alice and Bob measure in the H/V basis.

If Alice and Bob's photons are measured and both are found in |H>|H> (a quantum AND classical correlation), we know either that Victor performed a separable state measurement and got |V>|V> for his two photons; or that Victor performed a bell-state measurement and Alice and Bob's photons are now entangled in the bell-state |H>|H> - |V>|V> or |H>|H> + |V>|V>.

How can we differentiate between the two without knowing what kind of measurement Victor made (either a separable or bell-state)? That requires a classical communication channel, and thus renders your FTL communication scheme invalid.
Therefore we cannot attribute to any one measurement a 0 or 1 until we find out what measurement Victor performed.
 
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At the end of the day
At the end of the day, it doesn't matter how you twist, turn, try this that, or whatever, since there is no way for the sender to determine what they want to send, there is no way to send information. All you have are correlations - but that is not enough.

Some bright spark came up with a sneaky way of doing it if you could clone states. But guess what - they discovered a theorem that proves you cant clone states. Nature has conspired to respect relativity.

You don't have to predict anything.
Sorry but you do.

Imagine you are the person receiving information. You get spin up or down. How does this relate to what the person sending it wanted to do - in other words if you got spin up exactly what did the person sending it do to ensure he/she got spin up?

No referring to a paper. This is utterly basic - you must be able to answer it if you want to send information.

Thanks
Bill
 
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StevieTNZ:

Asked and answered.

Of course Alice and Bob can differentiate between the two even before Victor makes a choice. This is the delayed choice portion of the experiment. This is from the article:

On the other hand, when Victor performs the separable-state measurement on photons 2 and 3 and does
not swap entanglement, the correlation only exists in the |H〉/|V〉 basis and vanishes in the |+〉/|−〉 and
R〉/|L〉 bases. This is a signature that 1 and 4 photons are not entangled but in a separable state.
When Victor does perform a Bell-state measurement there's significant correlations between photons 1 and 4 in all three bases.

In short, Alice and Bob can detect when Victor makes a separable state measurement or when he makes a Bell state measurement based on the strong correlations between photons 1 and 4 on all three bases or the absence of strong correlations in the +/- bases or R/L bases.

Again, FTL communication can be achieved.
 
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In short, Alice and Bob can detect when Victor makes a separable state measurement or when he makes a Bell state measurement based on the strong correlations between photons 1 and 4 on all three bases.
But to determine a correlation you need to know what's sent.

This is utterly trivial and obvious.

If you cant see such can't be used to send information - shrug - there are some things people just don't get.

Thanks
Bill
 
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StevieTNZ:

Asked and answered.

Of course Alice and Bob can differentiate between the two even before Victor makes a choice. This is the delayed choice portion of the experiment. This is from the article:



When Victor does perform a Bell-state measurement there's significant correlations between photons 1 and 4 in all three bases.

In short, Alice and Bob can detect when Victor makes a separable state measurement or when he makes a Bell state measurement based on the strong correlations between photons 1 and 4 on all three bases or the absence of strong correlations in the +/- bases or R/L bases.

Again, FTL communication can be achieved.
They only know that there are significant correlations in all three bases, when they know whether Victor performed a bell-state measurement or not.

Don't believe everything you read. I disagree with
In short, Alice and Bob can detect when Victor makes a separable state measurement or when he makes a Bell state measurement based on the strong correlations between photons 1 and 4 on all three bases or the absence of strong correlations in the +/- bases or R/L bases.
and that is evident by my example I provided earlier, which equally applies to 45/135 and R/L bases.
 
They only know that there are significant correlations in all three bases, when they know whether Victor performed a bell-state measurement or not.
This was easily explained.

This is why Bob and Alice will be in the same place and they can check for separable or Bell state correlations between photons 1 and 4.

Also, atomic clocks will let you know if the information was transferred faster than light.
 
StevieTNZ:

You said:

Don't believe everything you read.
Why should I believe you over the study????

That doesn't make much sense.
 

DrChinese

Science Advisor
Gold Member
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When Victor does perform a Bell-state measurement there's significant correlations between photons 1 and 4 in all three bases.

In short, Alice and Bob can detect when Victor makes a separable state measurement or when he makes a Bell state measurement based on the strong correlations between photons 1 and 4 on all three bases or the absence of strong correlations in the +/- bases or R/L bases.

Again, FTL communication can be achieved.
Great idea, matrixrising! There is one small issue, however...

When Victor decides to entangle photons 1 & 4, he does so by projecting 2 & 3 into a Bell state. There are 2 possible Bell states for 2 & 3 (actually more but only these 2 matter). One is correlated, and the other is anti-correlated. And guess what? The correlated Bell state has 1 & 4 now being correlated (the same), while the anti-correlated Bell state has 1 & 4 now being anti-correlated (different). There is NO way for Victor to force one outcome over the other. So sometimes 1 & 4 are correlated, quite true, but that never happens more than half the time. The result is that 1 & 4 are ALWAYS showing random pairings. No information to be determined from a random set of pairs. So regardless of what Victor chooses to do, Alice and Bob see:

Alice: HTHHTHTHHTHTHHTHTTHTTH
Bob: TTHTHHTTTHTHTTHHTTTHHTH

Or similar. You definitely get an A for effort, though. :smile:
 
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StevieTNZ:

You said:



Why should I believe you over the study????

That doesn't make much sense.
It doesn't need to make sense. Not everyone agrees with Zeilinger's conclusions. You came here for a critique of your FTL scheme, and now you're questioning some of what I'm providing you with based on "authority of a study".

This was easily explained.

This is why Bob and Alice will be in the same place and they can check for separable or Bell state correlations between photons 1 and 4.

Also, atomic clocks will let you know if the information was transferred faster than light.
I ask you to carefully consider my example, and also take into consideration the measurement problem.
 
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Why should I believe you over the study????
What he means is if you study something that to you seems to be saying a very basic and fundamental tenant of physics, that if true would earn the originator an immediate Nobel prize, is wrong, then it is much more likely you have misunderstood something, and need to go back and think about things carefully.

The fact the paper you cited is NOT saying you can actually sent information FTL, but speaks in terms of correlations, should be the giveaway.

But for some reason you don't get the very fundamental fact about correlations - to send information this way you need to know what happened at the other end.

Dr Chinese has given the detail of why your proposal will not work - but when you understand that only correlations exist you can see that you can try all sorts of tricks and they will fail.

Thanks
Bill
 
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