# New quantum experiments and its implications

zonde
Gold Member
Even if Victor could entangle particle at will (which I gather is fundamentally impossible) FTL information is still impossible because:

Alice and Bob still need to compare to find out if Victor entangled the pair or not.

Is that correct?
It's wrong. You can place Alice and Bob side by side but Victor at some remote location. That way Alice and Bob can compare their results very quickly and receive information from Victor FTL (or backward in time).

It's wrong. You can place Alice and Bob side by side but Victor at some remote location. That way Alice and Bob can compare their results very quickly and receive information from Victor FTL (or backward in time).
you are right. I missed that.

So essentially all three (A, B and V) need to compare their results? Because Victor does not have control over which photons he can entangle.

however there another thought experiment where information can be sent FTL with some decent level of accuracy but not 100% accuracy:

When Victor tries entangling photons lets say One a Billion, on average, get entangled. However Victor cannot do it at will.

Now lets say Victor has 10 sets of a Billion entangled photons each. Thus a total of 10 billion photons.

If Victor does not entangle any of the 10 sets means ---> 0.
i.e. Victor does nothing, he just sits there and does not touch any of the photons. If he does not touch any of the photons they cannot get entangled, correct?

If Victor is able to entangle even one of the 10 billion photons means ---> 1

Alice and Bob are sitting close to each other and compare their 10 billion photons. If they find none is entangled then it means Victor is saying 0.

If even a single one is entangled its means Victor is saying 1.

This experiment was thought really quickly. There must be a flaw/catch somewhere. Where is it?

BTW the above experiment can be carried out with just two photons, we don't necessarily need four. For example the DCQE can be used.

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@San K:
From what I understand now, simply comparing the results of A and B is not sufficient. The results of A and B will be completely random: only when you know which pairs to look at (because the other two from the four photons become entangled or not), you can find the correlation.

Imagine another experiment, in which something similar happens: V has a switch. Then A and B will either see a light flash red or green. If V turns the switch into one position, A and B will flash the same color, if V turns the switch into the other position, A and B will flash a different color. Now imagine that the position of the switch is completely random.
Now A and B will see a 50/50 correlation (on average). However, when V discloses the position of the switch, they'll be able to predict with a 100% accuracy the other person's color.
(Note that the only similarity between the two experiments is that the information about the correlation is completely useless until V discloses his information)

I'm "disappointed" (or relieved) to see that this experiment, once again, fails to prove any of the promised magic of quantum physics.

You're quote "I am still convinced that there is a hidden variable theorem for all of quantum mechanics." tells me you need a little work on interpretational QM. I know that Bell's theorem and the violation of the CHSH inequalities does not convince alot of people about quantum nonlocality, but I would advise you that seeking out a hidden variables type explaination is not the right direction. If I could make two suggestions;

1>Read Ballentine's chapter on Bell's theorem and that will get you past any hidden variables type theory

if you've already done that then move on to the real single world interpretation of QM

2>Read Zeilinger's 1999 paper "A foundational principle for QM"(J. Found. Phys.). Believe me, as a believer in physical realism, there is no better explaination of the laws of QM then the information interpretation written by Zeilinger (and von Weisacker, Wheeler, etc)

I don't know much about the article you posted but I know that some other threads have some info on the matter (www.sciforums.com "retrocausality in action")

My explaination is that there is nothing strange or retrocausal happening in this experiment. The results of Alice and Bob's measurements can be later considerred "entangled" regardless of the outcome of their measurements at the time. It only requires that when Victor makes his measurement ( Bell state measurement) that the system (which knows the outcome of Alice and Bob's measurements already because it is the system after all) simply projects onto the appropriate symmetry state to make it seem like the results of Alice and Bob's measurements were entangled already, which they were not.

This is interesting to weak measurement theorists though, because it seems that Alice and Bob are priorly post-selecting Bob's measurement.

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@San K:
From what I understand now, simply comparing the results of A and B is not sufficient. The results of A and B will be completely random: only when you know which pairs to look at (because the other two from the four photons become entangled or not), you can find the correlation.

Imagine another experiment, in which something similar happens: V has a switch. Then A and B will either see a light flash red or green. If V turns the switch into one position, A and B will flash the same color, if V turns the switch into the other position, A and B will flash a different color. Now imagine that the position of the switch is completely random.
Now A and B will see a 50/50 correlation (on average). However, when V discloses the position of the switch, they'll be able to predict with a 100% accuracy the other person's color.
(Note that the only similarity between the two experiments is that the information about the correlation is completely useless until V discloses his information)

I'm "disappointed" (or relieved) to see that this experiment, once again, fails to prove any of the promised magic of quantum physics.
You are right Gespex, for a moment/day I forgot that A and B are random and always 50% correlated.

Interestingly even V cannot tell which photons are entangled till all three (V, A and C) compare plus they need a coincidence counter too?

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DrChinese
Gold Member
SPDC is stimulated by random vacuum fluctuations, and hence the photon pairs are created at random times. The conversion efficiency is very low, on the order of 1 pair per every 10^12 incoming photons.
Thanks! Wasn't really sure what the order of magnitude was.

You're quote "I am still convinced that there is a hidden variable theorem for all of quantum mechanics." tells me you need a little work on interpretational QM. I know that Bell's theorem and the violation of the CHSH inequalities does not convince alot of people about quantum nonlocality, but I would advise you that seeking out a hidden variables type explaination is not the right direction. If I could make two suggestions;

1>Read Ballentine's chapter on Bell's theorem and that will get you past any hidden variables type theory

if you've already done that then move on to the real single world interpretation of QM

2>Read Zeilinger's 1999 paper "A foundational principle for QM"(J. Found. Phys.). Believe me, as a believer in physical realism, there is no better explaination of the laws of QM then the information interpretation written by Zeilinger (and von Weisacker, Wheeler, etc)

I don't know much about the article you posted but I know that some other threads have some info on the matter (www.sciforums.com "retrocausality in action")

My explaination is that there is nothing strange or retrocausal happening in this experiment. The results of Alice and Bob's measurements can be later considerred "entangled" regardless of the outcome of their measurements at the time. It only requires that when Victor makes his measurement ( Bell state measurement) that the system (which knows the outcome of Alice and Bob's measurements already because it is the system after all) simply projects onto the appropriate symmetry state to make it seem like the results of Alice and Bob's measurements were entangled already, which they were not.

This is interesting to weak measurement theorists though, because it seems that Alice and Bob are priorly post-selecting Bob's measurement.
Thanks for your suggestions. I haven't read those articles yet, but at least the wikipedia article of CHSH fails to convince me as well. More specifically, this quote in wikipedia:
Note that in all actual Bell test experiments it is assumed that the source stays essentially constant, being characterised at any given instant by a state ("hidden variable") λ that has a constant distribution ρ(λ) and is unaffected by the choice of detector setting.
I fail to see why one would make the assumption that ρ(λ) has a constant distribution. It seems to me that it leaves a massive gap where hidden variable theorems may still exist, or is that mere ignorance on my side?

DrChinese
Gold Member
I fail to see why one would make the assumption that ρ(λ) has a constant distribution. It seems to me that it leaves a massive gap where hidden variable theorems may still exist, or is that mere ignorance on my side?
We expect random results. If the distribution was unbalanced, we would notice that in experiments pretty quickly. I.e. there are more H> than V> when we measure at 30 degrees. But even if the source does have some asymmetry in the hidden variables themselves, we can accept that too as long as there is a constant expectation value.

I would like to find that out too. For me derivation in paper (pg 14) is too short to follow it so I won't try to offer derivation for HV and VH separable states.
Hi Stevie,

I will write the state evolutions down and send you some scans. I hope I can do that by Wednesday.

Cheers,

Johannes
Will post the scan when they're sent through.

zonde
Gold Member
Will post the scan when they're sent through.
Will wait for it.

Do we expect to see photons #1 and #4 both in VV with photons #2 and #3 in HH (and vice versa)? And not #1 and #4 in HH, and #2 and #3 also in HH?

Received a PDF from Johannes Kofler re: the evolution of the separable state. But this is without the EOW's etc in action (which should produce the same result).

However if we have definite states prior to the first BS, even though after they're in superposition of travelling both paths, they reach the plates and are converted into L and R polarisation. When they reach the PBS's after the 2nd BS, wouldn't they have 1/2 probability of taking on V and H? *shrugs*

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Okay - so I don't understand a few things:

1) We start off with two pairs of entangled photons, #1 and #2, and #3 and #4 as |H>|V> - |V>|H>.
2) How do we get from that to describing #2 and #3 (before the 1st beam splitter) as |H>|H> + |V>|V> and |H>|H> - |V>|V>, when no entanglement swapping hasn't occured yet?

If we sent #2 and #3 through the interferometry, and they were definite H or V polarised, would we even end up with the results obtained? Shouldn't we describe #2 and #3 before the 1st BS as if they're still entangled originally, and calculate the result from that? But even then we wouldn't end up with |H>|V>(b") or |H>|V>(c") or |H>(b")|H>(c") or |V>(b")|V>(c").

Thanks! Wasn't really sure what the order of magnitude was.
post edit -this should have referenced San K post # 19. My apologies, Dr. Chinese.
Whereas the order of magnitude seems immense, all but these scant few down converted photons pass 'straight' through the mechanism while the 'entangled' down converted photons travel in a cone away from this straight line. Each pair of down converted photons split 180 degrees from each other. These are the entangled pairs of photons which are used in experiments. There is 'nearly' no messy noise that can enter an experiment in this process. IMO.

mathal

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