# Entanglement and delayed choice measurements

• B
• Alex Torres
In summary, the experiment demonstrated that future measurements can destroy the interference pattern of an entangled particle before it reaches the measurement device.

#### Alex Torres

A laser gun pumps one photon at a time that goes thru an SPDC process and two entangled photons are generated...photon A and photon B...

Photon A is sent to Bob who has a standard DS setup located a couple of meters away from the laser pump... since the photon is in superposition of states he should get fringes at the screen.

Photon B is sent to Alice who has a standard DS setup located at 50 meters farther away from the laser pump... since the photon is in superposition of states she should also get fringes at the screen.

Non-locality tells that if Alice place a detector by one slit to know which slit B will go thru, that future measurement in time automatically collapses the the wave function at Bobs station and he will get clumps.

The fact that a future measurement in time can destroy the interference pattern of an entangled particle before it reaches the measurement device was proven in the following experiment...

https://files.acrobat.com/a/preview/abdaa0fc-2e1a-4689-97f8-8d866fe89a63

Alex Torres said:
A laser gun pumps one photon at a time that goes thru an SPDC process and two entangled photons are generated...photon A and photon B...

Photon A is sent to Bob who has a standard DS setup located a couple of meters away from the laser pump... since the photon is in superposition of states he should get fringes at the screen.

Photon B is sent to Alice who has a standard DS setup located at 50 meters farther away from the laser pump... since the photon is in superposition of states she should also get fringes at the screen.

Non-locality tells that if Alice place a detector by one slit to know which slit B will go thru, that future measurement in time automatically collapses the the wave function at Bobs station and he will get clumps.

The fact that a future measurement in time can destroy the interference pattern of an entangled particle before it reaches the measurement device was proven in the following experiment...

https://files.acrobat.com/a/preview/abdaa0fc-2e1a-4689-97f8-8d866fe89a63
There is no separate 'past' or 'future' for an entangled pair because they share the same wave function. Read the section 'Making Sense of the Nonsensical' near the end of the paper.

The DCQE is well known in this forum ...

Mentz114 said:
There is no separate 'past' or 'future' for an entangled pair because they share the same wave function.

Well, that's exactly my point ...if we assume non-locality we have to throw away causality, if we throw away causality the speed of light is inconsequential ...but if you assume that no effect can travel faster than light speed you are holding yourself to causality, entanglement tells we can't hold on to both while explaining quantum events...

Please see the realization of Archer Peres thought experiment...Entanglement Swapping...

Alex Torres said:
Non-locality tells that if Alice place a detector by one slit to know which slit B will go thru, that future measurement in time automatically collapses the the wave function at Bobs station and he will get clumps.

The fact that a future measurement in time can destroy the interference pattern of an entangled particle before it reaches the measurement device was proven in the following experiment...

No, it does not and I have no idea why you keep on opening several threads continuing to discuss your erroneous understanding of he experiment although you have been explicitly told before that you are wrong and have been given literature references. coherence and entanglement in he same property are mutually exclusive. If you increase the degree of spatial coherence such, that you see an interference pattern, you will no be able to break Bell's inequalities with the same subset of photons and entanglement is broken. This is a general result and keeping on constructing more complicated scenarios instead of understanding the basic physics will not alter that.

The whole conditional coincidence count interference pattern for SPDC and two double slits is discussed by Walborn here:
https://arxiv.org/pdf/1010.1236.pdf around page 51. Do you have any explicit questions with respect to equations 94-98?

atyy and Mentz114
Cthugha said:
coherence and entanglement in the same property are mutually exclusive

Well, that could be another side of the same coin...

You should know about a pastiche of Victor, Alice and Bob about entanglement swapping, telling that the photons Alice and Bob receives will be entangled or not based on Victor's future measurement choice.

We all know that by measuring the quantum states alone in their respective particles, Alice and Bob won't be able to make any sense out of their individual results without a feedback from Victor or by using a classical information channel.

But about 4 years ago, it was proposed in this forum that Alice and Bob would be able to tell, (without using a classical information channel) that by measuring the quantum state fidelity (coherence) of their respective particles quantum states they would be able to tell if Victor's has actually swapped the entanglement or not without the need to resort to a classical information channel, given they already know in advance the fidelity values outcomes from swapping vs not swapping.

The point remained uncontested to date...(See last comment in page 4)

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Alex Torres said:
But about 4 years ago, it was proposed in this forum that Alice and Bob would be able to tell, (without using a classical information channel) that by measuring the quantum state fidelity (coherence) of their respective particles quantum states they would be able to tell if Victor's has actually swapped the entanglement or not without the need to resort to a classical information channel, given they already know in advance the fidelity values outcomes from swapping vs not swapping.

The point remained uncontested to date...

Sorry, but that point is far from uncontested. This thread consists of a troll deliberately misinterpreting a publication and 4 pages full of people telling him where he is wrong until the mods finally have mercy and close the thread. Fortunately, this forum is one of the few places, where real physics is discussed at a reasonable level and not a place for crackpot discussions. That topic contains several posts that clearly settle the point and there is little reason to resurrect that topic.

atyy, weirdoguy, Nugatory and 1 other person
Alex Torres said:
The point remained uncontested to date...(See last comment in page 4)

No, it didn't. @Cthugha has correctly described the actual message you should take away from that thread. The fact that the OP's name has a line through it, indicating that he is banned from PF, should also give you a clue.

Alex Torres said:
if we assume non-locality we have to throw away causality

No, you don't. You just have to correctly understand what causality means in the context of quantum field theory, i.e., quantum mechanics + special relativity. The correct meaning of causality is that measurements at spacelike separated events commute; i.e., their results do not depend on the order in which they are done. (It should be obvious that this is what relativity requires, since spacelike separated events do not have an invariant ordering). All quantum experiments that have been done meet this requirement, including the ones that show violations of the Bell inequalities and therefore confirm non-locality.

In short, your claim that entanglement and non-locality are incompatible with causality is not correct.

Cthugha said:
you have been explicitly told before that you are wrong and have been given literature references

Sorry for not reading your references before about the factors related to the coherence of the light source...thanks for posting them...if read before would have saved me from posting the wrong assumption as i did at the beginning of the thread ...

Correction done...let us go back to the last part of Walborn's DCQE...the delayed erasure, ...as far as i can see from the layman's description, even though Alice is located farther away than Bob she only needs to hold the polarizer in P's path thru the end of the experiment in order for Bob to get an interference pattern at the screen... isn't that enough of FTL signaling...? Or maybe there's something missing in this description...

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Alex Torres said:
let us go back to the last part of Walborn's DCQE...the delayed erasure, ...as far as i can see from the layman's description, even though Alice is located farther away than Bob she only needs to hold the polarizer in P's path thru the end of the experiment in order for Bob to get an interference pattern at the screen... isn't that enough of FTL signaling...?

This pattern is only present in coincidence counts, which is a filtered signal. The pattern at Bob's side does not change a little bit when the polarizer is inserted. You are just able to sort the total non-interference pattern into two partial interference patterns with the first appearing due to the fraction of photons that make it through the polarizers and the second due to the photons that are filtered at the polarizers. In the other DCQE experiment one had a superposition of interference patterns due to different wavevectors, where one particular pattern could be isolated in coincidence counting due to wavevector filtering on the other side. Here, you have a superposition of interference patterns due to different polarizations, where one particular pattern could be isolated in coincidence counting due to polarization filtering on the other side. You can replay this game with a lot of different quantities, but the principle is always the same: Filtering yields interference patterns in coincidence count rates, but never in the direct single photon count patterns.

This is a general story (you have been given a reference before that explains why that is necessarily the case) and there is no experiment that directly changes the pattern observed at a screen.

Alex Torres and atyy
Cthugha said:
This is a general story

Yep...just as suspected ...the Alice and Bob's pastiche at the end of the reference you gave earlier clearly depicts your point...thank you!

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Entanglement swapping requires Alice and Bob be connected with Victor by means of a classical information channel if they want to make sense out of the individual results they have at hands...is this process also referred to as "entanglement distillation"? ...I mean the quantum state correlations Alice and Bob have at hands is somehow "diluted", but by means of a classical channel they can obtain Victor's quantum state correlation and add them together to obtain a strong correlation ("distillation"?) that tells if their individual particles were entangled or not by Victors delayed choice measurememt...

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