Quantum erase explained with waves

[Cthugha]

No, that is an often used misconception about the wave-particle duality. Copenhagen does not state that the wave is a physical reality but that a photon partly acts like a classical wave.

First of all, that guy is not a supporter of Copenhagen and that guys considers the wave as real.
As has been pointed out several times, this way of understanding the wave-particle duality was all-out wrong. It is not tenable as there are situations in which the wave is non-local and as such non-classical. That problem is solved using quantum electrodynamics. The more modern versions of Copenhagen also take that into account.

Practical measurements shows that if you use wave equipment and -detectors, that with simple measurements one can use the classic wave formula, but without believing that it is a real wave. That is not philosophy but practice

Well, the classical wave model has very narrow limits. But, anyway, if you do not believe that it is a real wave: where is your problem and why do you always ask what the photon actually is when it goes through the slits? I am puzzled.

 

[DrChinese]

Here he implicitly claims that the wavefunction is a realistic entity and corresponds to some physical reality. This is philosophy and opinion.

Actually I too agree that the wave function corresponds to some physical "something". But yes, his argument is not really explicit (as you say) and there is still plenty of interpretation involved (also as you say). So much so that this paper, which really gives no specific new predictions or similar, doesn't really settle anything on the matter.

 

[Cthugha]

Actually I too agree that the wave function corresponds to some physical "something".

Maybe I should have emphasized that it is a perfectly valid opinion. Rereading my post it looks like a suggested otherwise. That was not my intention.

 

[DParlevliet]

If it is real or not is not important. The only thing what matters is that in simple measurements you can use the classical wave rules, but not together with particle rules. My point in the above measurement is that the waves are marked, not the particle. So after the slit you can measure through which slit a wave went, but not the particle. And because in absorption of the photon the two waves must be added (if you want interference) the marking is lost again. You did try to explain it with spin, which is a particle property, but I have not seen a consistent full explanation of the experiment.

And: every article claims that with marked waves the path of the particle in can be measured. But I have yet seen doing that.
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[Cthugha]

If it is real or not is not important. The only thing what matters is that in simple measurements you can use the classical wave rules, but not together with particle rules.

What is wrong about using the correct theory, QED, instead of a simplification that is known to be wrong in many cases?

My point in the above measurement is that the waves are marked, not the particle. So after the slit you can measure through which slit a wave went, but not the particle. And because in absorption of the photon the two waves must be added (if you want interference) the marking is lost again. You did try to explain it with spin, which is a particle property, but I have not seen a consistent full explanation of the experiment.

What do you mean by "if you want interference"? When you have perfect distinguishability of the paths, the superposition does not give rise to any interference terms. You add them of course, but that does not mean there is interference. This is why you actively need to do the erasing which projects the system into a state which does contain interference terms. For the same reason the marking is not lost. If you just perform the right measurement (spin-sensitive absorption, placing a QWP and a linear polarizer or whatever) just one of the field components survives. If you can get interference, it is not possible to perform a measurement where only one field component survives.

And: every article claims that with marked waves the path of the particle in can be measured. But I have yet seen doing that.
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Well, you deny the obvious. See for example "Observation of the Fresnel and Arago laws using the Mach-Zehnder interferometer" (Am. J. Phys. 76, 39 (2008)) for a demonstration that the addition of two light beams of orthogonal circular polarization does not give rise to interference.

 

[Dadface]

Well, you deny the obvious. See for example "Observation of the Fresnel and Arago laws using the Mach-Zehnder interferometer" (Am. J. Phys. 76, 39 (2008)) for a demonstration that the addition of two light beams of orthogonal circular polarization does not give rise to interference.

I have been trying to follow the discussion here but I'm confused by the point above. According to Pescetti and Piano:

Two light waves in orthogonal states of elliptical polarisations coming from the same unplolarised wave can interfere if brought into the same plane. Isn't this equivelent to the situation observed in the experiment under discussion where interference is observed when the "which way" information is erased?

 

[Cthugha]

Two light waves in orthogonal states of elliptical polarisations coming from the same unplolarised wave can interfere if brought into the same plane. Isn't this equivelent to the situation observed in the experiment under discussion where interference is observed when the "which way" information is erased?

Yes, that is quite right. The point I tried to make was that just adding the waves at some detector position without erasing anything will not give you any interference, so contrary to what DParlevliet claimed above the which-way info is not lost when the photon is absorbed (without erasing the which-way info).

 

[Dadface]

Yes, that is quite right. The point I tried to make was that just adding the waves at some detector position without erasing anything will not give you any interference, so contrary to what DParlevliet claimed above the which-way info is not lost when the photon is absorbed (without erasing the which-way info).

Thanks, that's clarified it.

 

[DParlevliet]

Well, you deny the obvious. See for example "Observation of the Fresnel and Arago laws using the Mach-Zehnder interferometer" (Am. J. Phys. 76, 39 (2008)) for a demonstration that the addition of two light beams of orthogonal circular polarization does not give rise to interference.

In this article the path of the photon is not measured.

 

[Cthugha]

In this article the path of the photon is not measured.

Eh? That was never the point of that paper. The point is that your claim:

And because in absorption of the photon the two waves must be added (if you want interference) the marking is lost again.

is wrong. You do not get interference if you add orthogonal states. That includes counter-circular polarizations as demonstrated. From this point on it is trivial that you have which-way info if you do not have interference. The interplay between both is given by the Englert-Greenberger duality relation.

 

[DParlevliet]

With adding waves I mean of course including polarization and taking into account classical Fresnel/Arago waves. But adding two opposite circular polarisations gives linear polarisation. That is basic goniometry.

What I mean is that in every article it is assumed that marking the wave of the photon makes it in principle possible to measure the path of the particle. But I have seen actually measuring that (because that is not possible, it would be against Copenhagen rules)

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[Cthugha]

With adding waves I mean of course including polarization and taking into account classical Fresnel/Arago waves. But adding two opposite circular polarisations gives linear polarisation. That is basic goniometry.

So? Interference is a redistribution of intensity due to cross-terms. These are not present for orthogonal states. The polarization is completely irrelevant for the question of which-way information.

What I mean is that in every article it is assumed that marking the wave of the photon makes it in principle possible to measure the path of the particle. But I have seen actually measuring that (because that is not possible, it would be against Copenhagen rules).

I absolutely do not get what you are aiming at. You get a single slit pattern corresponding to one of the slits. What else do you want? What kind of experiment do you expect. Something like the quantum delayed choice ("Entanglement-Enabled Delayed-Choice Experiment", Science 338, 6107, 637-640 (2012))? I absolutely do not get what else you would expect.

 

[DParlevliet]

The polarization is completely irrelevant for the question of which-way information.

But the Walborn article claims that if both waves have different circular polarization, there is which-way information.

 

[Cthugha]

But the Walborn article claims that if both waves have different circular polarization, there is which-way information.

Sigh. Let me rephrase. The fact that you can express the sum of two countercircular polarizations as a linear polarization is completely irrelevant for the question of which-way information. As long as you can associate the two pathways with orthogonal states, you have which-way information and no interference.

 

[DParlevliet]

As long as you can associate the two pathways with orthogonal states, you have which-way information and no interference.

But I have never seen an acticle where this which-way is actually measured.

 

[Cthugha]

The rest of the paracticing scientific world considers the measurements of a single slit pattern as seen in the DCQE experiments as a clear indicator of which-way info. What is it you want to see? What would you consider a which-way signature then?

 

[DParlevliet]

Measuring if this indicator is true. So a measuring setup which shows which way the particle went. The practicing scientific world will only consider this a clear indicator if it is measured, isn't it?

 

[Cthugha]

If you send photons just through one slit, you get a single slit pattern. If you send it through the second slit, you get a different pattern. Different position, different polarization. You get one of these patterns or the other if you do the right measurement and have which-way information. What else do you expect? You cannot do the measurement directly at the slit or the photon will never reach the final detection screen.

 

[DParlevliet]

If you send photons just through one slit, you get a single slit pattern. If you send it through the second slit, you get a different pattern. Different position, different polarization. You get one of these patterns or the other if you do the right measurement and have which-way information. What else do you expect?.

Which-way infomation is about a single photon. You don't see a pattern with a single photon, so no which-way infomation. What is needed is a measurement which can determine from a marked beam which way the single photon went.

 

[Cthugha]

That is a strawman. Quantum physics is a probabilistic theory. It does not make predictions about what happens in the case of a single photon state. All the predictions are for probabilities and testable on ensembles of identically prepared particles anyway.

 

[DParlevliet]

With multiple photons you know 50/50% goes throught one of the two slits. Both for standard double slit and Walborn e.a.

 

[Cthugha]

But you get very different patterns depending on whether you have which-way information or not. An interference pattern or two single slit pattern. If you do circular polarization filtering at the detection port, you get either the first or the second single slit pattern in the case with which-way informationbut you keep the interference pattern if you do not have which way information. That does not change for single photons.

 

[DParlevliet]

But you get very different patterns depending on whether you have which-way information or not.

Can you define what you mean with which-way information?

 

[Dadface]

Can you define what you mean with which-way information?

Can I have a try at this one?

With the two filters in front of the slits it is possible,in principle,to determine which slit a particular photon passes through. This results in the disappearance of the two slit interference pattern. When that possibility is erased we observe the single slit diffraction pattern(s) only. I'm still learning this stuff.

 

[Cthugha]

Can you define what you mean with which-way information?

Can I have a try at this one?

With the two filters in front of the slits it is possible,in principle,to determine which slit a particular photon passes through. This results in the disappearance of the two slit interference pattern. When that possibility is erased we observe the single slit diffraction pattern(s) only. I'm still learning this stuff.

Good one.

If you were interested in a quantitative measure, I consider the distinguishability in the Englert-Greenberger duality relation (http://en.wikipedia.org/wiki/Englert–Greenberger_duality_relation) as a good measure of how much which-way information is present.

 

[DParlevliet]

With the two filters in front of the slits it is possible,in principle,to determine which slit a particular photon passes through.

So a single photon isn't it?
And "in priciple": are there publications where this is actually done, so is the principle prooven?

 

[Cthugha]

I am still not sure what you are after. This is routinely done in student lab courses for classical light. Do you doubt that it works the same way for single photons?

"Wave-particle dualism and complementarity unraveled by a different mode" (Proc. Natl. Acad. Sci. 109, 24 (2012)) shows that along the way to some more complicated experiment. They show that you can get interference and spatial which-way info if you have two other possible "paths" - in this case different emission modes.

 

[vanhees71]

Of course, I've to read the paper first, but the title makes me already disappointed, since there is no wave-particle dualism in physics with the advent of modern quantum theory in 1925/26. Particularly photons have nothing in common with classical particles. It's not even possible to define a proper particle-position operator in a strict sense for massless particles with spin \geq 1.

Also the claim the emission of a photo electron in a photomultiplier proves the "particle nature of light" is unbelievable to be stated by people who are experts on photons ("quantum opticians"). It's well-known textbook knowledge that that photo-electric effect can be completely understood using the semiclassical approximation (electrons described by (non-relativistic) quantum theory, electromagnetic fields as classical external fields); see, e.g., Landau-Lifshitz vol. III. It's a nice application for time-dependent perturbation theory and usually given as a problem in recitations in Quantum Mechanics I!

 

[Cthugha]

Of course, I've to read the paper first, but the title makes me already disappointed, since there is no wave-particle dualism in physics with the advent of modern quantum theory in 1925/26. Particularly photons have nothing in common with classical particles. It's not even possible to define a proper particle-position operator in a strict sense for massless particles with spin \geq 1.

Kind of true. Some people tend to say wave-particle duality is outdated, other just choose to say that QED paves the way to wave-particle duality done right. Personally I side with the first camp, but I have grown pretty tired of discussing terminology instead of physics, when both camps mean the same thing.

Also the claim the emission of a photo electron in a photomultiplier proves the "particle nature of light" is unbelievable to be stated by people who are experts on photons ("quantum opticians").

I am puzzled. Did somebody make that claim in this thread? The smoking gun for photons being preparable in strongly particle-like states is of course photon antibunching.

 

[vanhees71]

I've tried to read the paper now. It's far better than I feared ;-)).

The main point seems to be the following: The experimenters shoot a T(01)-Gaussian mode coherent state with low intensity on a birefringencing BaO chrystal to produce single entangled photon pairs by spontaneous parametric downconversion. This mode has two maxima in its intensity distribution that can be seen as two lightemitting sources.

Now let's look at the signal photon first: The double slit is not necessary here but to strengthen the contrast. The point is that you can either measure in the near field and clearly can dinstinguish the two spots and one can show that then one has which-way information modulo the natural width of the spot. As long has one is close enough to the source that the resolution due to the finite width is large enough, you don't see interference effects in accordance with the complementary rules of quantum theory of mutual exclusive observables (here the phase of the em. wave field vs. the which-way information). If instead you detect the photons far away from the source such that the which-way information is lost due to the finite width of the T(01) mode, i.e., when these spots start to overlap in certain regions, you get interference patterns. So far the entire thing can be described as well with classical waves.

Now let's take into account that we also have the idler photons entangled with each signal photon. This is now the quantum theoretical situation that cannot be described anymore with classical electromagnetics, because here the entanglement of each photon pair is important. The experimenters indeed verified that with a high accuracy the which-way information, present in the near-field detection is one-to-one readable by detecting the idler photon due to its entanglement with the signal photon without ever touching the signal photon. So we have the which-way information present in the near-field observation setup also present with nearly 100% confidence by measuring the idler photon without ever touching the signal photon.

Now the point is that we have the which-way-information present in the near-field observation without touching the signal photons by measuring only the idler photons but at the same time can observe the interference pattern in the far-field observation of the signal photons.

In my opinion (now again we touch an "interpretation issue") again, that's not very astonishing from the point of view of the minimal interpretation of quantum theory. That's what also the authors state in their conclusion (on page 9317 last paragraph in the right column):

"However, we emphasize that even in our experiment the measurements of the whicht-slit information and interference still require mutually exclusive experimental arrangements. In order to observe the near-field coincidences [...] the detector D2 [measuring the signal photons] has to be just behind one of the slits, whereas the interference fringes [...] emerge only when we move D2 far away from the slits."

This shows that indeed there is no collapse of the quantum state due to the gain of which-way information that we would get in the near-field-observation setup by measuring it using the entangled idler photon but that we can still observe the interference pattern in the far-field observation of the signal photon. For me that's a very important measurement disproving once more (at least naive) collapse hypotheses a la some flavors of the Copenhagen/Princeton interpretation.

I would however strongly object against the first sentence of the conclusion section: "Quantum theory is build on the celebrated wave-particle dualism." There is no such wave-particle dualism as quantum theory (most strongly relativistic QFT, which has used in the present paper to describe the photons, by the way) tells us. The fundamental (elementary) "particles" are neither describable as classical (billard-ball like) little bullets as in classical mechanics nor as wave solutions of classical field equations but only by the rather abstract formalism of quantum theory itself with its strict indeterministic and probabilistic meaning of the quantum state as encoded in Born's rule. I'd call this the "reality" according to the current understanding of physics which is demonstrated with high significance by all known experiments, among them the one discussed in this paper. There is no "classical reality" indeed, but in my opinion, it's wrong to claim there is no reality according to quantum theory. "Reality" in the sense of physics can only mean what's objectively observable, and what's observed is the indeterministic and probablistic "quantum reality" and not "classical reality".