FTL Communication: Closing Bell Test Loopholes for Quantum Integrity

[Alex Torres]

TL;DR Summary

Anton Zeilinger quest in quantum communication

Have followed the work of Anton Zeilinger for some time, noticed it's mostly centered in a quest to close Bell test loopholes for the sake of quantum communication integrity but how about if by relaxing this he could overcome the need of the classical channel for communication...take for example his version of delayed choice entanglement swapping, how about if Alice and Bob are located at the same station with both deciding upon the same type of measurement...

Could they rule out the occurrence of negative and positive entanglement by deciding upon measuring only spin up or spin down...?

 

[DrChinese]

That doesn't work. Entangled systems are in a superposition of up/down (or other states that can take on binary type values). So all anyone ever sees is random outcomes. Not a lot of information to be communicated that way. That classical channel is needed to "decode" the hypothetical message.

 

[Nugatory]

No matter how you set things up and which measurements are made in which sequence, you won’t get FTL communication. However, if you could more precisely describe the thought experiment you’re considering, we might be able to explain what will happen.

 

[Alex Torres]

That doesn't work. Entangled systems are in a superposition of up/down (or other states that can take on binary type values). So all anyone ever sees is random outcomes. Not a lot of information to be communicated that way. That classical channel is needed to "decode" the hypothetical message.

That's true as long as the measurement made by Victor is random...but how if Victor always swaps entanglement upon Alice/Bobs pairs throughout the entire experiment run?

 

[Alex Torres]

No matter how you set things up and which measurements are made in which sequence, you won’t get FTL communication. However, if you could more precisely describe the thought experiment you’re considering, we might be able to explain what will happen.

https://arxiv.org/abs/1203.4834note that Zeilinger wants to avoid loopholes so measurements done by Alice/Bob are totally independent of each other, by relaxing this and letting Alice/Bob to agree upon the same measurement, let's say spin up/spin down...then it could be possible to completely rule out the occurrence of negative and positive entanglement requiring a classical communication channel with Victor to decode..

 

[Michael Price]

Summary: Anton Zeilinger quest in quantum communication

Have followed the work of Anton Zeilinger for some time, noticed it's mostly centered in a quest to close Bell test loopholes for the sake of quantum communication integrity but how about if by relaxing this he could overcome the need of the classical channel for communication...take for example his version of delayed choice entanglement swapping, how about if Alice and Bob are located at the same station with both deciding upon the same type of measurement...

Could they rule out the occurrence of negative and positive entanglement by deciding upon measuring only spin up or spin down...?

There's a simple rule to remember: entanglement never, NEVER allows you to transmit information.

 

[Alex Torres]

There's a simple rule to remember: entanglement never, NEVER allows you to transmit information.

That's true as long as you try to use quantum entanglement as the carrier of information but in Zeilinger s experiment the quantum correlation is a message itself, so the beauty of this is that the recipient Alice/Bob are receiving a pair of entangled photons, if Victor swaps entanglement to the entire batch then Alice/Bob will have a batch of photons 100% correlated...

 

[DennisN]

@Alex Torres,
classical information can not be transmitted this way. We can't beforehand set the state of the photon(s), because we have no control over that. We can only measure their state by letting them pass through e.g. polarizers and then detect them with photodetectors.

 

[DrChinese]

That's true as long as you try to use quantum entanglement as the carrier of information but in Zeilinger s experiment the quantum correlation is a message itself, so the beauty of this is that the recipient Alice/Bob are receiving a pair of entangled photons, if Victor swaps entanglement to the entire batch then Alice/Bob will have a batch of photons 100% correlated...

Sure they will be 100% correlated pairs, but you will never know that. Unfortunately they will be a random mixture of $\psi + {~ and ~} \psi -$ entangled pairs. I.e. ++/-- for half the pairs, and +-/-+ for the other pairs. Just a random bunch of bits that contain no useful information.

As mentioned already by the others in this thread.

 

[Alex Torres]

Sure they will be 100% correlated pairs, but you will never know that. Unfortunately they will be a random mixture of $\psi + {~ and ~} \psi -$ entangled pairs. I.e. ++/-- for half the pairs, and +-/-+ for the other pairs. Just a random bunch of bits that contain no useful information.

As mentioned already by the others in this thread.

Yes those results are in the papers...but isn't that due to the fact that Alice and Bob are making independently random measurements on their respective particles?...how about if they get together to make the same type of measurement on the entangled particles they receive...lets say spin up/spin down in both particles simultaneously...will this modification in the experiment setup rule out the occurrence of negative and positive entanglement and render a definite quantum correlation between Alice and Bobs particles?

 

[DrChinese]

From your reference, bottom of page 3:

"Victor may perform a Bell-state measurement which projects photons 2 and 3 either onto |Φ+〉23 or onto |Φ−〉23 . "

One state has the 1 & 4 photons matched (HH or VV). The other has them mismatched (HV or VH). So you need a classical signal to know which is which.

 

[Alex Torres]

From your reference, bottom of page 3:

"Victor may perform a Bell-state measurement which projects photons 2 and 3 either onto |Φ+〉23 or onto |Φ−〉23 . "

One state has the 1 & 4 photons matched (HH or VV). The other has them mismatched (HV or VH). So you need a classical signal to know which is which.

So, all measurements done by Victor, Alice and Bob are limited to check upon polarization of the 4 photons...is there any physical restraint in performing a different measurement to photons 1 & 4 instead?

Lets say spin up/down just for the sake of limiting the number of possible outcomes from measuring the entangled photons (1 & 4) to only 2 (up/down)...if my conception of entanglement is correct, photons 1 & 4 then should always correlate if Victor swaps entanglement for an entire experiment run without the need to post select...otherwise my conceptual view of entanglement as a holistic quantum state is completely wrong...thanks for shedding light on this one

 

[Alex Torres]

@Alex Torres,
classical information can not be transmitted this way. We can't beforehand set the state of the photon(s), because we have no control over that. We can only measure their state by letting them pass through e.g. polarizers and then detect them with photodetectors.

If you receive a batch of 100 entangled pairs of photons, can you determine a 100% quantum correlation by measuring the spin up/down of each pair??...if so, do you really need the sender to tell you they are all entangled?

 

[DrChinese]

If you receive a batch of 100 entangled pairs of photons, can you determine a 100% quantum correlation by measuring the spin up/down of each pair??...if so, do you really need the sender to tell you they are all entangled?

Depending on the source, the answer could be yes or no.

Yes: The most common source is parametric down converted photon pairs. Those are easy. Depending on whether they are Type I or Type II, you can see they are entangled (either all matched or all mismatched). So you can distinguish those from unentangled photon pairs easily. But... there is no good way to use those to send a message FTL. Because the pairs themselves travel to the receiver at c.

Entangled pairs (assuming Type I PDC)
HH
VV
VV
HH
HH
HH
VV
HH
etc.

Unentangled pairs:
VH
HV
HH
HV
HH
VV
HV
VV
etc.

No: As per your reference (via entanglement swapping), the pairs you see are a 50-50 mixture of matched (HH or VV) and mismatched (HV or VH) pairs. Yes, they are all entangled (or at least some are - but that's another discussion). But because the occurrence of + or - correlation is random (there is no way to control this), there is no opportunity to use entanglement to send the message. So regardless of whether someone sends entangled pairs or not, this is all you see:

Entangled pairs per your reference (state is $\psi \pm$):
HH
VV
HV
VV
VH
VH
HH
HV
etc.

Unentangled pairs:
HH
VV
HV
VV
VH
VH
HH
HV

I.e. nothing distinguishes one sequence from the other. Unless of course you get a classical signal telling you which is which.

 

[Alex Torres]

But because the occurrence of + or - correlation is random

Totally agree with this...but isn't that due to the way the rules for measurement were set in advance for the experiment?...from Alice results alone there are at least 6 different outcomes, and 5 from Bob, so that certainly renders their data useless for FTL signaling...so how about instructing Alice/Bob to perform the same measurement along the whole experiment run, (while Victor casts entanglement), and such measurement must also limit the total possible outcomes to only 2...the obvious question is: could this be possible??

 

[DrChinese]

Totally agree with this...but isn't that due to the way the rules for measurement were set in advance for the experiment?...from Alice results alone there are at least 6 different outcomes, and 5 from Bob, so that certainly renders their data useless...so how about instructing Alice/Bob to perform the same measurement along the whole experiment run while Victor casts entanglement, and also apply a measurement that limits the total possible outcomes from Alice/Bob to only 2...the obvious question is: could this be possible??

A Nobel awaits you for figuring that out.

However, at this time the answer is no. Any known technique to do that requires something which ends up limiting the communication speed to c.

And don't forget, the cases we have been discussing are ideal cases anyway. In reality, the "entangled" pairs may consist of pairs that are not actually entangled (after the swap). For the swap to be successful, photons 2 & 3 must be indistinguishable. Sometimes they are distinguishable.

Keep in mind that there are two initial sources: the source for 1 & 2 and the source for 3 & 4. Because down conversion is itself random, each source emits entangled pairs randomly and they do not particularly get emitted at nearly the same time - there is nothing that can make that happen with PDC (it is sometimes called Spontaneous PDC or SPDC to indicate this). They occasionally do, and that is what sets the stage for a successful swap. For example: a typical set of 2 pairs that can be swapped might occur somewhere around every 5-10 minutes. There are many teams working today to create sources of on-demand entangled pairs, and when that is realized, swapping will be much easier to control.

 

[Alex Torres]

Thanks for that input!

 

[vanhees71]

A Nobel awaits you for figuring that out.

I'd not promise that much. The Nobel prizes for QT has already been given to Heisenberg, Dirac, Schrödinger, and (much too late, but finally they got it right) Born.

As the example of General Relativity shows, there can be even the most profound discoveries in physics which are not awarded at all. Finally to get the Nobel Prize is also somewhat of luck. Einstein was unlucky since there was philosohpical prejudices involved. Of course, they couldn't omit Einstein completely, and so they gave the prize to him for his general work in theoretical physics, naming particularly his work on photons (which is ironic, since that's the only of his theories which didn't stand the further development, while his work of relativity nowadays is understood as the foundation of all physics and his statistical work was just groundbreaking, particularly his work on Brownian motion considering the thermal fluctuations for the first time) and explicitly stating that they give this prize independent of the question whether (general) relativity is correct of not, i.e., it's one (maybe the only one) Nobel which was explicitly not given for a specific achievement ;-)).

 

[Alex Torres]

I'd not promise that much. The Nobel prizes for QT has already been given to Heisenberg, Dirac, Schrödinger, and (much too late, but finally they got it right) Born.

As the example of General Relativity shows, there can be even the most profound discoveries in physics which are not awarded at all. Finally to get the Nobel Prize is also somewhat of luck. Einstein was unlucky since there was philosohpical prejudices involved. Of course, they couldn't omit Einstein completely, and so they gave the prize to him for his general work in theoretical physics, naming particularly his work on photons (which is ironic, since that's the only of his theories which didn't stand the further development, while his work of relativity nowadays is understood as the foundation of all physics and his statistical work was just groundbreaking, particularly his work on Brownian motion considering the thermal fluctuations for the first time) and explicitly stating that they give this prize independent of the question whether (general) relativity is correct of not, i.e., it's one (maybe the only one) Nobel which was explicitly not given for a specific achievement ;-)).

Come on, that's just figurative speech... my first reaction was to ....LOL at that comment... Dr Chinese has a good sense of humor...

...nevertheless, i firmly believe this experiment of Zeilinger has a lot of potential and i would criticize him for not using his resources at improving it, but he is so focused on closing loopholes instead that he might just overlooked the prospects of his own present achievements... don't know if he has a Nobel Price, but if not, he is at the brinks of it...

 

[vanhees71]

If you ask me, Zeilinger would for sure deserve a Nobel prize for his experimental work on the foundations of QT. For me he is also a role model many more people in this field should follow, namely just using Q(F)T do invent ever more interesting experiments and doing them without (too) much of philosophical gibberish and esoterics to sell it somehow ;-)).