Double slit: Human vs Machine Observer

  • Thread starter t_siva03
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  • #1
t_siva03
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Would someone help me to understand what would happen in this situation.
w→ x→ (y)→ z

Photon emitter is at point w emitting photons one at a time. Double slit is at point x. Screen is at point z.

There is a detector at point y (after photon has already passed through slits) which is capable of recording which slit the photon passed through. The detector marks this on a piece of paper within a closed box that is inaccessible to any human.

What would be seen on the screen, an interference pattern, or not? The question has to do with whether it is the actual instrument (detector at point y) that causes collapse of the wave function, or the observation by a sentient being (human). If the human does not see the results, and cannot ever see the result (let's say the paper is burned by the recording instrument after recording before anyone can see the result) would it still result in interference.

Thanks for your help.
 
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Answers and Replies

  • #2
San K
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No interference pattern.

The detector (or any other setup/device, for example polarizers, that can provide us which-way) causes the wave-function to collapse.

Human --- watching or not watching --- does not matter.

Human/living consciousness has nothing to do with it.

If the human (eye) can act as a detector -- then it matters.

In that case the human (eye) caused the collapse of the wave function.

Whichever device (that can, in principle, provide us which-way) comes first.....collapses the wave function.

Side note:

it is possible to have "some/partial" information about which-way and have a "partial" interference pattern

i.e. it is possible to have say.....80% probability the photon went through slit A and have an interference pattern that is distorted from the normal one.

as the probability/reliability (of which-way) is increased, the interference pattern gets fuzzier
 
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  • #3
Maui
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Different versions have been implemented (look up the delayed choice quantum eraser) and they all show that the detectors have nothing to do with the actual collapse but the availability of the which path information.
 
  • #4
StevieTNZ
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Definitely no interference pattern. However we cannot assert collapse has occurred yet. What QM predicts is merely entanglement of the photon with the detector by the slit, and the fundamental equation governing this experiment - the Schrodinger equation - predicts no wave function collapse.

So indeed it may be until a human looks at the detector that it goes through one slit or the other. Or it may be the case, although thought experiments don't support this idea*, that the apparatus causes the photon to go through one slit or the other.

* e.g. David Albert's book 'Quantum Mechanics & Experience' - chapter on self-measurement; GianCarlo Ghirardi's book 'Sneaking a Look at God's Cards' and thought experiment contained within.
 
  • #5
DrZoidberg
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Definitely no interference pattern. However we cannot assert collapse has occurred yet. What QM predicts is merely entanglement of the photon with the detector by the slit,

So if the "detector" is a human QM predicts that that person would be entangled with the photon? Just like Schrödinger's cat?
 
  • #6
Maui
767
2
Human --- watching or not watching --- does not matter.


This is taking it to a street level and it thus can become misleading. What the double slit shows is the uncertainty principle in action - you can not know(not determine, but know) to a high precision both the position(which of the two slits has been traversed) and direction(speed) of a particle(seen as interference pattern on the screen) at the same time.

http://www.nature.com/news/2011/110602/full/news.2011.344.html

So the experimentally verified cornerstone of quantum mechanics(the HUP) already poses quite a challenge for theories positing naïve realism.
 
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  • #7
craigi
615
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Would someone help me to understand what would happen in this situation.
w→ x→ (y)→ z

Photon emitter is at point w emitting photons one at a time. Double slit is at point x. Screen is at point z.

There is a detector at point y (after photon has already passed through slits) which is capable of recording which slit the photon passed through. The detector marks this on a piece of paper within a closed box that is inaccessible to any human.

What would be seen on the screen, an interference pattern, or not? The question has to do with whether it is the actual instrument (detector at point y) that causes collapse of the wave function, or the observation by a sentient being (human). If the human does not see the results, and cannot ever see the result (let's say the paper is burned by the recording instrument after recording before anyone can see the result) would it still result in interference.

Thanks for your help.

You won't get an interference pattern, because in principle you can use the smoke from the burning of the paper to work it out. No joke. Although, I'd be the first to admit that this logic is somewhat backwards.

If you're not already familiar with them, it's worth looking into quantum erasers though, which are capable of performing the function that you're looking to achieve.

To entertain the fantasy, as far as I'm aware, it is still unproven and arguably unprovable, whether a human observer is actually required at the end of the chain as the final stage in the detection process to bring the world into reality. Furthermore, it may actually be a very individual thing, where reality is entirely subjective. But you're not going to be able to prove either of those with an experiment like that.
 
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  • #8
StevieTNZ
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So if the "detector" is a human QM predicts that that person would be entangled with the photon? Just like Schrödinger's cat?

Indeed.
 
  • #9
Ibix
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So if the "detector" is a human QM predicts that that person would be entangled with the photon? Just like Schrödinger's cat?
When you ask someone else to open the cat's box, that person is "Wigner's friend", I believe.
 
  • #10
craigi
615
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So if the "detector" is a human QM predicts that that person would be entangled with the photon? Just like Schrödinger's cat?

Indeed.

Entanglement equating to measurement isn't the only interpretation. In fact, I don't believe it's even a particularly promiment viewpoint.

Traditionally, entanglement and measurement are completely different concepts. You'll certaintly find that the two terms are not used interchangably and a description of entanglement is very likely to rely on the concept of measurement.
 
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  • #11
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So if the "detector" is a human QM predicts that that person would be entangled with the photon? Just like Schrödinger's cat?

Of course. But that's not really of deep import to this situation.

And as everyone else mentioned you get no interference pattern.

But I sense perhaps a deeper issue - that you may be thinking because the word observation is often used it needs a conscious observer. It doesn't. In QM an observation is anything that leaves a 'mark' here in the commonsense macro world that exits independent of us regardless of if anyone is around to observe it or not - in the majority of interpretations anyway - this consciousness causes collapse thing is very much a minority view that's even less held than it once was. The high priest of it, the great mathematical physicist, Eugene Wigner, when he heard of some early work of Zurec on decoherence realized it was no longer required and abandoned it. Its still a valid interpretation, but its weirdness leaves many like me cold.

If you want to get to the bottom of this stuff I suggest Susskinds lectures:
http://theoreticalminimum.com/courses/quantum-entanglement/2006/fall

Thanks
Bill
 
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  • #12
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Entanglement equating to measurement isn't the only interpretation. In fact, I don't believe it's even a particularly promiment viewpoint.

Not quite so sure about that. It most certainly is not the only interpretation, but most modern writers on the subject seem to take that view - ie most modern interpretations seem to incorporate decoherence in them somewhere - Decoherent Histories, MWI, Ensemble Ignorance Interpretation.

The following gives a good overview on its effect on interpretations:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Thanks
Bill
 
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  • #14
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http://www.youtube.com/watch?v=TWEXkXwz7dk&list=PL993AF2864A61D274

Courtenay Raia sums up the situation with interpretations, and how to interpret the double slit with detections, nicely from 58:20 in the video above.

Sorry - couldn't get past the first couple of minutes.

QM is NOT magic, it does not break all the rules, and is not behind, well to be blunt, gutter trash, like The Secret.

From what I can gather she is a history professor. I think Susskind is a better source of what QM REALLY says.

Had a bit more of a look at it - she has a dirty big slide entitled Quantum Mysticism with reference books like the Tao of Physics - sorry - she is way off the mark.

The REAL issue with QM is not this QM is magic stuff etc etc. The REAL, the FUNDAMENTAL issue, is we have all these different interpretations that are equally as valid and no way experimentally to tell them apart. Some like De-Brogle-Bohm are most definitely NOT strange in the kind of world view it engenders - they have other issues - but the world view it gives us is not that different to the everyday commonsense view we have. We have other interpretations that fix up those issues but create others like a weird conscious observer created reality. What we don't have is an interpretation that satisfies everyone - that's it - that's all.

Thanks
Bill
 
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  • #15
craigi
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http://www.youtube.com/watch?v=TWEXkXwz7dk&list=PL993AF2864A61D274

Courtenay Raia sums up the situation with interpretations, and how to interpret the double slit with detections, nicely from 58:20 in the video above.

I watched a couple of hours of that. She's an excellent speaker and very entertaining to watch, but I think it's clear that she'd be very concerned to be considered in any way, an authority on quantum physics or its interpretations.
 
  • #16
t_siva03
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Thanks to everyone for all of your replies. The delayed choice quantum eraser experiments make me think even more that some "human knowledge" is required for wavefunction collapse. http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

In this case we have two different instruments acting on a photon and the act of "measurement" does not cause the collapse since when both splitters are present there is no collapse. This leads to a couple of conclusions in my understanding.

1. The act of measurement does not matter since the presence of two splitters cancels out the wavefunction collapse
2. Only the potential knowability matters (i.e. having the potential to look and see for sure which pathway was taken)

This importance of knowability for the collapse to occur makes me think that a sentient observer must be present since the quantum eraser experiment shows that an instrument alone cannot cause the collapse if the information is later erased.
 
  • #17
craigi
615
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Thanks to everyone for all of your replies. The delayed choice quantum eraser experiments make me think even more that some "human knowledge" is required for wavefunction collapse. http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

In this case we have two different instruments acting on a photon and the act of "measurement" does not cause the collapse since when both splitters are present there is no collapse. This leads to a couple of conclusions in my understanding.

1. The act of measurement does not matter since the presence of two splitters cancels out the wavefunction collapse
2. Only the potential knowability matters (i.e. having the potential to look and see for sure which pathway was taken)

This importance of knowability for the collapse to occur makes me think that a sentient observer must be present since the quantum eraser experiment shows that an instrument alone cannot cause the collapse if the information is later erased.

If you want to incorporate sentience into your interpretation, first you're going to need to be able to define sentience. It's not as easy as it first appears.

In the process of doing so, I expect that you'll realise that the only reliable sentience, in that respect is oneself, à la Decartes, if you pardon my French or that an arbitrary macroscopic object performs the role equally well. In which case you abandon the role of sentience and look to decoherence for a solution to the measurement problem.
 
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  • #18
Cthugha
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In this case we have two different instruments acting on a photon and the act of "measurement" does not cause the collapse since when both splitters are present there is no collapse. This leads to a couple of conclusions in my understanding.

1. The act of measurement does not matter since the presence of two splitters cancels out the wavefunction collapse

This is incorrect. You do not cancel out collapse. The beam splitters never actually perform a measurement. The measurement always happens at the detectors as does the collapse. There is no irreversible interaction prior to the detectors and irreversible interactions are the things that count and constitute measurements. You cannot erase a measurement that has been performed, only the possibility to do a measurement giving you some certain kind of information. These are very different things.

2. Only the potential knowability matters (i.e. having the potential to look and see for sure which pathway was taken)

This is correct, but it is evidence for the opposite of what you make out of it. "Potential knowability" is just a measurement (irreversible interaction) taking place.
 
  • #19
StevieTNZ
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This is incorrect. You do not cancel out collapse. The beam splitters never actually perform a measurement. The measurement always happens at the detectors as does the collapse.

That statement comes across as if the measurement problem is solved.
 
  • #20
Cthugha
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It was not intended to. I was just aiming at contrasting reversible interactions (like using a half wave plate, shifting the phase of a light beam and such stuff) to irreversible interactions (absorption, incoherent scattering and so on). Leaving all the questions of the measurement problem aside, it is pretty well agreed upon, that the latter actually constitute a measurement, while the former do not.
 
  • #21
t_siva03
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People have posted in other threads about a setup where you have a detector at the slits, and then a monitor which displays the results. They claim that if the detector is turned on and the monitor is on then there will be no interference, but if the detector is on but the monitor is turned off then the interference re-emerges. No one provided a reference to the experiment however. How can this be explained if it is true?
 
  • #22
Cthugha
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People have posted in other threads about a setup where you have a detector at the slits, and then a monitor which displays the results. They claim that if the detector is turned on and the monitor is on then there will be no interference, but if the detector is on but the monitor is turned off then the interference re-emerges.

This is like an urban myth which gets repeated again and again by some people who heard that stuff somewhere. Turning monitors off does not change interference patterns. Neither do the other variations of this urban myth like saving the experimental data on a hard disk and smashing it before having a look at it.

If you come across these claims again, just ask these people for the peer-reviewed publication they got this from. That usually creates uncomfortable silence.
 
  • #23
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This importance of knowability for the collapse to occur makes me think that a sentient observer must be present since the quantum eraser experiment shows that an instrument alone cannot cause the collapse if the information is later erased.

A sentient observer is NOT required in virtually all interpretations - most explain it without reference to such.

Thanks
Bill
 
  • #24
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That statement comes across as if the measurement problem is solved.

The measurement problem is very interpretation dependent - in some like De-Brogle-Bohm it doesn't even exist, in others its a non issue, in still others its the central issue, in others it has been morphed into something else. Speaking of such things without reference to a specific interpretation is fraught with danger.

Thanks
Bill
 
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  • #25
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People have posted in other threads about a setup where you have a detector at the slits, and then a monitor which displays the results. They claim that if the detector is turned on and the monitor is on then there will be no interference, but if the detector is on but the monitor is turned off then the interference re-emerges. No one provided a reference to the experiment however. How can this be explained if it is true?

Cthuga is correct - its a myth, because the formalism of QM, this is the formalism without any interpretation, (it would be better to say, with very minimal interpretation - but no need to go into that here) says it happens regardless. Those of the consciousness causes collapse brigade are driven to a pretty weird view of the world for all sorts of reasons, only one of which is - exactly what counts as a conscious observer? If the results of the double slit experiment were stored on a computer, the original apparatus destroyed, then viewed a few centuries later is that when it collapsed? If so then it would render computer science basically gibberish - you can probably maintain such a view with some very complicated semantic shenanigans - but start sprouting it in front of a computer science class and they will likely leave roaring their heads off.

Its not that such views are not a valid interpretation - people that championed it like Wigner and Von Neumann were not fools, its just that a lot of water has gone under the bridge since it was first proposed and its issues are now better understood, as well as the issue that led them to it in the first place - without going into exactly what that was - if you are interested feel free to start a thread about it. As I mentioned previously in the thread Wigner changed his mind when he heard of some early work on Decoherence by Zurek:
http://cds.cern.ch/record/882828/files/0509042.pdf
'In his later years Wigner changed his position to an exactly opposite viewpoint, in order to avoid solipsism and the role that consciousness plays, closely mirroring that postulated by Shimony. Wigner considered it to be necessary to admit state reductions independently of an observer’s consciousness, and makes a concrete suggestion for an amendment of the Schrödinger equation which is intended to describe a physical process of state reduction. A state reduction is now felt by Wigner to occur as an objective event in the physical realm before the von Neumann chain reaches the consciousness of an observer.'

(Note - I gave that link simply as an overview of the issue - not because I advocate the particular position it takes that collapse may occur in the eye - that too would lead to all the issues I mentioned previously with regard to storing the results in a computer)

The other advocate, the great mathematician and polymath, Von Neumann, died too early to be exposed to these developments, but I suspect he would have changed his mind as well.

Thanks
Bill
 
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  • #26
DParlevliet
161
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Or look at it from another side. Suppose a thought experiment where in stead of detectors both slits are covered with a very thin transparent foil with a layer of atoms which absorbs all photons and directly transmits it again. Will there be interference after the slits? I guess: yes.
 
  • #27
.Scott
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If you want to incorporate sentience into your interpretation, first you're going to need to be able to define sentience. It's not as easy as it first appears.
But not as difficult as it first appears either. I would claim that sentience, which is not restricted to humans, is a valid topic for discussion in physics - quantum physics in particular. However, the Forum moderators consider it too speculative for discussion here.

As for whether it is needed for a QM observation, let's presume not.
 
  • #28
.Scott
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Or look at it from another side. Suppose a thought experiment where in stead of detectors both slits are covered with a very thin transparent foil with a layer of atoms which absorbs all photons and directly transmits it again. Will there be interference after the slits? I guess: yes.
They have performed this experiment with atoms carrying microwave photons passing through tine microwave resonant cavities. As long as the microwave photons did not retain the which-way, the interference pattern remained.

My guess is that it would depend on whether the material was transparent or translucent. Translucent would imply that the momentum had changed. But even that should not destroy the pattern.
 
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  • #29
craigi
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Or look at it from another side. Suppose a thought experiment where in stead of detectors both slits are covered with a very thin transparent foil with a layer of atoms which absorbs all photons and directly transmits it again. Will there be interference after the slits? I guess: yes.

Nope. For a number of reasons.

1) You lose the phase in the absorption - emission process. This differs from reflection by the free electrons in a metal.
2) Information is retained within the foil about the path of the photon.
 
  • #30
DParlevliet
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In glass the forward licht also has interference with the back-reflected light (Feynman, QED). Reflection is based on absorbed-emitted photons. Therefore I expect it will be the same in my case.
 
  • #31
craigi
615
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In glass the forward licht also has interference with the back-reflected light (Feynman, QED). Reflection is based on absorbed-emitted photons. Therefore I expect it will be the same in my case.

The difference with reflection is that it's from electrons which are free to take on any vibrational state, so re-emit a photon that is coherent with the incident photon, leaving no record of it's reflection. The electrons in orbit around atoms can't do this.
 
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  • #32
DParlevliet
161
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So there is a difference between a reflection back and "forward"?
If you reflect on a small angle, it is almost forward
Or photons which in the glass are absorbed and retransmit an photon forward
Let us forget the foil. The atoms are the "foil" (it is a thought experiment)
 
  • #33
craigi
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So there is a difference between a reflection back and "forward"?
If you reflect on a small angle, it is almost forward
Or photons which in the glass are absorbed and retransmit an photon forward
Let us forget the foil. The atoms are the "foil" (it is a thought experiment)

You can reflect on a small angle, no problem. Stand infront of a mirror, then keep moving to the side. It still reflects.

The point is that there's different ways that photons interact with materials. Some presevere coherence, others don't. Scattering is different to specular reflection. Firing things through foil, scatters them. Bouncing things off mirror like surfaces reflects them specularly.

It's not the atoms, as such, that determine specular reflection. It's the surface. The interactions between photons and atoms are different.

A thought experiment isn't supposed to ignore actual physics and make up new physics. It's supposed to be an experiment that you could theoretically perform with actual physics, given hypothetical technology, time and resources.
 
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  • #34
DParlevliet
161
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Alright, so now we have a case (1) where photons are not detected (they are emitted by the atoms with the same energy), but no interference.
Then case 2: mirrors are placed at both slits. The light comes from the side and is reflected by the mirrors into the slits. No other light can enter the slits. There is no detection. I suppose there will be interference now?
Compare case 2 with detectors at the slits. In both cases the photon interferes with matter and passes a/the photon. Then the conclusion should be that a detector (and case 1) works in a way that it changes something in the photon, making it unable to interfere.
Could it be possible that a photon can only interfere with its own wave, not with another photon/wave with the same properties. In detectors (and case1) new photons are emitted. But that would mean that in a mirror not a new, but the same photon is re-emitted (or wave reflected).
 
  • #35
craigi
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Alright, so now we have a case (1) where photons are not detected (they are emitted by the atoms with the same energy), but no interference.
Then case 2: mirrors are placed at both slits. The light comes from the side and is reflected by the mirrors into the slits. No other light can enter the slits. There is no detection. I suppose there will be interference now?
Compare case 2 with detectors at the slits. In both cases the photon interferes with matter and passes a/the photon. Then the conclusion should be that a detector (and case 1) works in a way that it changes something in the photon, making it unable to interfere.
Could it be possible that a photon can only interfere with its own wave, not with another photon/wave with the same properties. In detectors (and case1) new photons are emitted. But that would mean that in a mirror not a new, but the same photon is re-emitted (or wave reflected).

Detectors end coherence, mirrors, lenses, electromagnetic and gravitational fields don't. You need coherence for interference patterns (but not for interference).

Photons can occupy the same position and state and do interfere with each other. Individual photons interfere with themselves also.

In the cases that you mention, I would say that the photon interacts with matter rather 'interferes with matter'.

Mirror-like surfaces do absorb and emit new photons. The important thing about these photons is that they preserve the phase relationship, frequency and leave no record of path information. This means that they remain coherent and interfere with the incident photon in a way that is consistent with the probabilty distribution of an interference pattern (and all other photons which originate from processes that preserve coherence). Some surface interactions reverse the phase but coherent interference still takes place, in analogy to classical wave mechanics.
 
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