# Implications of Quantum Eraser, Delayed Choice, etc.

The behavior of the particle at the slits seemingly depends on what the photon encounters after the particle has passed through the slit(s). Does this prove that:

1. Particles can go back in time and do stuff?

2. Has quantum erasers been done using electrons or molecules like buckyball?

3. Does this experiment prove that particles are not really particles but just related to the wavy field that can form interference?

4. Does this prove that particles don't travel in between but only the detection event is important?

5. What's the explanation why if which-path information is suppressed, intereference form, does this has to do with decoherence?

6. Or is this more of information theory that any time which path information is known, it automatically hide the interference without regards to decoherence or others? In other words, it's a rule of nature that this must be done.. but how is the rule implemented? Or what is the corresponding symmetry of nature preserved for this rule if there is any?

Quantum erasers and stuff can give us a clue about what really are particles and what would be the correct interpretations. Heck, if there are no particles at all, then most interpretations can be done away with. But we know particles like buckyball exist. But how can buckyballs pull such stunts as quantum eraser? Does it really go back in time or are buckyballs really not there before detection? What's a rational explanation for the quantum eraser mystery? I can't understand how HUP (Heisenberg Uncertainty Principle) can solve it. Can it? How. Please explain. Thanks.

whitsona

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The behavior of the particle at the slits seemingly depends on what the photon encounters after the particle has passed through the slit(s).
This is not true. It is purely conjecture not based on any evidence.

5. What's the explanation why if which-path information is suppressed, intereference form, does this has to do with decoherence?
The truth is actually quite obvious and less mysterious that often presented. It is of the "eating your cake and having it" kind. You cannot detect a photon just after it has passed the slits and still expect the same photon to reach the screen. At the very least your efforts will disburb/deflecting the photons. Therefore, by attempting to detect photons at the slits, you have introduced a disturbance which perturbs your interference pattern!
6. Or is this more of information theory that any time which path information is known, it automatically hide the interference without regards to decoherence or others? In other words, it's a rule of nature that this must be done.. but how is the rule implemented? Or what is the corresponding symmetry of nature preserved for this rule if there is any?
Check out these experimental reports which might dispel some of the misconceptions:

Lorenzo Basano and Pasquale Ottonello. American Journal of Physics -- March 2000 -- Volume 68, Issue 3, pp. 245. Interference fringes from stabilized diode lasers
- Interference fringes produced by a pair of intracavity stabilized diode laser beams, each impinging separately on one aperture of a double slit, are recorded on a linear charge-coupled device array.

Sillitto R and Wykes, C. 1972, Phys. Lett., An interference experiment with light beams modulated in anti-phase by an electro-optic shutter, 39A, 333–4
- Clear interference observed with one photon at a time, and one slit open at a time.

E. J. S. Fonseca, P. H. Souto Ribeiro, S. Pádua, and C. H. Monken. Phys. Rev. A 60, 1530–1533 (1999), Quantum interference by a nonlocal double slit
- Spatially separated slits and beams, full path information, clear interference pattern.

This is not true. It is purely conjecture not based on any evidence.

The truth is actually quite obvious and less mysterious that often presented. It is of the "eating your cake and having it" kind. You cannot detect a photon just after it has passed the slits and still expect the same photon to reach the screen. At the very least your efforts will disburb/deflecting the photons. Therefore, by attempting to detect photons at the slits, you have introduced a disturbance which perturbs your interference pattern!

Check out these experimental reports which might dispel some of the misconceptions:

Lorenzo Basano and Pasquale Ottonello. American Journal of Physics -- March 2000 -- Volume 68, Issue 3, pp. 245. Interference fringes from stabilized diode lasers
- Interference fringes produced by a pair of intracavity stabilized diode laser beams, each impinging separately on one aperture of a double slit, are recorded on a linear charge-coupled device array.

Sillitto R and Wykes, C. 1972, Phys. Lett., An interference experiment with light beams modulated in anti-phase by an electro-optic shutter, 39A, 333–4
- Clear interference observed with one photon at a time, and one slit open at a time.

E. J. S. Fonseca, P. H. Souto Ribeiro, S. Pádua, and C. H. Monken. Phys. Rev. A 60, 1530–1533 (1999), Quantum interference by a nonlocal double slit
- Spatially separated slits and beams, full path information, clear interference pattern.
But you can even use Quasar millions of light years away as the source. In Wheeler Delayed Choice Experiment, the source is Quasar and the slits are the so called galactic lens. Here you can't say your measuring device can disturb the galactic slits. Somehow the photons can go back in time and behave according to the rules. What's the convensional explanation anyway of "shut up and calculate" physicists on this Wheeler Delayed Choice Experiment?

But you can even use Quasar millions of light years away as the source. In Wheeler Delayed Choice Experiment, the source is Quasar and the slits are the so called galactic lens. Here you can't say your measuring device can disturb the galactic slits. Somehow the photons can go back in time and behave according to the rules. What's the convensional explanation anyway of "shut up and calculate" physicists on this Wheeler Delayed Choice Experiment?
What part of "you can't detect your photons and still have them" is not clear? Who said anything about disturbing slits? Point to an actual experiment, rather than a thought experiment and I will give you an explanation of it.

What part of "you can't detect your photons and still have them" is not clear? Who said anything about disturbing slits? Point to an actual experiment, rather than a thought experiment and I will give you an explanation of it.
No. It seems the explanation is because the photons are entangled. This is because the setup is done in such a way that, according to http://grad.physics.sunysb.edu/~amarch/ [Broken]

"In this set up, the "which-way" measurement does not alter the momentum or position of the photons to cause destruction of the interference pattern. We can think of the loss of interference as being due only to the fact that the photons are entangled and that the presence of the quarter wave plates changes this entanglement. "

Do you agree with it? Or do you still believe that the setup can really disturbed the interferences as the above site refutes?

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The behavior of the particle at the slits seemingly depends on what the photon encounters after the particle has passed through the slit(s). Does this prove that:

1. Particles can go back in time and do stuff?

Thanks.
i my opinion, no....because

i think that... when either (one) of the photons is detected/strikes the screen, the entanglement is broken, the wave function collapses.

the position of the photon is locked, also the other photon's path is now "fixed".....

the other photon will behave in such a way (i.e. take such a path) that is consistent with the position of the first photon on the screen/detector.

No. It seems the explanation is because the photons are entangled. This is because the setup is done in such a way that, according to http://grad.physics.sunysb.edu/~amarch/ [Broken]

"In this set up, the "which-way" measurement does not alter the momentum or position of the photons to cause destruction of the interference pattern. We can think of the loss of interference as being due only to the fact that the photons are entangled and that the presence of the quarter wave plates changes this entanglement. "
- The quoted article says:
To make the "which-way" detector, a quarter wave plate (QWP) is put in front of each slit. This device is a special crystal that can change linearly polarized light into circularly polarized light. The two wave plates are set so that given a photon with a particular linear polarization, one wave plate would change it to right circular polarization while the other would change it to left circular polarization.

With this configuration, it is possible to figure out which slit the s photon went through, without disturbing the s photon in any way ...
If you believe that, could you please explain to me how a quarter wave plate can change the properties of the photon without disturbing it any any way!? Once you change the properties of the photon with the QWP, you shouldn't be surprised that it doesn't behave the same way as before. The interference pattern is produced because the photon with certain properties interacts with a slit which has certain properties. If you disturb one of the actors and observe a different result, what is the mystery in that?

The article adds to the lie:
In case you might be suspicious of the quarter wave plates, it is worth noting that given a beam of light incident on a double slit, changing the polarization of the light has no effect whatsoever on the interference pattern. The pattern will remain the same for an x polarized beam, a y polarized beam, a left or a right circularly polarized beam.
This is true on the surface until you realize that the experiment they are describing is very different. In one case, all the photons impinging on the slits are passing through the same QWP without any coincidence counting. But in their case, they have two different QWPs , one for each slit and in addition they have coincidence counting. By simply introducing the QWP, they have changed the effective flight-time/path-length for one of the photons which means they must introduce corrections to their coincidence counting. There is no evidence that they are doing that.

Then the mysticism continues
Increasing the strangeness of this scenario, the next step is to bring back the interference without doing anything to the s beam. A polarizer is placed in the p beam, oriented so that it will pass light that is a combination of x and y.

...
How does photon s know that we put the polarizer there?
Photon s and photon p are entangled. Photon p must be able to communicate to s through some means that is unknown to us.
Oh, so they have now compensated for the flight-time differences by introducing a delay on the p-beam, and mysteriously the interference reappears. Wow it must be because the photon knows our motives. NOT.

You may ask. What about delayed erasure in which the p-beam path length is increased so that the s-photon is detected first? To which I will have just one question for you: Were they able to see an interference pattern before the p-beam was observed?

Do you agree with it? Or do you still believe that the setup can really disturbed the interferences as the above site refutes?
There are certain facts about the experiments:
1 - No interference pattern is ever observed by considering only the s-photons alone. Yet if you look only at the subset of s-photons detected which are coincident with p-photons, they form an interference pattern. Obviously, the remaining photons not-coincident with p-photons will show an opposite interference photon with troughs aligned to the peaks of the coincident subset. That is why together, they do not form a pattern.
2 - Coincidence counting is very important in filtering out the s-photons.
3 - Changes in flight-time/path-length will therefore have profound effect on visibility of interference patterns. Disturbing any other property of the photons which enables you to effectively separate/filter the two sets of photons, will eliminate the interference.

I am not convinced that this experiment has anything to do with erasure of which-way information. If anything, it is a demonstration that quantum effects such as correlations between individual photons are very sensitive to manipulations. Therefore by trying to manipulate the photons in order to extract one kind of information, we are destroying the other information that is important for us to be able to filter out the two sets of photons. Therefore no interference. You do not need to resort to backward causation and photons reading human minds to explain this.

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DrChinese
Gold Member
I am not convinced that this experiment has anything to do with erasure of which-way information. If anything, it is a demonstration that quantum effects such as correlations between individual photons are very sensitive to manipulations. Therefore by trying to manipulate the photons in order to extract one kind of information, we are destroying the other information that is important for us to be able to filter out the two sets of photons. Therefore no interference. .
For the benefit of readers of this statement: This is a personal opinion of billschnieder and is not a generally accepted scientific viewpoint. This member makes a habit of mixing comments which follow normal PF guidelines and other items which are better served up from the top of his soapbox.

bill: Haven't we had this conversion enough times already? Please stick to the standard. You know enough to help others out, you don't need to convert them in the process. We could always use your assistance (within the guidelines).

whitsona
DrChinese
Gold Member
The behavior of the particle at the slits seemingly depends on what the photon encounters after the particle has passed through the slit(s).
This is an accurate summary. It does "seem" to act this way, as the time ordering makes no difference. In fact, this is generally true of observations on entangled systems. There are a variety of interpretations of this, and you can get an idea of those from one of the "Interpretations" threads.

whitsona
This is an accurate summary. It does "seem" to act this way, as the time ordering makes no difference. In fact, this is generally true of observations on entangled systems. There are a variety of interpretations of this, and you can get an idea of those from one of the "Interpretations" threads.
How about the explanation here that the photons are simply entangled.

Is it mainstream view? If not. What is the mainstream view?

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How about the explanation here that the photons are simply entangled.

Is it mainstream view? If not. What is the mainstream view?
The mainstream view is that QM framework developed by the end of 1926 (so don't even need uncertainty principle) exactly predicts the results of this experiment.

To go further you might admit that the wave functions involved encode the experimental setup nonlocally, but the basic QM framework doesn't insist on this, you'd need some further compelling argument for the nonlocal encoding argument to get it accepted mainstream.

BillSchnieder's argument that the quarter wave plates influence the flight time of the photons so as to require recalibration of the coincidence counters is wrong, the coincidence counters are not so accurate and have quite a wide time window in comparison to the flight time of the photons in this experiment.

In fact, the Walborn experiment is an excellent and simple demonstration that billschnieder is wrong in thinking that you cannot get interference patterns if you disturb the "flightpath" of the photons.

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The mainstream view is that QM framework developed by the end of 1926 (so don't even need uncertainty principle) exactly predicts the results of this experiment.

To go further you might admit that the wave functions involved encode the experimental setup nonlocally, but the basic QM framework doesn't insist on this, you'd need some further compelling argument for the nonlocal encoding argument to get it accepted mainstream.

BillSchnieder's argument that the quarter wave plates influence the flight time of the photons so as to require recalibration of the coincidence counters is wrong, the coincidence counters are not so accurate and have quite a wide time window in comparison to the flight time of the photons in this experiment.

In fact, the Walborn experiment is an excellent and simple demonstration that billschnieder is wrong in thinking that you cannot get interference patterns if you disturb the "flightpath" of the photons.
How did QM predict the results of the experiment? Maybe it's because it treats everything like a wave only in the Delayed Eraser? Maybe particles are really just momentum of the fields and doesn't have any permanence. Is this how QM predict the results?

Maybe at some point we have to give up and jus shut up and calculate. No matter how hard we tried. We can't explain why the math of General Relativity work. Maybe the math of QM work because somehow the whole reality is just *like* a simulation inside a computer, here only measurements make sense. One can't recover any ontological reality because there is literally none.

Btw, in Wheeler Delayed Choice Experiment where the source is Quasar, would it still work if electrons or buckyballs are used instead of photons? I'm thinking photons are not ordinary because there is a real wave of electromagnetic field (light). But in matter wave, what's waving is probability wave, in light.. the electric/magnetic field is waving. So there's a big difference. Now if buckyball made up of 430 atoms can still behave like photons from the Quasar where the path of them millions of years ago determined by how you set up your instrument now. Then it's now time we have to eliminate the reality of particles. Since we know particles dont travel back in time, then there are no particles.

The experiment hasn't been convincingly demonstrated with photons from quasars, but I would not expect it to fail if the situation was perfect. With buckyballs etc it is too difficult with current technology to avoid decoherence of the particles under invesitigation.

A simple explanation of delayed choice experiments is that nature has non-local wavefunctions but
(effectively) local evolution laws (regarding causality etc), I can't discuss here how that might work since the forum moderators forbid it, but delayed choice experiments cannot be explained with classical physics ideas

DrChinese
Gold Member
How about the explanation here that the photons are simply entangled.

Is it mainstream view? If not. What is the mainstream view?
This is mainstream analysis. I often provide this as a reference. Also this, which is more or less the source for that:

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Would it be correct to say that the delayed-choice quantum eraser experiments don't actually change the past, as the particle going through either one or both slits (before we choose to find out which-way info, or 'erase') never existed until we 'erase' or find out which-way info?

DrChinese
Gold Member
Would it be correct to say that the delayed-choice quantum eraser experiments don't actually change the past, ...
I don't know if the past is being changed or not. In some sense you might say the past is not "locked down" until a later time. I don't think the existence of the particle itself is the issue.

Would it be correct to say that the delayed-choice quantum eraser experiments don't actually change the past, as the particle going through either one or both slits (before we choose to find out which-way info, or 'erase') never existed until we 'erase' or find out which-way info?
it's easier to allow that nature knows about the whole experimental setup (even if separated by galactic distances) than allow that the past can be changed. (so easier to assume wave-functions are non-local)

it's easier to allow that nature knows about the whole experimental setup (even if separated by galactic distances) than allow that the past can be changed. (so easier to assume wave-functions are non-local)
Why is the explanation the past can be changed or the wave function simply non-local is interchangeable? Can anyone show a simple illustration how this can be?

Hmmm, I guess if the particle went through both slits, then which-way info was obtained, then the particle changes its behaviour to going through one slit - which is in a sense 'creation of the past'.

But it could always be the other way around - the particle goes through one slit, the which-way info is erased, then the particle changes its behaviour to going through both slits.

But creation of one past history, would the alternative be real before?

Bruce Rosenblum says:
Changing the unobserved past, or creating it, not much different.
Unobserved? -> Not real, then?

The behavior of the particle at the slits seemingly depends on what the photon encounters after the particle has passed through the slit(s). Does this prove that:

1. Particles can go back in time and do stuff?

2. Has quantum erasers been done using electrons or molecules like buckyball?

3. Does this experiment prove that particles are not really particles but just related to the wavy field that can form interference?

4. Does this prove that particles don't travel in between but only the detection event is important?

5. What's the explanation why if which-path information is suppressed, intereference form, does this has to do with decoherence?

6. Or is this more of information theory that any time which path information is known, it automatically hide the interference without regards to decoherence or others? In other words, it's a rule of nature that this must be done.. but how is the rule implemented? Or what is the corresponding symmetry of nature preserved for this rule if there is any?

Quantum erasers and stuff can give us a clue about what really are particles and what would be the correct interpretations. Heck, if there are no particles at all, then most interpretations can be done away with. But we know particles like buckyball exist. But how can buckyballs pull such stunts as quantum eraser? Does it really go back in time or are buckyballs really not there before detection? What's a rational explanation for the quantum eraser mystery? I can't understand how HUP (Heisenberg Uncertainty Principle) can solve it. Can it? How. Please explain. Thanks.
1. No, particles cannot go back in time. What this means is that the measuring device contributes to the experimental result. In Wheeler's original delayed choice experiment, the choice of measuring device determines whether or not we obtain which-way information or not.
2. I don't know, but there is no reason why, with the appropriate apparatus, delayed choice and erasure experiments cannot be done with particles other than photons.
3. Particles are always detected as particles. We must detect many different particles, but all prepared in the same way, in order to see interference effects. The corresponding wavefunction is a complex vector in a Hilbert space, and is not a "real" wave that propagates in space-time. (There are others who believe otherwise.)
4. A quantum experiment requires a measurement result. (Bohr) If we assume that the particle exists prior to measurement, as was done in the EPR experiment, then we get erroneous results. Taking the results of quantum experiments at face value, quantum particles do not have trajectories and they exist only at the instant they are detected. For this reason, Wheeler calls particle detection, "an elementary act of creation."
5. If we do not know which way the particle went, then the wavefunction is $$\psi _A + \psi _B$$ and there is interference. If we do know which way the particle went, then the wavefunction is $$\psi _A$$ or $$\psi _B$$, and there is no interference. This can be explained in terms of incoherence.
6. Nothing is being "hidden" or "erased". Some experiments exhibit interference. Others do not. Different experiments have different results. The delayed choice experiments confirm Bohr's assertion that we can change the apparatus at any time, even long after the particle should have passed the slits. The apparatus in place at the instant of particle detection determines whether we have interference, or not.

There is no classical explanation for such things. Many would like a quantum theory that fits into the classical scheme of things. But, quantum mechanics was invented to explain such non-classical experiments. I know this is hard to accept, but IMHO there is no classical way out!

BillSchnieder's argument that the quarter wave plates influence the flight time of the photons so as to require recalibration of the coincidence counters is wrong, the coincidence counters are not so accurate and have quite a wide time window in comparison to the flight time of the photons in this experiment.
Unfortunately you are uninformed about this issue. I will welcome you to look at the experiment described in PHYSICAL REVIEW A 71, 052103(2005) [http://people.whitman.edu/~beckmk/QM/qe/qe.pdf] [Broken]. Specifically look at figure 4 where they show how the number of coincidence counts between arms A and B vary as the pathlength difference between the two arms is changed. Also note that the figure shows changes within 4 microns and it takes less than half a micron difference to drop the coincidence counts from maximum to minimum.

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DrChinese
Gold Member
I will welcome you to look at the experiment described in PHYSICAL REVIEW A 71, 052103(2005) [http://people.whitman.edu/~beckmk/QM/qe/qe.pdf] [Broken]. Specifically look at figure 4 where they show how the number of coincidence counts between arms A and B vary as the pathlength difference between the two arms is changed. Also note that the figure shows changes within 4 microns and it takes less than half a micron difference to drop the coincidence counts from maximum to minimum.
Are you guys talking about the same thing?

I read it that the polarization interferometer (Fig. 2) path length was sensitive, but the path to the coincidence detector itself was not.

I guess the point is whether the insertion of a plate inside the polarization interferometer "slows" the photon down, is that it?

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But, quantum mechanics was invented to explain such non-classical experiments. I know this is hard to accept, but IMHO there is no classical way out!
Just because you do not have a classical explanation does not mean none exists, or none can be found.

On the contrary, there is a published classical explanation QE with photons:
Event-by-Event Simulation of a Quantum Eraser Experiment
Jin, F.; Zhao, S.; Yuan, S.; De Raedt, H.; Michielsen, K.
Journal of Computational and Theoretical Nanoscience, Volume 7, Number 9, September 2010 , pp. 1771-1782(12)

We present a computer simulation model that is a one-to-one copy of a quantum eraser experiment with photons (P. D. D. Schwindt et al., Phys. Rev. A 60, 4285 (1999)). The model is solely based on experimental facts, satisfies Einstein's criterion of local causality and does not require knowledge of the solution of a wave equation. Nevertheless, the simulation model reproduces the averages as obtained from the wave mechanical description of the quantum eraser experiment, proving that it is possible to give a particle-only description of quantum eraser experiments with photons. We demonstrate that although the visibility can be used as a measure for the interference, it cannot be used to quantify the wave character of a photon. The classical particle-like simulation model renders the concept of wave-particle duality, used to explain the outcome of the quantum eraser experiment with photons, superfluous.

http://arxiv.org/abs/0908.1036
I'm shocked that many well meaning scholars lazily gravitate towards mysticism as the first choice explanation for any non-obvious phenomenon.