# Entangled photons thought experiment

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## Main Question or Discussion Point

Following scenario:

One million entangled photon pairs are created and are sent to Alice's and Bob's laboratory such that Alice receives 1 million photons(or any other large number) of which each is entangled to one of the million photons Bob receives, their laboratories being at a distance to each other.

Bob is to just have the 1 million photons run through a double slit and check if there is an interference pattern or not.

Also, Bob's laboratory is further away from the point the photons were entangled compared to Alice's laboratory, giving Alice some time to perform some tasks/experiments on the photons reaching her, before the photons reaching Bob go through the double slit.

My question is:

Can Alice affect if Bob will see an interference pattern or not in any way, by doing any kind of experiments on the 1 million photons reaching her, before the photons reaching Bob go through the double slit?

I leave the way of how the photons have been entangled and what kind of experiments Alice could do to affect the pattern Bob will see open to your imagination/expertise. I am only interested to know if Alice could or could not affect the pattern Bob would see.

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stevendaryl
Staff Emeritus
My question is:

Can Alice affect if Bob will see an interference pattern or not in any way, by doing any kind of experiments on the 1 million photons reaching her, before the photons reaching Bob go through the double slit?
It depends on what you mean by "affect". What Alice cannot do is change what Bob sees in a way that would allow Bob to conclude that Alice had done something. Anything that Bob sees is consistent with Alice doing nothing.

It depends on what you mean by "affect". What Alice cannot do is change what Bob sees in a way that would allow Bob to conclude that Alice had done something. Anything that Bob sees is consistent with Alice doing nothing.
I think i was pretty clear about it. Affect as in "manipulate" the photons on her side, such that their entangled counterparts on Bob's side would display an interference pattern necessarily, or no interference pattern necessarily depending on what Alice did or did not to the photons.

And yes, it seems like if Alice could do that, then she could send information to Bob that way. So your answer is basically "No", Alice cannot affect if Bob will see or will not see an interference pattern, meaning that Bob will always or will never see an interference pattern on his side, no matter what Alice does to the photons on her side.

stevendaryl
Staff Emeritus
And yes, it seems like if Alice could do that, then she could send information to Bob that way. So your answer is basically "No", Alice cannot affect if Bob will see or will not see an interference pattern, meaning that Bob will always or will never see an interference pattern on his side, no matter what Alice does to the photons on her side.
Right. Alice can't do anything that Bob would notice. However, I forget the details, but I think it is possible to set things up so that for certain subsets of the photons, there will be an interference pattern, and for other subsets there will not be. But in such a case, it's impossible for Bob to know ahead of time which subset to look at (although Alice will know).

Right. Alice can't do anything that Bob would notice. However, I forget the details, but I think it is possible to set things up so that for certain subsets of the photons, there will be an interference pattern, and for other subsets there will not be. But in such a case, it's impossible for Bob to know ahead of time which subset to look at (although Alice will know).
But if Alice could do that, being able to have a subset interfere or not, then to me this looks like she could send information that way. Because she could also decide to not affect the photons at all such that they either all interfere or all do not interfere(not sure which one would be the case).
So if Alice was to not be able to send any information, at least as far as i can tell, then the photons reaching Bob would either ALL have to interfere or all would have to not interfere. Or to be more precise, they would have to always act the same way, even if it meant that half of them would always interfere while the other half would not.
Anything else would allow Alice to send information FTL IMO.

stevendaryl
Staff Emeritus
But if Alice could do that, being able to have a subset interfere or not, then to me this looks like she could send information that way. Because she could also decide to not affect the photons at all such that they either all interfere or all do not interfere(not sure which one would be the case).
I should try to find the details, but if I'm remember correctly, Alice can't CAUSE one set or another to show interference, she can just figure out which set will show interference, based on her observations.

I should try to find the details, but if I'm remember correctly, Alice can't CAUSE one set or another to show interference, she can just figure out which set will show interference, based on her observations.
So it would be correct to state, that even if Alice would run the photons through a double slit herself, prepared in such a way that it would result in no interference pattern on her side, it would not affect the later outcome on Bob's side. The outcome on Bob's side would solely depend on how the entangled photons were prepared at the source.
So while Alice could end up seeing no interference pattern at all while Bob does, even though the photons were entangled?

stevendaryl
Staff Emeritus
So it would be correct to state, that even if Alice would run the photons through a double slit herself, prepared in such a way that it would result in no interference pattern on her side, it would not affect the later outcome on Bob's side. The outcome on Bob's side would solely depend on how the entangled photons were prepared at the source.
So while Alice could end up seeing no interference pattern at all while Bob does, even though the photons were entangled?
Here's a discussion of how interference can be created and destroyed. I haven't completely absorbed to the extent of being able to answer your questions adequately:

https://en.wikipedia.org/wiki/Quantum_eraser_experiment

But note the comment:
Bob can only distinguish the two peaks in his data after he got access to Alice's results
Alice might view her actions as affecting Bob's interference pattern (or destroying his interference pattern), but Bob can't actually see this effect without knowing what Alice knows.

vanhees71
Gold Member
2019 Award
Right. Alice can't do anything that Bob would notice. However, I forget the details, but I think it is possible to set things up so that for certain subsets of the photons, there will be an interference pattern, and for other subsets there will not be. But in such a case, it's impossible for Bob to know ahead of time which subset to look at (although Alice will know).
You can build a quantum eraser:

https://arxiv.org/abs/quant-ph/0106078

I think it's understandable if you know about classical optics and polarization states as well as simple optical elements like quarter-wave plates and employ a very naive photon picture just reducing everything to the polarization states only!

Of course, all this doesn't imply anything esoteric (no action at a distance, no retrocausation) as long as you stick to the minimal statistical interpretation. The statistical properties of the various ensembles, being defined by choosing partial ensembles from the total ensemble of photon observations according to the once and for all fixed measurement protocols by A and B, and these statistical properties is all that is predicted by QT, and that's what's observed in the experiment. It's fascinating and very "quantum" but it's not esoteric!

The part i am not understanding is as follows.

Given both Alice and Bob allow the entangled photons to go through the double slits undisturbed, i ASSUME that both would just see an interference pattern as expected to be seen in the usual double split experiments when not messing with the photons. It would be nice if someone could confirm this or deny it.

So this leaves the cases in which Alice runs the photons through polarizes/double slits or whatever else one could come up with.
If however above description for the case of not disturbing the photons at all is correct, THEN,as far as i can think, no matter what Alice would do to the photons on her side, Bob would have to see an interference pattern still, because if he didn't, simply by Alice deciding to either mess with the photons or not, she could pass yes/no messages to Bob.

stevendaryl
Staff Emeritus
The part i am not understanding is as follows.

Given both Alice and Bob allow the entangled photons to go through the double slits undisturbed, i ASSUME that both would just see an interference pattern as expected to be seen in the usual double split experiments when not messing with the photons.
I'm not sure about the details, but as a rule of thumb, entangled photons do not produce interference patterns. In "quantum eraser" experiments, it's a little more complicated than that...

I found this paper but i cannot evaluate it's validity.

It's a similar scenario to the one i am trying to understand

http://exvacuo.free.fr/div/Sciences/Dossiers/FTL/R Jensen - Is faster-than-light communication possible using entangled photons.pdf

part of the paper

Figure 1. Dopfer’s experiment. Entangled photons are emitted by a source. For every pair, the photon going left enters a double slit and is detected. The photon going right is detected by a “Heisenberg detector” H after passing through a lens. When H is placed at distance f behind the lens, an interference pattern is obtained (left). When H is at distance 2f, no interference pattern, or a “blob” is obtained (right).

It relies on comparing the two signals, both of which are limited to the speed of light!

DrChinese
Gold Member
Following scenario:

One million entangled photon pairs are created and are sent to Alice's and Bob's laboratory such that Alice receives 1 million photons(or any other large number) of which each is entangled to one of the million photons Bob receives, their laboratories being at a distance to each other.

Bob is to just have the 1 million photons run through a double slit and check if there is an interference pattern or not.

Also, Bob's laboratory is further away from the point the photons were entangled compared to Alice's laboratory, giving Alice some time to perform some tasks/experiments on the photons reaching her, before the photons reaching Bob go through the double slit.

My question is:

Can Alice affect if Bob will see an interference pattern or not in any way, by doing any kind of experiments on the 1 million photons reaching her, before the photons reaching Bob go through the double slit?

I leave the way of how the photons have been entangled and what kind of experiments Alice could do to affect the pattern Bob will see open to your imagination/expertise. I am only interested to know if Alice could or could not affect the pattern Bob would see.
The explanation is that entangled photons do not produce an interference pattern as you might expect. The easiest way to understand this is that they are not coherent.

Strilanc
Regardless of what Alice does on her end, Bob sees a bland lack-of-interference pattern.

The only way to find an interference pattern is for Alice to measure her copy in a particular way, have her tell Bob the result, then have Bob combine the results together to find that Alice's results split Bob's results into two complementary interference patterns.

An equivalent way to think about this is that Alice isn't controlling Bob's outcome, she's getting a noisy prediction of what it was or will be. When Bob's photon hits a point associated with a peak in 'Erased Case 1' and a trough in 'Erased Case 2', Alice is much more likely to measure the result corresponding to 'Erased Case 1'.

The explanation is that entangled photons do not produce an interference pattern as you might expect. The easiest way to understand this is that they are not coherent.
Even if they did, you still couldn't send information that way, because Bob has no way of knowing whether or not the pattern ( or its form ) is due to anything Alice did on her side. In a sense, the very meaning of "entanglement" already precludes the instantaneous exchange of information, in the absence of any other channels.

The explanation is that entangled photons do not produce an interference pattern as you might expect. The easiest way to understand this is that they are not coherent.
Yes, that is probably the case, but it is not unchallenged it seems.

Someone on stackexchange asked a similar question which was surprisingly answered by R.Jensen, the original author of the paper i linked above and seemingly wasn't challanged by anyone even though it had almost 400 views.

One answer to this is that the argument that any FTL signal will be drowned out by noise is insufficient. For example, you can filter out the noise due to singles in the Dopfer experiment by using a three-photon GHZ state. The one behind the 2-slit observing the interference pattern (or none) will receive 2 photons of the trio, the third one going to the Heisenberg Detector--D1 in fig.4.6. The only noise will be from "receiver-only" doubles as well as triples where the opposite of the trio never reaches D1...
see more here http://physics.stackexchange.com/questions/79427/why-does-the-dopfer-epr-experiment-require-coincidence-counting/81567#81567

he then links to another paper which describes an experiment with entangled photons and a mach zender inferometer, with all the mathematics included, claiming that FTL communication is possible once again. The paper can be found here http://vixra.org/pdf/1103.0095v1.pdf

quoting the introduction

It has been claimed that it is impossible to communicate instantaneously via a two-particle correlated system (Dušek, 1999) (although some experiments seem to indicate otherwise (Zeilinger, 1999)). The argument is based on the fact that the probability pi of obtaining an eigenvalue ai, after observable operation A is done on one photon of a correlated pair, is unaffected by operations done on the second photon of the pair. This argument repudiates attempts at faster-than-light information transfer using probabilities of such eigenvalues, however it does not eliminate methods of faster-than-light information transfer which rely on measurement of the uncertainty of eigenvalues.
I am looking if such an experiment has actually been performed still.

He is not the first one to claim such a thing. There exists another physicist, John G Cramer, who was funded by the public to perform a similar experiment claiming FTL communication pictured here http://www.physics.ohio-state.edu/~lisa/CramerSymposium/talks/Cramer.pdf (also included the Birgit Dopfer experimental setup in a more detailed version). Unfortunately, i cannot find any information on if he ever actually performed the experiment or any conclusive results on it. Which is strange in itself. It almost looks like he ran with the bag.

edit: Looks like i was too quick on Cramer. He actually admitted that it did not work.

"We analyzed it up, down and sideways, and concluded that what happens is, yes, you have a switchable interference pattern," Cramer said. "But because you have no coincidence measurement, you can't look at just one interference pattern. You have to add up two patterns. And they always add up to no signal."

Translation: When you analyze the quantum signal from earlier in time, you have to include an "anti-signal" in your calculations. Thus, the future leaves no fingerprints on the past. "Nature appears to be well-protected from the possibility of nonlocal signaling," according to Cramer and his co-author, Nick Herbert.
A more detailed explanation can be found in this paper http://arxiv.org/pdf/1409.5098.pdf

The Birgit Dopfer paper (1988) which seems to have triggered most of this, can be found here, https://users.isy.liu.se/jalar/kurser/QF/assignments/Dopfer1998.pdf but it is available in German only it seems.

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Strilanc
Someone on stackexchange asked a similar question which was surprisingly answered by R.Jensen, the original author of the paper i linked above and seemingly wasn't challanged by anyone even though it had almost 400 views.
The fact that the answer has a positive vote count is more significant than the fact that it has views. I have no idea why it has a positive upvote count.

The problem with the paper is really obvious: the author thinks that the quantum eraser leaves the system in the state $|+\rangle + |-\rangle$ with certainty, when actually it leaves the system in the state $|+\rangle + |-\rangle$ 50% of the time and the state $|+\rangle - |-\rangle$ 50% of the time. So the author thinks they have a pure state $|+\rangle + |-\rangle$ in one case and a maximally mixed state 50% $|+\rangle$ + 50% $|-\rangle$ in the other case (though they don't seem familiar with that terminology). But actually it's a maximally mixed state in both cases.

The rest of the paper is about distinguishing a pure state from the maximally mixed state. Which is trivial, and doesn't need to be explained in a paper.

That's why the paper is on vixra (it couldn't even make it to arxiv): it's junk with trivial flaws.

DrChinese
Gold Member
Yes, that is probably the case, but it is not unchallenged it seems.
An unfiltered entangled source won't produce an interference pattern, that is something that is experimentally verifiable and hopefully not in question. The coincidence filtering (a la quantum erasers) is a whole 'other story and involves a lot of complexity. You can go 'round and 'round on that. :)

vanhees71
Gold Member
2019 Award
That's why the paper is on vixra (it couldn't even make it to arxiv): it's junk with trivial flaws.
Then it shouldn't be discssed here on physics forums either!