Photons from separated sources can be entangled - after they were detected

[Frame Dragger]

Let us see what the theory of decoherence says on this question, augmented by either many-world or Bohmian interpretation. According to such a view, we can say that:
1. There is no true collapse at all.
2. There is an effective collapse which occurs locally when interaction with the environment takes place.
3. When there are many particles, there are many effective local collapses, each associated with another particle.
4. Since dynamics is deterministic, these different effective collapses are not independent. Instead, if you know the result of one effective collapse, you can also predict something about the results of other effective collapses.
5. It doesn't matter which effective collapse occurs first.

Well, that is the upside of believing in a Pilot Wave and particles coming in ensembles. I don't see how that is any easier to swallow that SQM.

 

[Demystifier]

Well, that is the upside of believing in a Pilot Wave and particles coming in ensembles. I don't see how that is any easier to swallow that SQM.

Why exactly do you find it difficult to swallow?

 

[SpectraCat]

OK, maybe we agree on some of these things and I didn't realize it.

I don't think we disagree on the statistical predictions. So that is good. I have a few quibbles but I don't think we are far enough apart on some of them to keep hammering.

The issue that is open between us is the timing of collapse. Does it occur upon first measurement? Or does it relate to the context of the entire setup. I say - absolutely - it is the entire context that is important. On the other hand, I readily admit that for *some* contexts - such as basic entanglement followed by polarization measurements - it can be interpreted as occurring "as if" based on "first measured". So I saying that is a special case which is a bit deceptive. You could just as easily interpret collapse as occurring on the SECOND measurement as occurring on the first in that case.

...Since order does not matter.

Hmmm ... I will write up a more detailed response to this when I have more time, but for now I just want to say that I can't see how your above description could hold based on our earlier discussion.

Given the staring conditions (A & B, and C & D entangled) and the experimental setup, I agree that A will be entangled with either B or D whenever it is detected (ignoring possible external perturbations). It all depends on whether or not Charlie has acted before A is measured by Alice. If he has, then A will be entangled with D, if he has not, then A will be entangled with B. That is my view, and by "acted" here, I mean, that Charlie has allowed B and C to interfere at his fiber beamsplitter.

Two points that I will expand on in a future post:

1) I am still trying to find a reference to justify my separation of the interference and detection events in Charlie's BSM. However, I contend that it is the interference that is essential here, because the particles are detected in all configurations. Basically, if B & C are not entangled because A & D have been measured, then there is no entanglement after the interference event either.

2) Regarding the "delayed choice" aspect of this experiment, I am not sure that it is not appropriate to compare this with the "standard" delayed-choice double-slit experiment, such as the Scully paper. In the vein of the Wheeler gedanken, it seems interesting to ask the question in this case, what if Charlie inserted the fiber beamsplitter *after* B & C passed that point, but before they were detected. I guess by your contextual analysis, A & D would still be found to be entangled. Hmmm ... I will have to think about this some more ...

 

[Frame Dragger]

Why exactly do you find it difficult to swallow?

All of my objections to dBB have been raised in the 'end of local realism?' thread, as have nearly all of my objections to SQM. I am as yet, unconvinced by lengths each theory must go to, if they are to remain valid. SQM tries to stick to the math MOST of the time becuase the implications otherwise are too paradoxical, disturbing, or just plain weird.

dBB is the only of an ensemble (yes, a joke) of theories that WERE LHV theories. Now dBB is a Non-Local Hidden Variable Theory, and while SQM would require enormous refuation, a few experiments could kill dBB. Feet of clay are not a good place for a theory to begin. If HR is confirmed, dBB dies... if DCQE experiments lead to convincing evidence of atemporal collapse dBB is hurting. If the Higg's boson is found, dBB is going to have to find an alternate explanation for scalar fields that exists in tandem with the force carrier for that field.

What experiment is dBB (as a whole) going to run that in one shot does the same damage to SQM? QM as a whole can't just die, because dBB's predictions must be in line with QM for that theory to remain valid. What specific evidence of a pilot wave, or ensemble action... what NON post hoc explanation for an experiment carried out to prove a point in SQM will dBB offer?

Sell me on it. I don't find SQM terribly compelling, in no small part because SQM without an interpretation hurts my head. Still, it seems to be the best way to the approach the science.

EDIT: I should add, purely as a layperson, that the notion of a pilot wave seems like a desire to maintain the guiding hand of fate, if not god in matters. It's a fascinating idea that seems like a needless complication. QM is a partial theory, and knows it. dBB is trying to be complete, and it's clear we're FAR from that. Again... these are my completely 'gut' related concerns.

 

[Demystifier]

a few experiments could kill dBB.

I disagree.

If HR is confirmed, dBB dies...

What is HR?

if DCQE experiments lead to convincing evidence of atemporal collapse dBB is hurting.

Not true.

If the Higg's boson is found, dBB is going to have to find an alternate explanation for scalar fields that exists in tandem with the force carrier for that field.

Classical Higgs field can be described as a coherent state of Higgs particles, just as classical electromagnetic field can be described as a coherent state of photons.

 

[Frame Dragger]

I disagree.


What is HR?


Not true.


Classical Higgs field can be described as a coherent state of Higgs particles, just as classical electromagnetic field can be described as a coherent state of photons.

-I disagree with your disagreement!

-Hawking Radiation

-I think so

-That strikes me as a stretch for EM, and it just feels like more of one when you're trying to formulate the scalar 'mass'.

 

[Demystifier]

QM is a partial theory, and knows it.

Many would disagree.

dBB is trying to be complete, and it's clear we're FAR from that.

You are probably right on that. But how to find the complete theory without even trying? dBB is the best try so far. So it could be closer than SQM to the final goal. Or maybe it's on the wrong track, but again, how to know it without even trying?

 

[Demystifier]

-Hawking Radiation

Hawking radiation is not in contradiction with dBB.

 

[Demystifier]

-I think so

Do you have an argument?

-That strikes me as a stretch for EM, and it just feels like more of one when you're trying to formulate the scalar 'mass'.

I don't understand what is that supposed to mean.

 

[torquil]

According to QM, the path lengths of the various measuring devices can be set to any length in principle. And length implies time as well.

Why are you allowed to change the order of events? That is certainly not the case usually in quantum mechanics. I'm not sure this is correct, even if relativity has been mentioned in this thread.

Doesn't a simple application of nonrelativistic quantum theory disprove this, e.g. using the Schrödinger equation? Examining the state at a time before the BSM is made should easily tell you that photons A and D are uncorrelated?

EDIT: Sorry I didn't notice that there had been a long discussion. My viewpoint has probably already been mentioned.

Torquil

 

[DrChinese]

Why are you allowed to change the order of events? That is certainly not the case usually in quantum mechanics. I'm not sure this is correct, even if relativity has been mentioned in this thread. ... Examining the state at a time before the BSM is made should easily tell you that photons A and D are uncorrelated?

The results appear random when you look at A & D. You can't make any sense of them until you look at the B & C results - which can either be a BSM or ordinary polarization measurements (or anything else depending on a later choice).

 

[DrChinese]

Hmmm ... I will write up a more detailed response to this when I have more time, but for now I just want to say that I can't see how your above description could hold based on our earlier discussion.

Given the staring conditions (A & B, and C & D entangled) and the experimental setup, I agree that A will be entangled with either B or D whenever it is detected (ignoring possible external perturbations). It all depends on whether or not Charlie has acted before A is measured by Alice. If he has, then A will be entangled with D, if he has not, then A will be entangled with B. That is my view, and by "acted" here, I mean, that Charlie has allowed B and C to interfere at his fiber beamsplitter.

Two points that I will expand on in a future post:

1) I am still trying to find a reference to justify my separation of the interference and detection events in Charlie's BSM. However, I contend that it is the interference that is essential here, because the particles are detected in all configurations. Basically, if B & C are not entangled because A & D have been measured, then there is no entanglement after the interference event either.

2) Regarding the "delayed choice" aspect of this experiment, I am not sure that it is not appropriate to compare this with the "standard" delayed-choice double-slit experiment, such as the Scully paper. In the vein of the Wheeler gedanken, it seems interesting to ask the question in this case, what if Charlie inserted the fiber beamsplitter *after* B & C passed that point, but before they were detected. I guess by your contextual analysis, A & D would still be found to be entangled. Hmmm ... I will have to think about this some more ...

Picking up on our earlier discussion around whether the collapse occurs when the photon encounters the PBS or upon detection:

1. Would there be any way to tell the difference? I think we might agree that there is no way to detect this.

2. Is an observation reversible? Clearly if an observation is erased, then there effectively was no observation. If it is not reversed, then the observation occurred. We probably agree on this too.

3. If so, the only difference in our position is probably one of semantics. Because the decision to erase (or not) can be delayed until the point of detection, but not after. So to me, that signals the point of no return and therefore the point of observation. Whereas to you, the observation DID occur at the PBS as long as it wasn't erased.

 

[DrChinese]

A couple of comments. Making sure that everyone has seen this additional reference from Zeilinger's group, 2008:

http://arxiv.org/abs/0809.3991

"Entanglement swapping allows to establish entanglement between independent particles that never interacted nor share any common past. This feature makes it an integral constituent of quantum repeaters. Here, we demonstrate entanglement swapping with time-synchronized independent sources with a fidelity high enough to violate a Clauser-Horne-Shimony-Holt inequality by more than four standard deviations. The fact that both entangled pairs are created by fully independent, only electronically connected sources ensures that this technique is suitable for future long-distance quantum communication experiments as well as for novel tests on the foundations of quantum physics. "

 

[DrChinese]

Now, regarding the above: Zeilinger et al say that the experiment does "establish entanglement between independent particles that never interacted nor share any common past." This is actually a slight - but fair - distortion. The photons do not interact IF you define entangled photons as independent entities. In QM, however, entangled photons do share a state equation and therefore should not truly be considered independent in spacetime.

Therefore, with entanglement swapping, there is a meeting of entangled pairs.

 

[Demystifier]

In QM, however, entangled photons do share a state equation and therefore should not truly be considered independent in spacetime.

Therefore, with entanglement swapping, there is a meeting of entangled pairs.

I completely agree with this.

 

[Frame Dragger]

I completely agree with this.

For me that ultimately means that intuition and LR are just out of the window.

EDIT: I also agree.

 

[DrChinese]

Here is a great new experimental report on the subject. Entanglement from fully independent sources are entangled, in a cleaner and much more accurate fashion that some of the earlier references provided. Here S=2.54, and Bell's Inequality is violated by 27 standard deviations.

http://arxiv.org/abs/1005.1426

Experimental non-local generation of entanglement from independent sources

Authors: Xian-Min Jin, Jügen Röch, Juan Yin, Tao Yang (2010)

"We experimentally demonstrate a non-local generation of entanglement from two independent photonic sources in an ancilla-free process . Two bosons (photons) are entangled in polarization space by steering into a novel interferometer setup, in which they have never meet each other. The entangled photons are delivered to polarization analyzers in different sites, respectively, and a non-local interaction is observed. Entanglement is further verified by the way of the measured violation of a CHSH type Bell's inequality with S-values of 2.54 and 27 standard deviations. Our results will shine a new light into the understanding on how quantum mechanics works, have possible philosophic consequences on the one hand and provide an essential element for quantum information processing on the other hand. Potential applications of our results are briefly discussed. "

 

[billbray]

Dr. Chinese, you are the master of this stuff -so you would know - Michio Kaku held a laser on a beam splitter and said the photons at the end of the beams were superpositioned. I tried this trick in my lab with a light meter and measured about half the intensity of each outgoing beam when compared to the ingoing beam.

It doesn't seem to me that the resulting beams are superpositioned. Unless you have a better way to do the experiment with a laser, beam splitter, and light meter?

 

[Frame Dragger]

Dr. Chinese, you are the master of this stuff -so you would know - Michio Kaku held a laser on a beam splitter and said the photons at the end of the beams were superpositioned. I tried this trick in my lab with a light meter and measured about half the intensity of each outgoing beam when compared to the ingoing beam.

It doesn't seem to me that the resulting beams are superpositioned. Unless you have a better way to do the experiment with a laser, beam splitter, and light meter?

Great scientist, lousy shows that he's on, so the explanations tend towards the painfully simplistic or misleading.

 

[DrChinese]

Dr. Chinese, you are the master of this stuff -so you would know - Michio Kaku held a laser on a beam splitter and said the photons at the end of the beams were superpositioned. I tried this trick in my lab with a light meter and measured about half the intensity of each outgoing beam when compared to the ingoing beam.

It doesn't seem to me that the resulting beams are superpositioned. Unless you have a better way to do the experiment with a laser, beam splitter, and light meter?

Cool that you did this!

So we know the output beams should be 1/2 the intensity, so no surprise there.

Perhaps Kaku was trying to say that before they are detected, they are in a superposition. That would be accurate in my view.

 

[Lord Crc]

As someone who has only read pop-sci stuff about QM, I'm a bit slow on how to interpret the setup proposed in the first post. If anyone could help me out I'd be very grateful as I find these things very fascinating! My main confusion lies with what Alice and Bob will measure.

Let's say Alice sets her detector to theta degrees and say Bob also sets his to theta degrees. If photons A and D are entangled then they will measure the same 100% of the time (or 0% if the photons are anti-correlated). Now, if A and D are NOT entangled, then, since the pairs AB and CD are independent, they would measure the same 50% of the time (ie independent uniformly random distributions).

Assuming the above is correct, then I fail to see how Charlie's information is needed for Alice and Bob to figure out if the photons are entangled. That is, I understand they can't do so on a single pair basis, but assume that Charlie ensures that the pairs are always entangled. Then how is this different from the situation where Charlie doesn't exist and Alice and Bob performs the measurements on a single pair of entangled photons?

However if it results in the same situation as a single pair measurement, then it would seem that Charlie has no free will, assuming he doesn't change his mind too often :)

Due to this I assume that I missed a turn somewhere, invalidating my subsequent assumptions. Anyone care to shed some light? Thanks in advance.

 

[zonde]

Here is a great new experimental report on the subject. Entanglement from fully independent sources are entangled, in a cleaner and much more accurate fashion that some of the earlier references provided. Here S=2.54, and Bell's Inequality is violated by 27 standard deviations.

http://arxiv.org/abs/1005.1426

Experimental non-local generation of entanglement from independent sources

Authors: Xian-Min Jin, Jügen Röch, Juan Yin, Tao Yang (2010)

"We experimentally demonstrate a non-local generation of entanglement from two independent photonic sources in an ancilla-free process . Two bosons (photons) are entangled in polarization space by steering into a novel interferometer setup, in which they have never meet each other. The entangled photons are delivered to polarization analyzers in different sites, respectively, and a non-local interaction is observed. Entanglement is further verified by the way of the measured violation of a CHSH type Bell's inequality with S-values of 2.54 and 27 standard deviations. Our results will shine a new light into the understanding on how quantum mechanics works, have possible philosophic consequences on the one hand and provide an essential element for quantum information processing on the other hand. Potential applications of our results are briefly discussed. "

I would say that experiment from Zeilinger's group is much better then this one. Here this non-locality argument is quite dubious as all photons go through the same interferometer.

And if you compare them from perspective of practical application it's clear that experiment from Zeilinger's group can be used to develop quantum repeaters where this one can't. I think it's quite reasonable way how to make distinction between true quantum "non-locality" and feigned quantum "non-locality".

EDIT: Thought that in last sentence I should have used "true/feigned non-local generation of entanglement" instead of "true/feigned quantum non-locality".

 

[DrChinese]

As someone who has only read pop-sci stuff about QM, I'm a bit slow on how to interpret the setup proposed in the first post. If anyone could help me out I'd be very grateful as I find these things very fascinating! My main confusion lies with what Alice and Bob will measure.

Let's say Alice sets her detector to theta degrees and say Bob also sets his to theta degrees. If photons A and D are entangled then they will measure the same 100% of the time (or 0% if the photons are anti-correlated). Now, if A and D are NOT entangled, then, since the pairs AB and CD are independent, they would measure the same 50% of the time (ie independent uniformly random distributions).

Assuming the above is correct, then I fail to see how Charlie's information is needed for Alice and Bob to figure out if the photons are entangled. That is, I understand they can't do so on a single pair basis, but assume that Charlie ensures that the pairs are always entangled. Then how is this different from the situation where Charlie doesn't exist and Alice and Bob performs the measurements on a single pair of entangled photons?

However if it results in the same situation as a single pair measurement, then it would seem that Charlie has no free will, assuming he doesn't change his mind too often :)

Due to this I assume that I missed a turn somewhere, invalidating my subsequent assumptions. Anyone care to shed some light? Thanks in advance.

I think you have summarized things pretty well. The issue is that clearly, sometimes A & D ARE in fact entangled. And that depends on what happens with B & C. There is no questioning the effect really. It is just: how do you interpret it? (And no interpretation is even required!)

 

[zonde]

Assuming the above is correct, then I fail to see how Charlie's information is needed for Alice and Bob to figure out if the photons are entangled. That is, I understand they can't do so on a single pair basis, but assume that Charlie ensures that the pairs are always entangled. Then how is this different from the situation where Charlie doesn't exist and Alice and Bob performs the measurements on a single pair of entangled photons?

Two photons from two different pairs interact at Charlie's beam splitter. Now there are two possibilities either both photons appear at the same output of Charlie's beam splitter or they appear at different outputs. Entangled pairs at Alice and Bob are those where corresponding photons at Charlie are detected in different outputs. And Charlie have to communicate this information to Alice and Bob so that they could create appropriate subsample. If Alice and Bob do not check they pairs against Charlie's data they do not see any entanglement.

 

[Lord Crc]

Entangled pairs at Alice and Bob are those where corresponding photons at Charlie are detected in different outputs.

Ah there's my misunderstanding. I was under the impression Charlie could determine which pair to entangle, not that he merely observed which ones were which. That changes things a bit :)

Thanks to both of you. While the experiment seems a bit less weird now, it's never the less very fascinating to contemplate.