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Photons Detector not yielding which-path info.

  1. Jul 16, 2008 #1
    Photons Detector not yielding "which-path" info.

    In Fabric Of The Cosmos, I am just getting through the part where Greene discusses a multi-path setup (7.5?) involving a series of beam splitters and downconverters, whereby photons striking two of four detectors will yield definitive "which-path" information, but, due to the configurations of the optical components, "hits" on the other two detectors will not yield definitive which-path info, and therefore, we preserve the interference pattern that one would expect without any measurement intervention.

    It seem that the system "knows" that our experimental setup can not, or does not, provide which-path info and therefore behaves accordingly.

    So, what if the detectors were present, but not "ON" or functioning in any way. What would we expect then?

    What if they were on, but the wiring was sabotaged such that the result would never be reported reliably?

    What if the result were available, but only sent to a computer that was programmed to irretrievably delete the data and never make it available to living beings?

    Or what if a long delay line were in the system such that the result were not known for some time after the photon struck the detector. Would the interference pattern break down at the instant the photon struck the detector, or only after the result was "known.?"

  2. jcsd
  3. Jul 16, 2008 #2
    Re: Photons Detector not yielding "which-path" info.

    Anton Zelinger discusses this very issue in Experiment and the Foundations of quantum physics (google it). The issue is whether we arrange the experiment in such a way as it is possible - in principle - to determine the which-path information. Even if the detector is off, or if we get the information for a nanosecond but then discard it, the photons behave as though we had the which-path info, and there is no interference pattern.

    In other words, nature really doesn't care what the human observes - which should not at all be surprising!

    There are "delayed choice" experiments which attempt to show that when we (think we) have the which path information, but then it is destroyed, the interference pattern reemerges. These experiments are very tricky to devise. They only work when it is absolutely impossible - in principle - to recover the information because, say, a beam splitter has sent a photon to one of two random directions which can never be known. But you can't make a quantum eraser by sabotaging the wires or telling the computer to disregard the information. :) The eraser has to ruly be "quantum".

    Now a fascinating quantum eraser experiment would be a quantum computer eraser, which randomly - based on a _quantum_ event - discarded the information such that there was no trace of it in the computer's quantum memory. I suspect such an eraser would work.
  4. Jul 16, 2008 #3
    Re: Photons Detector not yielding "which-path" info.

    I'm not familiar with the particular setup described in your post, and I haven't read any of Brian Greene's books. But I know that for the original double-slit experiment, when the detectors are on, but the screen that the observer looks at is off, you get an interference pattern. If you keep everything the same but turn the screen on, you don't get an interference pattern. See this guy's lecture: http://www.youtube.com/watch?v=_OWQildwjKQ&feature=related This shows that only a living creature (and possibly only a human) can collapse the wave function.

    I also read somewhere (can't find the source now) that when everything in the setup was identical except that the lighting of the room was just dim enough to prevent the human observers from seeing the screen, the wave function didn't collapse, when otherwise it would have. Again, this shows that it is the human being that collapeses the wave function.
  5. Jul 16, 2008 #4
    Re: Photons Detector not yielding "which-path" info.

    This guy's presentation is incorrect. (BTW, is he in a church??)

    When the photon has been detected by one of the detectors, an irreversible measurement is made. That measurement "collapses the wavefunction." If the detector is disconnected from the screen, that does not change the fact that the measurement was made. The state of the system has been altered and merely disregarding the information is not the same as putting the system back into the superpositioned state, whereas with a quantum eraser it is.

    Again, that is also impossible. Whatever source you read that in in bogus.
  6. Jul 16, 2008 #5
    Re: Photons Detector not yielding "which-path" info.

    So, dim lighting and human "awareness" aside...

    Where in the "block diagram" of the detector is the wave function caused to collapse?

    I don't know the schematic of the detectors that are used, but it is probably safe to assume that there is some sort of light sensitive element which then sends some charge or control signal to something that will indicate that the photon is/was present.

    At which moment in time does the wave collapse? Is it upon the launch of this control signal, or only when the information is recorded or preserved? It just seems to me that there is some finite element of chronology that would yield answers.
  7. Jul 16, 2008 #6
    Re: Photons Detector not yielding "which-path" info.

    It collapses the instant the photon is absorbed by an atom in the detector.

    The difference between a detector and a mirror or beamsplitter is that, in the latter case, the photons are either reflected or refracted purely, and the state of the device is not permanently altered. (Beautifully, the state actually is altered momentarily, while the electrons "wobble" around a bit, but then goes back to normal - very quantum eraser-like).

    A photosensitive detector, however, must alter its state when a photon strikes it in order to be a detector, at which point there's been an unmistakeable collapse of the state of the detector: it was in a superposition of detect/no-detect, and later becomes "detect". I believe most of them use the photoelectric effect, and so a small current is generated when the photon is absorbed. Another photon might well be reemitted and might well continue past the slit, but it will have a well defined position and therefore won't make an interference pattern.

    In other words, even if some kind of photon is still passed on throught he slit, it won't be in the same state it was before - no matter whether someone "looks at" the detector or not. That's why the video posted is so off the mark. It will be in a definite state - "Detected by left detector" or "detected by right detector" and will behave accordingly. It most certainly will not remain in left/right superposition.
  8. Jul 17, 2008 #7
    Re: Photons Detector not yielding "which-path" info.

    Since no one else seems to be biting on this (to me) very interesting subject, and because I love to hearmyself talk (:smile:) I'd like to add some more random observations:

    This question really goes to the heart of quantum interpretation. What is the wavefunction? People originally thought it was continuous charge distribution, but quickly realized that couldn't be right, because the charge of an electron or EM field of a photon would then be felt (in small amounts) everywhere. Born figured out that it was probability amplitude, and that the absolute square of the amplitude was the probability density function which, when integrated over a space, gives you the actual odds of finding the particle in that space.

    Easy enough right? The big problem then becomes why and how does the wavefunction change when there's a measurement? If it's all just raw statistics with no physical meaning, why *doesn't* whether the detector is connected to the monitor matter? If it were all just probability and Bayes' theorem, then the human observer would be necessary. But he's not. So there's something more at work.

    We've got to look for something that physically occurs when the wavefuncion "collapses" to figure out why that happens. And a good starting point is the contrast between a photon striking a mirror and a photon striking a detector. In many experimental setups, one collapses the wavefunction, the other does not. (i.e. you can use mirrors or lenses or beamsplitters or what have you - lets just say "glass" - to bounce photons around as much as you want and still get an interference pattern if you do it right). Yet in both cases (glass vs. detector) electrons are being perturbed by the photon, and new photons are emerging as a result. The same kind of *stuff* is happening. What's the difference?

    In the case of glass, the electrons are perturbed, they wobble, and then emit a new photon - conserving energy and momentum. The photon out is (for our purposes) identical to the one that came in. Critically, the photon leaves the glass behind with no trace of it ever having been there. And so maybe it wasn't - because in QM, if there was no evidence that it happened, it might as well not have happened. In any event, we *cannot* know, in principle, whether a photon ever struck that piece of glass. And so bouncing around in a lens or mirror doesn't destory the photon's superpositioned state - because it may not have hit that mirror at all. Put another way, the state of the glass after the photon is gone is identical to the state of the glass before the photon got there - so the state of the glass makes no contribution (and therefore is not entangled with) the state of the photon. (It doesn't matter how many molecules are involved in the photon's "journey" through the lens or bouncing off the mirror - they all wind up in the exact state they were in before.)

    Contrast this with a photon hitting a detector. Assuming the detector uses the photoelectric effect, a small current is generated by the strike of the photon. The current takes the form of electric charge - electrons - moving around in a wire, jumping around, getting excited, atoms getting ionized, etc. Billions and billions of molecules are affected even by that tiny little bit of current that was generated as it propagates through the wire.

    Electricity is still technically a QM phenominon though, so isn't it possible the wire returns to the state it was in after the current has passed through it? No way - all wires have resistence, which means heat, and even the tiny current from a single photon will generate *some* heat. But even if you ignored that, the current winds up activating electronic circuitry, and once that starts happening there's no going back. The transistors involved contain gazillions of molecules that are interacting and exchanging electrons and generating heat and doing all the things transistors do - all because that photon struck the detector.

    So the screen is shut off. All those molecules have still been affected by the photon's strike. They *remember* it. There's evidence of it, and if you really wanted to find it, you could. And therefore the wave function has collapsed. Put another way, the state of the detector - a macroscopic object - has substantially been altered by the hit of the photon. Knowing the state of the detector tells us the state of the photon. The photon has now become entangled with a macroscopic object.

    The fancy word for all of that is decoherence. If you let the wave function keep evolving, accounting for every molecule of the wire and each molecule in each transistor, not to mention if you could (somehow) account for the heat as well, you'd find that the wave function would be an absolute mess - but - it would apporximate, very closely, the state of 100% certainty that the photon struck that detector. The state of the detector has become so intertwined with the state of the photon that there's no way the detector could be in that state if the photon hadn't hit there (compared to glass, where the state of the glass means nothing about whether the photon hit). And whether the human looked at the result wouldn't really matter all that much. To be sure, if the human looked, he'd become entangled with the photon too, but the detector's in the state that it's in, and it ain't going back no matter what, so no one really cares whether the human looks.

    Decoherence is the subject of a lot of research right now, but IMHO is the leading candidate, if not the outright winner, for explaining the "quantum/classical" transition. (Unfortunately, it's also the leading problem in developing quantum computers). Decoherence doesn't explain everything either - technically speaking, the system is still in a state of superposition, but the other state is so miniscule as to be undetectible. But where is it? That's the subject of debate as well.
  9. Jul 17, 2008 #8
    Re: Photons Detector not yielding "which-path" info.

    Good self talk, Peter0302. Thanks for jumping further with this. You went a long way towards helping my understanding of wave collapse. As you say "We've got to look for something that physically occurs when the wavefunction "collapses" to figure out why that happens.

    The issue surrounds the delayed choice quantum eraser experiment (google that and look at the wiki for a great explanation)

    If I understand it's fundamental message, they use a network of detectors and down converters such that in 50% of the pathways, the photon strikes the detector, but the interference pattern persists, even though the detector was active and presumably causes, heat, current, collapse, etc, but ALAS, it does not because:

    due to the config of the detectors, there are some paths that will not yield to the experimenter any definitive which path info.

    So, in this experiment, it appears that the physical impact the detector brings to the photon-detector system is the same as in the more basic experiments, but the interference pattern is still present.

    Do I have this right, and if so, why would that be?
  10. Jul 18, 2008 #9


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    Re: Photons Detector not yielding "which-path" info.

    Yes and no. The direct output of the detector will nerver show an interference pattern in delayed choice quantum eraser experiments. You only get the pattern by coincidence counting, so the subset of detected photons, which do not provide which-way information gives you the interference pattern.

    The reason for this behaviour lies in the state of the system after the collapse. At first you have a state, where there is a photon present and the detector is not in an excited state. After the detection/collapse the photon has gone and the detector is in an excited state. Due to the fact, that there is no which-way information, the state after the collapse is still a superposition of the two possible paths the photon could have taken and therefore the interference pattern is still present.
  11. Jul 18, 2008 #10
    Re: Photons Detector not yielding "which-path" info.

    The DCQE is a very interresting and complicated experiment. First off, you're dealing with entangled photons, so it's a little more complicated than a simple double slit. And the rule is that the measurements you make on entangled photons have to be consistent. So if you detect which-path on one, you have to get the same result when you measure the other. And therefore you can't see an interference pattern from either if you detect which-path from either. That's basic HUP.

    So with that in mind what's going on in that experiment? We've got 5 detectors total - D0 through D4 and two entangled photons, a signal and an idler. D0 gets the signal photon first. That photon lands where it lands, so that's set in stone right? Well, apparently no. Because apparently what happens at D1-D4 actually has a correlation with what happens at D0. I say correlation - not causation - because we don't know what caused what. If we start talking about causation, we are almost trapped into talking about REVERSE causation, so let's not talk about that for now. :) Let's just talk about quantum states.

    What does entanglement mean? It means that you cannot talk about the state of "A" without saying something about the state of "B". They are mutually dependent. They're like complimentary angles: if angle A is x degrees, complimentary angle B is 90-x degrees. Period. Causation? Who knows. Not important right now.

    So, the signal and idler photons are entangled and D0 always gets the first hit. Now here's the kicker: a hit at D0 tells you _nothing_ about which slit the photon came out of. As you can see from the drawing (figure 2 in the paper, attached), the photon will hit at D0 regardless of which slit it came out of. So while the states of the two PHOTONS are mutually dependent, the state of the detector D0 actually doesn't tell you anything about which-path, and so it doesn't collapse the wavefunction! There's been no measurement of which-path yet. If that was the end of the experiment, it would be impossible, in principle, to determine which path, and so a plot of photons across the x-axis of D0 would show an interference pattern.

    But there's the idler photon. The idler photon has a choice: it can either go straight to the detector - D3 or D4 (D4 is not shown in the figure for some reason) depending on the slit - or the eraser - D1 or D2. So if it hits D3, we know it came from slit A, and if it hits D4 we know it came from slit B. If either of those two things happen, BAM the wavefunction is collapsed. There's now a correlation between the irreversible state of the macroscopic detector (D3 or D4) and the idler photon and, because of entanglement, the signal photon as well. So the wavefunction collapses - or the system decoheres - when a hit is registered at D3 or D4 and the plot from D0 shows no interference pattern.

    BUT if the idler photon goes to the eraser - D1 or D2 - the wavefunction never collapses. Because of the set up, a hit at D1 or D2 is as ambiguous as a hit at D0. The beamsplitter mucks it up. A hit at D1 is still a 50/50 chance of having come from slit A or B, as is a hit at D2. So the plot from D0 shows the interference pattern because the system stays in the superpositioned state.
    Between the fact that the hit at D0 told you nothing about the slit, and the fact that a hit at D1 or D2 told you nothing about the slit, there's now no opportunity - ever - to figure out which slit the photons emerged from.

    I'm avoiding any interpretive discussion because inevitably we'll start arguing over causality. How can what happened in the future affect the past!?! I don't have an answer that I can prove, I can only take some guesses and speculate, no more than anyone else can do right now.

    I can tell you that we can't send information into the past using this method, for reasons that are even more copmlicated than the above explanation, but to put it simply, the pattern at D0 never emerges until it's been compared with what happens at D1-D4, which obviously cannot occur until after the experiment has run its course. If you substituted the random beamsplitters BSA and BSB with a switch which caused the erasure to occur, and went back later and looked at the data you'd think you were sending messages into the past. But you can't _read_ the messages until the experiment's over, so they do you no good. :)

    Attached Files:

    Last edited: Jul 18, 2008
  12. Jul 19, 2008 #11
    Re: Photons Detector not yielding "which-path" info.

    Momentum conservation requires that for any change in the photon's momentum, another particle/group of particles (from the mirrors, slits, etc.) must change their momentum as well. So the immediate cause of the fact that a photon is detected in one place or another is known and it consists in the photon's interaction with whatever we put in front of it. I don't think that a good understanding of DCQE (or any other quantum experiment) can be achieved without a discussion about the microscopic structure of each element that takes part in the experiment. For example it would be interesting to know how the field around one slit changes when the other is closed, or a detector is placed near it.

    I think that the requirement for reverse-causality and other such effects comes from the wrong assumption that a change in the experimental setup has no influence whatsoever on the way the photon is scattered during the experiment.
  13. Jul 19, 2008 #12
    Re: Photons Detector not yielding "which-path" info.

    But that's the whole point of designing a delayed choice quantum eraser. The choice whether to erase the which-path information or to destroy it is made after the experiment has been set up. There are no mvoing parts in the experiment. Therefore there's nothing inherent about the physical set up that causes the interference pattern or not - the choice is made after the signal photon has been detected.

    So whatever info such a microscopic analysis might yield still cannot change the fact that there is nothing physically different about the set up in the erase case vs. the non-erase case. It's entirely dependent on what the photons do.
  14. Jul 21, 2008 #13
    Re: Photons Detector not yielding "which-path" info.

    In my opinion this experiment adds nothing to the classical double-slit experiment, but only complicates things by introducing down-converters, beam-splitters and detectors. Just like in the "simple" experiment, the photons for which you have "which-path" information do not produce interference, the others do.

    I think there is an objective difference between the possible paths that a particle can take and this difference comes from the fact that any object produces an EM field. A solid object consists of charged particles and those particles generate an EM field (even if the average value for a neutral object is null). Even before living the source the particle is surrounded by these fields that could provide a sort of map of what the particle will encounter. A wall with one hole produces a different field than one with two holes. A wall with two holes and one detector produces a different field than a wall with no detector. A detector that is switched on, produces a different field than a detector that is off. A detector that can be switched on after one minute is different than a detector that is permanently off (it needs a source of energy for example) etc. My guess is that when all these factors will be taken into consideration a non-puzzling explanation will emerge. Otherwise we'll learn nothing from this or any other quantum experiment just like those trying to device ingenious mechanisms to produce energy from nothing.

    It is obvious that a particle for which the "which path" information has been (or even will be) extracted does not produce interference, just like it is obvious that energy must be conserved. What it is laking is not another, more complex experiment, but a microscopic explanation of this observed fact. For energy conservation we have such an explanation (each interaction between two particles must conserve energy). What is the equivalent of it for the "which-path knowledge" law? We don't even have a clear statement of the problem at microscopic level.
  15. Jul 21, 2008 #14
    Re: Photons Detector not yielding "which-path" info.

    We do, it's the Schrodinger Equation, and right now it's the best we've got. Unfortunately it defies all our notions of common sense and local realism, which is why all these "interpretations" pop up.
  16. Jul 21, 2008 #15
    Re: Photons Detector not yielding "which-path" info.

    Yeah, but the microscopic devices (source, detectors, beam-splitters, etc.) do not receive a full quantum description so the interactions I'm speaking about are simply ignored. A bigger problem is that the standard QM does not even allow such a treatment because of the measurement problem. So we have from the start a highly truncated, statistical description of the experiment.
  17. Jul 21, 2008 #16
    Re: Photons Detector not yielding "which-path" info.

    You're absolutely right, but my point is that the way DCQE is designed, such a treatment becomes irrelevant, because neither the items you're talking about - beam splitters, detectors, slits, etc. - nor their configuration change in the erase case vs. the non-erase case. There's no moving parts. The detectors (D0-D4) are all identical. The only difference is the odds, and so the odds are all that matter. DCQE does do something more than the classical double slit - it proves that the experimental environment is irrelevant, and that the schrodinger equation is the only thing at play. Even more importantly, the "choice" that drives everything comes after the first photon is detected. Yet there's an unmistakeable correlation between what happens at D1-D4 (later) and what happened at D0 (earlier). So even if we could scrutinize the set up microscopically, we could never account for this apparant retrocausality in any classical model.

    That highly truncated, statistical description turns out to be the only determinative factor, irrespective of space or time!

    Now, if you want to disregard the particle model, then sure, the photons behave exactly like waves and so why is this a shock? Well, unfortunately you can't escape the fact that they're particles, because SPDC (the method they use to make entangled photons) is one photon at a time.
  18. Jul 21, 2008 #17
    Re: Photons Detector not yielding "which-path" info.

    Also, ueit, that's not entirely true that the elements are never given microscopic treatment. Using QED and decoherence, you can show exactly what happens when a photon hits a beamsplitter versus a detector (what I was talking about in post #10). With QED you can show that when it hits a beamsplitter, the electrons wobble around and then return to their original state with no evidence of the interaction. With decoherence, you can show that, by contrast, the system enters a chaotic and thermodynamically irreverislbe state when the photon hits the detector and a current starts flowing toward the coincidence counter and starts affecting the transistors in the counter. All this is perfectly consistent with the "statistics-only" approach.

    They didn't perform such an analysis in DCQE for reasons I stated in post #16.
  19. Jul 22, 2008 #18
    Re: Photons Detector not yielding "which-path" info.

    Not all particles are created identical. Their momenta is slightly different. Not all of them will hit the mirrors in exactly the same place, and even if they do, the microscopic configuration of the mirror is continuously changing. So, by design, the experiment is dividing the incoming particles in two groups (which-path known and which-path unknown). In the end one can separate those groups and find out what it was expected, that the second group does show interference.

    I don't think so, see above.

    Schroedinger's equation, while correct, does not tell us if we have a case of retrocausality or not, it only gives us a statistical prediction of the experimental result. It is not proven that a local-causal interpretation conflicts with it.

    I don't think we know what "drives" what. It might be that the distribution of matter arround the source "stimulates" it to produce particles with different properties. For example, a matter distribution that corresponds to an experiment that registers which-path info will determine the source to produce particles that do not produce interference.

    I think that a microscopic treatment could show how the interactions between all charged particles taking part in the experiment might produce the effect I was speaking above.

    I disagree.
  20. Jul 22, 2008 #19
    Re: Photons Detector not yielding "which-path" info.

    You know what, you're right. It could be the moons of jupiter at work for all we know and even if we were to disprove your completely non-scientific and unsupported contention that the molecules inside the mirrors, detectors, etc. might be determining whether the photon creates the interference pattern or not, you'd undoubtedly cite something else that could be at work. I can't prove a negative. But at some point you have to pick the most manageable theory that actually produces results, and this is it for now.

    By the way, if you watch the Feynman lectures on QED, he discusses this possibility in the context of whether a photon gets reflected or refracted off the surface of water. He said they can direct the photon to individual molecules to test precisely what you're saying - and it still winds up being random. So if you don't believe me believe America's greatest physicist.
  21. Jul 22, 2008 #20


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    Re: Photons Detector not yielding "which-path" info.

    Going back to the Kim et al experiment,

    I think the key point in all this is the relative phase of pi between the two patterns corresponding to no path information. What introduces the relative phase difference? I guess this comes from the crystal creating the SPDC, right? What's the physical reason for this phase difference?
    It's so strange that QM is "saved" by this relative phase. If it was not of this phase, then QM would collapse (no pun intended) because it would be possible to chooses after the reception of the signal photons if there should be an interference pattern or not.
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