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Double slit experiment with detectors not recording

  1. Jul 7, 2010 #1
    In the double slit experiment, a beam of electrons forms an interference pattern on a screen after passing through two slits. If you place a detector on one or both of the slits, the electrons do not form an interference pattern.

    In this video:

    Physicist Thomas Campbell makes the following claim (at 2:45) if you leave the detectors turned on, but you throw away the data from the detectors without looking at it, you get a wave interference pattern on the screen behind the slits.

    Is this true?
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Jul 7, 2010 #2
    Here is a link to an experiment that was done regarding a "Delayed Choice Quantum Erasure"

    Last edited by a moderator: Sep 25, 2014
  4. Jul 7, 2010 #3
    How is that even possible? I though that the fundamental idea of quantum mechanics is that the act of measurement collapses the wavefunction of the electron and thus makes it behave as a particle, irrespective of whether we acknowledge the presence of the detector being there or not.
    The detector here is essentially measuring the position observable X, whose only eigenfunctions are Dirac Delta functions, which gives the wavefunction a highly localised behavior and thus make it exhibit particle nature. How does it matter if we are not looking at the data of the detector? If the detector is there, then the electron should behave as a particle.
  5. Jul 7, 2010 #4


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    No. Once the data is detected by an irreversible process, deleting the data does not induce any changes. It does not matter whether you throw away the data or look at it.

    The only thing you can erase are reversible markers. For example you can make the paths in the double slit experiment distinguishable by using polarizers at each slit. Afterwards it is possible to change this polarization without destroying the photon, so you can shift the polarization of the beams originating from both slits such, that they are the same again. As no irreversible process happened, the interference pattern will reappear. In this case you could get which-way information and destroy the interference pattern if you measured the photon at the right position and time, but as you never measure, it persists.

    This is very different from actually measuring and throwing away the data, which will never give a persisting interference pattern.
  6. Jul 8, 2010 #5
    So basically this guy, who is calling himself a physicist, is just a crackpot and doesn't know what he's talking about!
  7. Jul 8, 2010 #6


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    Well, it is very hard to find information about that guy. The only info I found is as follows:

    "Tom holds a Bachelor of Science in Physics and Math from Bethany College and a Master of Science in physics from Purdue University, as well as having done doctoral-level work at the University of Virginia. He is the physicist described as “TC” in Bob Monroe’s Far Journeys. Tom began researching altered states of consciousness with Bob in the early 1970s. He and Dennis Mennerich helped to design experiments and develop the technology for creating specific altered states. They were also the main subjects of Bob’s investigations at that time. For the past thirty years, Campbell has been focused on scientifically exploring the properties, boundaries, and abilities of consciousness. During that same time period, he excelled as a working scientist—a professional physicist dedicated to pushing back the frontiers of cutting-edge technology.

    Using his mastery of the out-of-body experience as a springboard, he dedicated his research to discovering the outer boundaries, inner workings, and causal dynamics of the larger reality system. In February of 2003, Tom published the My Big TOE trilogy. The acronym “TOE” is a standard term in the physics community that stands for “Theory Of Everything” and has been the Holy Grail of that community for fifty years. My Big TOE represents the results and conclusions of Tom’s personal and scientific exploration of the nature of existence. This overarching model of reality, mind, and consciousness merges physics with metaphysics, explains the paranormal as well as the normal, places spirituality within a scientific context, and provides direction for those wishing to personally experience an expanded awareness of All That Is."

    This sounds very much like crackpottery. But besides: if he had evidence that throwing away data alters the interference pattern, why wouldn't he publish it in a scientific journal?
  8. Jul 8, 2010 #7
    There is another crackpot?:

    Professor David P. Jackson
    Dickinson College
    Dept. of Physics and Astronomy
    Carlisle, PA 17013
    (717) 245-1073

    This project is funded in part by the National Science Foundation (CCLI grant DUE-0737230) and Dickinson College.

    Here is the experimental setup and their conclusions:

    "The mystery of the Quantum Eraser is that when we tag the photon so that we can determine which path the photon travels it seems to know that we are watching and only travels one way. However when we erase path information and we can no longer determine which path the photon travels the interference returns!

    To say it simply: The photon seems to know when we are watching and behaves differently when we can say where it has traveled."


    But if this were true, it would be possible to send signals ftl. Makes you wonder how fairly those grants are distributed.
    Last edited: Jul 8, 2010
  9. Jul 8, 2010 #8
    Yes, this guy is trying to support his theory of quantum universal consciousness theory of everything. He has to embellish on the meaning in order to claim physical consistency such a crackpot theory.

    The problem with the proliferation of such crackpots is that at the edge of theoretical physics it's hard to explain to many people what is wrong with it, and even involves some things none of us yet understand. It too often allows crackpots to have their cake and eat it to.
  10. Jul 8, 2010 #9
    I don't see anthing wrong with Jackson's experiment above, even though the phrasing is too cute, but the other youtube one is very confused and without specific experimental details you can't understand what he's talking about regarding the detectors.

    However, he's almost certainly wrong if he's claiming conscious observation of the data caused the interference to disappear.

    For a proper peer-reviewed double-slit with erasure experiment see http://grad.physics.sunysb.edu/~amarch/ [Broken]

    This has interesting details, such as the fact that the which-way information can seemingly be obtained after the photon is detected and still destroy the interference! (Though, if you're happy with a non-local interpretation of quantum mechanics that's not too surprising, as a FTL mechanism can explain how the photon still "knows" about the which-way marker)
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  11. Jul 8, 2010 #10


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    This is not really crackpottery, but - well - very simplifying language. He uses a very specialized meaning of tagging and watching which can lead to very misleading interpretations, but he gets the experimental results straight.

    This is not really true. The information is not really obtained after detection of the first photon because all information is available only in coincidence counting, when both photons are already detected. What they do is more like a clever kind of filtering process than changing the past.
    I gave a (atmittedly very simplifying) rough account of the basic physics behind DCQE (however, based on Kim's paper) in a different thread a rather long while ago, in case you are interested:

    The Walborn paper just introduces a different mechanism of introducing, removing and recovering indistinguishability of both possible photon paths.
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  12. Jul 8, 2010 #11
    Thanks for the clarification, I assumed a ftl mechanism was required, but using this coincidence counting method does seem to complicate the analysis.
    Last edited by a moderator: Apr 25, 2017
  13. Jul 9, 2010 #12


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    Well, if you go into details, you will indeed find that one must assume some nonlocal model to explain some of the experiments performed with entangled particles (depending on your preferred interpretation this can be immediate collapse of the wave function or several different proposed mechanisms - there are enough threads in these forums discussing the different interpretations). You just do not get a possibility for ftl information transfer.
  14. Jul 9, 2010 #13
    But I don't understand that why does he say that "when we leave the detector on, but don't make any measurements, that is, the magnetic head is moving, but there is no magnetic tape to record the readings of the moving head, then we see an interference/diffraction patter". Has this been proved in any experiment? The part where measurement affects the state of the system is understood, but why does he narrows down the measurement process only to consciousness, while the same effects can also be produced whenever electromagnetic waves interact with matter of any form, whether living or not.
  15. Jul 9, 2010 #14


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    No, it has not been proved in any experiment and it is just a plain wrong claim. Whether there is an consious observer at the end of the detection chain or not does not make any difference.
  16. Jul 9, 2010 #15
    Yes, at least no classical information transfer, due to the non-deterministic selection of the quantum state.

    I do wonder if a more ideal experiment with single (pairs) of photons (or other particles) could be realised, perhaps with the "which-way" particle detected over a much larger distance (so we're talking about several seconds or even minutes) than the "interference" particle so that coincidence counting wouldn't be required and perhaps a full detection screen for the interference pattern could be employed.

    ie make the ftl mechanism explicit, since the "which-way" particle won't even be detected for several seconds/minutes after the "interference" particle hits the detection screen. That way you would be sure that some ftl "wave-function" mechanism was operating.
  17. Jul 9, 2010 #16


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    The interference seen is a genuine two-photon interference effect comparable to Hong-Ou-Mandel interference. Therefore coincidence counting is always required to see the interference pattern even if the longer distance taken by the which-way particle corresponds to minutes.
    It is only the two-photon state which is coherent. The which-way arm signal is as incoherent as it gets and will therefore on its own never show any meaningful interference pattern (unless you break entanglement).
  18. Jul 9, 2010 #17
    I don't understand, are you referring to the double-slit experiment I posted? The interference is created by single photons going through a double slit, the entangled partners are used to obtain which-way information only.
  19. Jul 9, 2010 #18


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    This is the most common misconception about DCQE experiments. It is not true that the interference pattern in DCQE experiments is caused by single photon interference and the other photon is just used for which-way information. If that was true, you would never need coincidence counting.

    In fact, the light reaching the double slit is spatially incoherent. This means that its wavevector is rather undefined and by just looking at this arm you NEVER see any interference. Low coherence corresponds to a large spread in wavevectors. If you just shone light with well defined wavevector at the double slit, you would see a clear interference pattern. If you just shone light with a different well defined wavevector at the double slit, you would see a different clear interference pattern. However, as the spread in wavevectors is large, you will see a superposition of all possible interference patterns, which corresponds to no interference pattern at all. By using a small detector at one end and a large detector at the other end, you can choose some well defined wavevector at the small detector side. As you have entangled photons, the coincidence counting now acts like a filter. The entangled counterparts detected at the other side which are correlated to the ones detected at the small detector will now also be a subset with a well defined wavevector and therefore this small subset will have a well defined interference pattern. However, it is by no means possible to choose such a subset without doing coincidence counting.
  20. Jul 9, 2010 #19
    In the experiment I'm referring to the coincidence counts are used just to ensure both entangled photons have passed through the apparatus.

    http://grad.physics.sunysb.edu/~amarch/ [Broken]

    This experiment uses single photon pairs via SPDC.

    I think you're thinking of another experiment.

    But correct me if I'm mistaken.
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  21. Jul 9, 2010 #20


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    Sorry, but: no, they are not. As that page says: "It is not accurate to consider these photons as separate entities, but rather as one. They can travel very far away from each other, but they will not loose their correlation." Even if you completely removed all stray light and had no other background photon detections, you would not see an interference pattern without doing coincidence counting.

    This is right, but i would change the emphasis to:
    This experiment uses single photon pairs via SPDC.

    There are indistinguishable two-photon probability amplitudes leading to the same results. It is not necessary that these two photons ever meet for two-photon interference to occur.

    No, I am considering the same experiment. Doing coincidence counting in such experiments is NEVER a means of just reducing the background noise due to stray light and other photons which are not part of an entangled pair.

    In fact, single and two-photon interference are even complementary as has been shown in "Demonstration of the complementarity of one- and two-photon interference" by Abouraddi et al. (Phys. Rev. A 63, 063803 (2001)), also available on Arxiv.
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