Delayed choice quantum eraser experiment question

[Jayjay]

trying to wrap my head around the delayed choice quantum eraser experiment. There is one thing that I don't quite get, and it's perhaps best illustrated with this picture from the relevant Wikipedia article:

If the beam splitters (which I think are semi-reflective mirrors that let through ~50% of the photons, and reflect the other half) at BS_b and BS_a are replaced with actual mirrors, what would the pattern be at detector D₀? An interference pattern, or two solid blocks, per two different slits that the photons go through?

What if the BS mirrors are removed, and all photons go to the "eraser"? Would there be an interference pattern then?

 

[barbos]

That's a loaded picture. Last time I checked wikipage on it it was a less loaded than that.
I will need some time to answer your specific question so I will answer more general one:
All these different variants of quantum eraser paradoxes designed in such a way that you assume that particle "decides" to be wave or particle at certain usually early time. In reality this "decision" happens at the last possible moment much later than you think - during actual measurement when it interact with macroscopic detector.

So to answer your specific question, you should treat your particle as pure wave as long as possible that is until actual macroscopic measurement takes place.

 

[Treedbear]

...
If the beam splitters (which I think are semi-reflective mirrors that let through ~50% of the photons, and reflect the other half) at BS_b and BS_a are replaced with actual mirrors, what would the pattern be at detector D₀? An interference pattern, or two solid blocks, per two different slits that the photons go through?

(Not having taken a course in Quantum theory, but having read a bit...) When a photon is detected at D₃ and/or D₄ its path is known. Therefore if BS_b and BS_a are replaced with mirrors the path will always be known and there will never be interference. The photon will go through one slit or the other.

What if the BS mirrors are removed, and all photons go to the "eraser"? Would there be an interference pattern then?

I assume that by "the BS mirrors" you mean only BS_b and BS_a. There will be no way to tell which slit the photon went through so there will always be interference.

 

[Jayjay]

(Not having taken a course in Quantum theory, but having read a bit...) When a photon is detected at D₃ and/or D₄ its path is known. Therefore if BS_b and BS_a are replaced with mirrors the path will always be known and there will never be interference. The photon will go through one slit or the other.

What if the BS mirrors are removed, and all photons go to the "eraser"? Would there be an interference pattern then?

I assume that by "the BS mirrors" you mean only BS_b and BS_a. There will be no way to tell which slit the photon went through so there will always be interference.

If that were the case, then you could build a time machine; a device that transmits information in the past. Just increase the length of the path to the detectors enough (say, one light-day) and then observe the pattern. If it is interference, then you know that one day from now the mirrors are off. If it shows no interference, then you know that the mirrors will be on.

My understanding is that it is not possible. So whatever the pattern is, it has to be the same in both cases, and you can only detect the "interference" or "non-interference" afterwards by looking at subset of photons based on what detector they hit.

 

[barbos]

They don't measure interference patern at any detectors. They measure correlations between different detectors. Detectors are not some kind of cameras, they are single pixel photo-detectors
Data at D0 is not affected by anything.

 

[Treedbear]

...
If that were the case, then you could build a time machine; a device that transmits information in the past. Just increase the length of the path to the detectors enough (say, one light-day) and then observe the pattern. If it is interference, then you know that one day from now the mirrors are off. If it shows no interference, then you know that the mirrors will be on.

My understanding is that it is not possible. So whatever the pattern is, it has to be the same in both cases, and you can only detect the "interference" or "non-interference" afterwards by looking at subset of photons based on what detector they hit.

That's above my pay scale. But I'll think on it. I have some problems in understanding the experimental conclusions also.

 

[Treedbear]

They don't measure interference patern at any detectors. They measure correlations between different detectors. Detectors are not some kind of cameras, they are single pixel photo-detectors
Data at D0 is not affected by anything.

D₀ is a scanning detector. I think that means that although it only detects one photon at a time (and then correlates it with the other non-scanning detectors) it also records the location at which it was detected, thereby constructing an image. It probably doesn't record every photon that goes through the slits, and how it scans the target area might be rather arbitrary, as long as it scans everywhere equally. I think which of the paths the idler photon takes through the BS's is random (for all practical purposes). It's just a matter of collecting a whole bunch of data points and sorting them out afterwards. The Wikipedia article shows 4 examples and how they correspond with where the "idler" photon is detected.

ETA: The "delayed" aspect of the experiment is designed to demonstrate the principle that what occurs after the photon strikes D₀ can effect where the photon strikes D₀. According to the article the effect is produced at the moment the idler photon strikes one of the idler detectors, ~ 8 nsec. after striking D₀.

 

[barbos]

Yes, D0, lets you determine which hole photon went through, so it's a 2 pixels telescope. These people need better illustrations. There are other parts of illustration which makes no sense.
This is actually a common problem with these eraser experiments. They should provide idealized version of their experiments instead of badly illustrated and loaded picture.

OK, I quickly scanned the wikipage and D0 is a single pixel but movable which means it lets you select photons which come from one hole or another.

 

[Treedbear]

nm

 

[Treedbear]

Here's how I think it works:
Detector D₀ can look at each pixel individually in an 2-dimensional array of pixels that cover the target area. It doesn't matter if this is a mechanical scanning technique or an array of pixels as in a digital camera. Just one pixel is observed at a time, each for a specific period of time, and in a manner such that all the pixels are sampled once and only once. Simple. If a photon is detected during this period of time there should be a corresponding "idler" photon detected 8 nsec later at either one or two of the other detectors. D₁ or D₂ (but not both) will always detect the photon. This can't reveal which slit the photon went through, but helps eliminate electronic noise from detector D₀. Either D₃ or D₄ (but not both). Either would reveal which slit the photon went through and theoretically there will be no interference at D₀ for that pixel. If neither then the path is undetermined and there will be interference at D₀ for that pixel. In this way the pixels can be saved in separate image files according to whether the predicted result is interference or not.

There are certain technical requirements. The sampling period for each pixel must be short enough and the photon rate (i.e.; light intensity) must be low enough such that it is unlikely that more than one photon will be detected during the sampling period of any one pixel. Also the area covered by each pixel should be small for the same reason. Likewise there should be a high enough pixel density.

The one qualm I have about the experiment is that the article attributes the effect to the time delay between the photon striking D₀ and one or more of the idler arm detectors. The detectors themselves are 8 nsec farther from the slits than is D₀. But I would think if there is an event that determines whether there is interference or not it is at BS_a or BS_b where the photon is randomly diverted to one of the detectors that will reveal its path. I'd like to know whether that distance is larger than or even smaller than it is to D₀.

 

[Treedbear]

(Not having taken a course in Quantum theory, but having read a bit...) When a photon is detected at D₃ and/or D₄ its path is known. Therefore if BS_b and BS_a are replaced with mirrors the path will always be known and there will never be interference. The photon will go through one slit or the other.



I assume that by "the BS mirrors" you mean only BS_b and BS_a. There will be no way to tell which slit the photon went through so there will always be interference.

If that were the case, then you could build a time machine; a device that transmits information in the past. Just increase the length of the path to the detectors enough (say, one light-day) and then observe the pattern. If it is interference, then you know that one day from now the mirrors are off. If it shows no interference, then you know that the mirrors will be on.

My understanding is that it is not possible. So whatever the pattern is, it has to be the same in both cases, and you can only detect the "interference" or "non-interference" afterwards by looking at subset of photons based on what detector they hit.

I can take a stab at a partial answer (aka, a guess). Keep in mind that the data points are used to compute average probabilities for where a photon will appear. The quantum world is based on probabilities. And the correlation is only relevant to one particular idler photon detected after a precise time delay corresponding to a particular photon appearing at a particular pixel. So having the ability to detect every idler photon emitted from the BBO tells you nothing without maintaining extremely fine precision in timing. The phrase "quantum decoherence" comes to mind. As the idler detectors are moved farther away the interference pattern will become more refined.

 

[barbos]

A delayed choice quantum eraser experiment, first performed by Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih and Marlan O. Scully,[1] and reported in early 1999, is an elaboration on the quantum eraser experiment that incorporates concepts considered in Wheeler's delayed choice experiment. The experiment was designed to investigate peculiar consequences of the well-known double slit experiment in quantum mechanics as well as the consequences of quantum entanglement.

The delayed choice quantum eraser experiment investigates a paradox. If a photon manifests itself as though it had come by a single path to the detector, then "common sense" (which Wheeler and others challenge) says it must have entered the double-slit device as a particle. If a photon manifests itself as though it had come by two indistinguishable paths, then it must have entered the double-slit device as a wave. If the experimental apparatus is changed while the photon is in mid‑flight, then the photon should reverse its original "decision" as to whether to be a wave or a particle. Wheeler pointed out that when these assumptions are applied to a device of interstellar dimensions, a last-minute decision made on earth on how to observe a photon could alter a decision made millions or even billions of years ago.

While delayed choice experiments have confirmed the seeming ability of measurements made on photons in the present to alter events occurring in the past, this requires a non-standard view of quantum mechanics. If a photon in flight is interpreted as being in a so-called "superposition of states," i.e. if it is interpreted as something that has the potentiality to manifest as a particle or wave, but during its time in flight is neither, then there is no time paradox. This is the standard view, and recent experiments have supported

This is from wikipage. I completely agree with that. As I said before paradox arises from wrong assumptions. They assume that photon decides to behave certain way. If your assumptions lead to paradox you need to change these assumptions.

In standard interpritations wave functions is not considered a real physical object that's why it is allowed to appear as if it "violates" causality and speed of light limit during collapse.

 

[DBT]

I see the delayed choice quantum eraser experiment is being used by some to support their contention that the physical world is consciousness dependent, that without a conscious observer the World remains in a diffuse state that is probability wave function.

 

[barbos]

I see the delayed choice quantum eraser experiment is being used by some to support their contention that the physical world is consciousness dependent, that without a conscious observer the World remains in a diffuse state that is probability wave function.

I am pro MWI which simply says every outcome actually happens and does not require any additional conscious observers.

 

[DBT]

I see the delayed choice quantum eraser experiment is being used by some to support their contention that the physical world is consciousness dependent, that without a conscious observer the World remains in a diffuse state that is probability wave function.

I am pro MWI which simply says every outcome actually happens and does not require any additional conscious observers.

The rationale being used for the necessity of a conscious observer basically being that wave collapse only occurs when 'observed' - which presumably implies wave function remains, causing interference patterns as waves, not particles.

 

[barbos]

I am pro MWI which simply says every outcome actually happens and does not require any additional conscious observers.

The rationale being used for the necessity of a conscious observer basically being that wave collapse only occurs when 'observed' - which presumably implies wave function remains, causing interference patterns as waves, not particles.

I understand the rationale, I just don't buy it, not even for non MWI versions.

 

[DBT]

The rationale being used for the necessity of a conscious observer basically being that wave collapse only occurs when 'observed' - which presumably implies wave function remains, causing interference patterns as waves, not particles.

I understand the rationale, I just don't buy it, not even for non MWI versions.

Can you say why....just curious, not arguing for or against any particular interpretation.

 

[barbos]

I understand the rationale, I just don't buy it, not even for non MWI versions.

Can you say why....just curious, not arguing for or against any particular interpretation.

I don't believe conscious observer needs to be necessary for universe to function.

 

[Treedbear]

...
The delayed choice quantum eraser experiment investigates a paradox. If a photon manifests itself as though it had come by a single path to the detector, then "common sense" (which Wheeler and others challenge) says it must have entered the double-slit device as a particle. If a photon manifests itself as though it had come by two indistinguishable paths, then it must have entered the double-slit device as a wave. If the experimental apparatus is changed while the photon is in mid‑flight, then the photon should reverse its original "decision" as to whether to be a wave or a particle. Wheeler pointed out that when these assumptions are applied to a device of interstellar dimensions, a last-minute decision made on earth on how to observe a photon could alter a decision made millions or even billions of years ago.
...

This is from wikipage. I completely agree with that. As I said before paradox arises from wrong assumptions. They assume that photon decides to behave certain way. If your assumptions lead to paradox you need to change these assumptions.

In standard interpritations wave functions is not considered a real physical object that's why it is allowed to appear as if it "violates" causality and speed of light limit during collapse.

I agree that there is a seeming paradox revealed by the results of the delayed choice quantum eraser experiment in the article. But if you skim through this other article that I cited on quantum decoherence you will see that in the case of multiyear delays involving interstellar distances the correlation between "signal" and "idler" photons is gradually lost as it "leaks" into the environment over time and distance.

In interpretations of quantum mechanics -
Before an understanding of decoherence was developed the Copenhagen interpretation of quantum mechanics treated wavefunction collapse as a fundamental, a priori process. Decoherence provides an explanatory mechanism for the appearance of wavefunction collapse ...

Decoherence does not claim to provide a mechanism for the actual wave function collapse; rather it puts forth a reasonable mechanism for the appearance of wavefunction collapse. The quantum nature of the system is simply "leaked" into the environment so that a total superposition of the wavefunction still exists, but exists — at least for all practical purposes — beyond the realm of measurement. ...

 

[Jayjay]

And here is the answer (best one I've seen anyway):



So basically, regardless of how the detectors 1-4 are set up, the first detector will always show an indistinguishable blur, no inteference pattern and no two bands either.