Quantum behavior experiment flawed?

[vanhees71]

The screen is substituted by a photo detector which can be placed at different positions. How else do you think they made the Figures in their paper? Of course they do coincidence measurements with the two photodetectors, and that's why you can't simply use a screen for the signal photon. All this is completely irrelevant to the question whether or not there's an interference term of the partial waves going through the one or the other slit.

I don't see any flaw in the arguments of the authors. For sure they are well aware about the fact that indeed there's no retrocausality nor causal interactions between space-like separated registration events. From where in the paper to you infer the claim the authors were not aware of this basic facts about QED?

 

[tistemfnp]

The screen is substituted by a photo detector which can be placed at different positions. How else do you think they made the Figures in their paper?

Quoting @DrChinese "Arrrghh". A photo detector photo screen per se is not able to fix the position of the other partner while a detector with a coincidence counter will do exactly that.

You don't see a flaw as you obviously do not understand that fixing the other partner (x-position of D_P ) changes something in the experiment. Edit *)

A real photo-detector screen would add all virtual interference patterns and as they have random phases, related to the position in x of detector D_P, it would not show interference.

The difference between causality and correlation is well explained in this video.

Edit: Please have a look at the work of Dopfer, figure "Abbildung 4.5". Please specifically note the comment on "Quelle" D1: f i x. If you change the position of D1 in x, the virtual interference pattern on D2 will also change its position in x. If you put a real photo screen instead of a detector D2, you have no control on fixing the position of D1 in x! Hence, what you need to do in order to get the output of a real screen is to take the outcome of many such experiments, where you change the position of detector D1 in x. This will remove the interference pattern, as it adds up different phases.

 

[vanhees71]

I still don't understand your arguments. Are we discussing about the same experiment? I refer to the Walborn quantum eraser experiment.

https://arxiv.org/abs/quant-ph/0106078
https://doi.org/10.1103/PhysRevA.65.033818

In this experiment the interference pattern is measured by counting photon rates of a detector as a function of its position. The detection position is given by the position of the detector, no matter whether you use a single photon detector, a photoplate or silicon-pixel detector or whatever you want.

 

[tistemfnp]

In this experiment the interference pattern is measured by counting photon rates of a detector as a function of its position.

Half of the truth. In this experiment a virtual interference pattern is measured by counting photon rates of a detector as a function of its position coincident with counts of another detector of which the position is fixed.

The detection position is given by the position of the detector, no matter whether you use a single photon detector, a photoplate or silicon-pixel detector or whatever you want.

And how would a photoplate be able to only register detections coincident with detector D_P ? You will not see any interference pattern on a photoplate, as you won't be able to selectively look at a virtual interference pattern depending on the position of D_P.

If at detector D_S you change the position in x, and fix the position of D_P (which is the case in the Walborn experiment), you will see an interference pattern I₁. If you change the x-position of D_P, you will see an interference pattern I₂. The interference pattern I₂, is also shifted in the x-position. So if you repeat the Walborn experiment N times with N different, random x-positions of D_P, you will get N different interference patterns I_n with arbitrary phases in x-direction. Adding them up will cancel out interference and you will see approximately a Gaussian-like shape of the distribution of detections. Exactly this is what you see if you look at the screen behind the double-slit using a photoplate. Clearer?

Likewise, changing elements in the path and or changing path lengths, does not effect the outcome of the experiment. The outcome of the experiment is determined by the conservation of momentum alone, independent on the order of events in time. Nothing is 'erased', only the state of the idler was changed. The (virtual!!) screen pattern does not change because something changed retrospective in time, but because the correlation changed and a different type of (again: virtual) coincidence-pattern is observed.

To sum that up:
1. Nothing is erased in the Quantum "eraser". Instead, different correlations are observed.
2. Entangled photons do not show (self-)interference, as this would break conservation of momentum. Only virtual interference can be observed for a fixed position in x of the partner for a certain position in z of the partner (see Dopfer), in coincidence with the detection of the partner.Btw., isn't it much more cool, that for entangled photons we don't see two maxima on the screen behind a double slit, but only one? As if it wasn't two slits but one? This has an application: you can determine if photons are entangled by looking at what happens after a double-slit: interference --> simple photons; no interference and one maximum --> entangled photons. I like that .

 

[vanhees71]

Half of the truth. In this experiment a virtual interference pattern is measured by counting photon rates of a detector as a function of its position coincident with counts of another detector of which the position is fixed.
And how would a photoplate be able to only register detections coincident with detector D_P ? You will not see any interference pattern on a photoplate, as you won't be able to selectively look at a virtual interference pattern depending on the position of D_P.

That's why you use the movable detector and not a photoplate.

If at detector D_S you change the position in x, and fix the position of D_P (which is the case in the Walborn experiment), you will see an interference pattern I₁. If you change the x-position of D_P, you will see an interference pattern I₂. The interference pattern I₂, is also shifted in the x-position. So if you repeat the Walborn experiment N times with N different, random x-positions of D_P, you will get N different interference patterns I_n with arbitrary phases in x-direction. Adding them up will cancel out interference and you will see approximately a Gaussian-like shape of the distribution of detections. Exactly this is what you see if you look at the screen behind the double-slit using a photoplate. Clearer?

Yes, it was clear all the time.

Likewise, changing elements in the path and or changing path lengths, does not effect the outcome of the experiment. The outcome of the experiment is determined by the conservation of momentum alone, independent on the order of events in time. Nothing is 'erased', only the state of the idler was changed. The (virtual!!) screen pattern does not change because something changed retrospective in time, but because the correlation changed and a different type of (again: virtual) coincidence-pattern is observed.

To sum that up:
1. Nothing is erased in the Quantum "eraser". Instead, different correlations are observed.

That's in a sense true: You select different subensembles. What's erased is the information imprinted in the corresponding states, i.e., the "preparation procedures". As Zeilinger puts it QT is foremost about "information".

2. Entangled photons do not show (self-)interference, as this would break conservation of momentum. Only virtual interference can be observed for a fixed position in x of the partner for a certain position in z of the partner (see Dopfer), in coincidence with the detection of the partner.

Of course, they show "self-interference". Otherwise you'd never see a double-slit interference patterns. I don't know, what you mean by "virtual interference". In the Dopfer experiment you observe a "real interference" in the setup, where no which-way information is present and you don't see interference, if it's not present, depending on the position of the "Heisenberg lens". That's the whole point of this experiment!

Btw., isn't it much more cool, that for entangled photons we don't see two maxima on the screen behind a double slit, but only one? As if it wasn't two slits but one? This has an application: you can determine if photons are entangled by looking at what happens after a double-slit: interference --> simple photons; no interference and one maximum --> entangled photons. I like that .

This I also don't understand.

E.g., in the Walborn experiment with no QWPs present in the slits you see an interference pattern with the signal photons. It doesn't matter that these are entangled with the other photon from the parametric down conversion. If you don't know about the idler photons you simply have unpolarized but coherent photons making a double-slit interference pattern. Without the coincidence measurements on both photons you can never figure out that the signal photons are entangled with another photon. To observe entanglement you need these two-photon observations!

 

[tistemfnp]

To observe entanglement you need these two-photon observations!

In SPDC every photon emitted at down conversion frequency is entangled, as this is the principle of SDPC. Photons with the higher frequencies are blocked. Any photon with no coincidence is still entangled, you just didn't catch its partner in the measurement setup. And entangled photons will not show interference as long as you don't fix their partner in a certain position. That should be clear from the cited figure of Dopfer.

Everything said. From my side, I wouldn't know what else to say. Sorry.

 

[vanhees71]

But in Fig. 2 of the Walborn paper

https://doi.org/10.1103/PhysRevA.65.033818

you see interference fringes. How can that be, if your claim is true? Maybe I don't understand which precise setup you are discussing?

You are always referring to Dopfer. Is it about her thesis? Then which of the two experiments in which setup you discuss?

 

[tistemfnp]

you see interference fringes

No you don't. You see statistical coincidence with the other photon of the pair, which is fixed. I am referring to Dopfer, because everyone with minimum optical expertise would immediately catch the situation (including te Walborn situation) looking at the referred figure.

Answering these questions correctly would immediately clarify the situation:
1) What changes can be observed with the "interference pattern" if detector D1 is shifted in x?
2) What would happen when overlapping different "interference patterns" from different experiments where D1 was shifted in x?
3) Is a detector D1 necessary if we use a photoscreen instead of D2?

 

[vanhees71]

Once more: You contradict the observed facts in the paper by Walborn et al.

I cannot discuss your argument about Dopfer, because you don't tell me, which setup you are referring to in her thesis.

(1) In her "Heisenberg microscope experiment", as expected, she gets double-slit interference fringes if the detector $D_1$ is positioned in the focal plane (she reports 97% contrast) while she gets no double-slit interference fringes if $D_1$ is positioned in the image plane. It depends on the setup, whether you see fringes or not. Of course for any position of $D_1$ between these extremes, you get interference fringes with more or less contrast. This indeed needs very basic facts about the opticl properties of lenses and basic Fraunhofer diffraction theory.

(2) Quantum theory discusses results of experiments really done, not results of fictitious experiments. You need to give a detailed description of which experiment you have in mind in this question to discuss it.

(3) I don't know what you mean here either. With a photoscreen you cannot make coincidence meausurements to begin with.

 

[PeterDonis]

Thread closed for moderation.

 

[berkeman]

After a Mentor discussion, the OP is on a 10-day vacation from PF, and this thread will remain closed. Thank you everybody for trying to help the OP.