Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mandel & Faster than Light Communication ?

  1. May 23, 2005 #1
    In Mandel et Al's most famous experiment (Fig 6 at ‎[URL]http://student.science.nus.edu... one-photon and two-‎photon interference.pdf) the signal beams from two coherent downconverters are ‎observed to interfere only if the two corresponding idler beams are allowed to ‎interfere.‎

    Let's say that the a blocking object is inserted or removed in front of idler beam 1 at ‎point A. Let us also say that the signal beams are detected at point B.‎
    The direct distance between A and B is x light seconds, and the distance travelled by ‎the idler 1 light beam from the 1 downconverter plus the distance travelled by the ‎signal 1 beam from the 1 downconverter to B is y light seconds.‎

    When the object is inserted/removed at A, how long does it take for the interference ‎pattern at B to disappear/appear ?‎

    Is it
    ‎1) Instantaneously
    ‎2) x seconds later, or‎
    ‎3) y seconds later ?‎

    ‎(I know that the detector needs to move back and forth to see interference patterns, ‎but assume it can move back and forth and record very quickly).‎

    The above is question 1.‎

    Question 2:‎
    Since light travels at the speed of light, and from Relativity, something travelling at ‎the speed of light experiences simultaneously different times (unlike all less-than-‎light-speed particles/waves), does it make any sense to say that light communicates ‎and interferes with itself at what an outside inertial observer would call different ‎times, due to those different times being experienced by the light beam ‎simultaneously (ie can General Relativity's understanding of the light beams ‎experience of simultaneous time explain some of the Quantum results seen in ‎Question 1) ?‎
    ‎(and is there a good [very] basic paper which explains the link ?)‎
    Last edited by a moderator: Apr 21, 2017
  2. jcsd
  3. May 25, 2005 #2
    The link didnt seem to work for me, but its sound like it might be the usual delayed choice experiment, which cant be used for faster then light communication.

    Furhermore I was under the impression that things moving with the speed of light didnt experience time at all, since they follow delta tau = 0 lines, eg their clocks dont ever change, time is standin still for these things.
  4. May 25, 2005 #3
    That's not quite right.

    Bell showed that communication does travel faster than light, but we just can't use that random-to-random communication to piggyback any information on.

    Here's another URL to the same paper - See Fig 6:
    http://www.google.com/url?sa=U&star...on%20interference.pdf&e=9707[/PLAIN] [Broken]
    IFABOVEDOESN'TWORKTHENUSEwww.google.com/url?sa=U&start=1&q=http://student.science.nus.edu.sg/~g0203645/Atomic%2520Molecular%2520and%2520Optical%2520Physics/Quantum%2520effects%2520in%2520one-photon%2520and%2520two-photon%2520interference.pdf&e=9707 [Broken]

    But in Mandel's now classically quoted experiment, blocking Idler beam 1 does cause a noticeable change elsewhere at the Signal detector where the interference pattern disappears.

    What is the time gap between blocking the idler path, and noticing the signal interference pattern disappearing.

    Is it

    1] Instantaneous (like Bohm's pilot wave) - but this has been proven impossible by Eberhard in similar but not identical cases

    2] The direct straight line distance between the cause and effect. Cause is the place where the idler beam is blocked, and effect is the signal beam detector (A & B in my previous post)

    3] The actual distance travelled by the light beams between the cause and effect (which is longer than 2] as the light beams take a tortuous route in the experiment - this is the sum of two paths, the idler beam from downconverter to block, and the signal beam from downconverter to detector).

    or 4] The distance between where the two idler beams would have first met each other on the one hand, and the effect on the other. This is similar to 3], but with an addition - ie this is the sum of two paths, the idler beam from downconverter to the point it would have first met the other idler beam, and the signal beam from downconverter to detector. After all, according to Mandel, it is the idler beams meeting each other and destroying latent information which allows the interference pattern to occur at the signal detector. (missed this one out the first time).

    We can gain (can we not) significant understanding of the 'spooky' quantum effect by determining if the answer is 2), 3) or 4).

    Can anyone help me understand which it is.

    Last edited by a moderator: May 2, 2017
  5. May 25, 2005 #4


    User Avatar
    Science Advisor
    Gold Member

    The effect you describe travels no faster than c. So case 2 is the correct answer.

    It is easy to be confused by the description of the setup. Because it is fundamental that the signal and idler paths are co-mingled. You must also consider that the idler detector plays a role.

    In all of the cases Mandel describes in which it appears something happens FTL, you will find that coincidences are an issue - so it is not FTL communication. In all of the cases in which the behavior does not require comparing results from signal and idler, the communication is at c or less.
  6. May 25, 2005 #5

    But NO on two counts.

    1) Mandel has shown that the Idler detector plays no part in what occurs. It is only the theoretical threat of detection, irrespective of whether the detector is there or not, that makes the Signal interference pattern appear or disappear.

    2) Bell showed that we live in a non-local universe. Somehow, entanglement is causing FTL communication, it is just a communication that we can never use to send information.

    Was your answer 2 based on dogma, or do you have a model to explain why the answer should be 2.

    I have a feeling the answer is one of the options other than 2 (2 makes least sense if you think about it).

    Looking forward to your reply

  7. May 25, 2005 #6


    User Avatar
    Science Advisor
    Gold Member

    Well, look... This is a system. It is a bit difficult to artifically dissect it as you are attempting. Your "choices" are confusing, so I tried to pick one that came as close as possible. The blocking must occur prior to the detection for the interference pattern to be eliminated. Turning on and off the blocking does not allow you to change the pattern faster than c. So if you are trying to assert something specific, please just go ahead and say it.

    The fact is that there is no known or demonstrable FTL signalling occurring in any of the experiments Mandel describes in this paper. It MIGHT be acceptable to say there is an FTL effect, although that is certainly debatable too. The standard dogma, of course, is that there is instantaneous collapse of the wave function relating the two photons. These experiments do not describe any new effects over and above what you could have predicted years ago. They do repackage it nicely so that variations on quantum behavior can be studied in more detail using the entangled photons.
  8. May 25, 2005 #7
    I would say that the correct answer is (3). Although the system may be quantum in nature, the interference or lack of it can be explained in terms of wave optics.
  9. May 25, 2005 #8
    Who says ? If you are you relying on Eberhard's proof, then is it valid to this case ? Aspect's experiment seems to indicate that your understanding doesn't match out what quantum reality has to offer.
    You have put your finger on the problem. Instantaneous for the two photons, which are at two non-identical positions (ie space divided), means FTL !
    That was exactly Bell's 1964 revolution.
    Sorry. No way. The world we live in is much stranger than that. Look at Mandel's experiments and results. Changing something at A (blocking one beam) changes another beam at B which has absolutely no connection by wave optics at all to A. This bears no relation to wave optics whatsoever. I wish the world we lived in was that simple.

    So, does anyone have an answer from 1, 2, 3 or 4 ? Thanks if you do

  10. May 25, 2005 #9
    Bell and Aspects exeperiments show (if you trust em, which I do) that "local reality" is nonexistent, they dont show that any signals travel faster then light, eg they dont show that there is a non-local reality just that there arent a combined "local reality", there might be a local nonreality :). Furthermore since these exeperiments (Mandell etc) all follow the laws of QM they must all obey the reduced density matrix idea, that is all that one can locally know about a system is encoded in the reduced density matrix, which in turn is independent of anything you do to the other parts of the system, since that part has been traced out. So if there is indeed FTL between the particles, which hasnt been shown to take place, it can never be used to send any information faster than light. Atleast according to QM, and relativity ofcourse. Since the relativity principle nowadays says that you cant send information faster then light. Signals can go FTL, but you cant use them to anything "usefull" =). Also its interessting to note that since we cant send information faster than light we can never observe or in otherways assertain that particles communicate FTL, we cant catch em in the act so to speak. For if we could, we could use it to send information FTL.
    Last edited: May 25, 2005
  11. May 25, 2005 #10


    User Avatar
    Science Advisor
    Gold Member

    Your Mandel question is completely equivalent to asking how fast an interference pattern disappears when one of the two slits is blocked in a traditional double-slit setup. There is absolutely no difference in how the interference occurs - it is a superposition of 2 possible paths to the signal detector.

    Eberhard's proof has nothing to do with the matter that I can see. The question you are asking can be experimentally tested.

    I do not follow the reference to Aspect. Aspect showed that Bell's Inequality is violated. What am I missing?
  12. May 25, 2005 #11


    User Avatar
    Science Advisor
    Gold Member

    Reading the answers again, yeah maybe 3 is better. Anyway, as you say, this is essentially classical at this point and there is nothing weird going on between the blocking and the detection.
  13. May 25, 2005 #12
    Assuming the reduced density matrix is a full and correct description of quantum mechanics - which it might not be.

    Either way, which is the answer ? 1, 2, 3 or 4 ?


    p.s.: And I wish I could change the title of this thread. FTL communication was meant to imply, as in Bell's Proof, that the measurement of the wave-function at a point A simultaneously changes the wave-function of an entangled photon at some other space-separated point B, causing observable results. I know we can't send information on this change, but the change happens.
  14. May 25, 2005 #13


    User Avatar
    Science Advisor
    Gold Member

    This is standard CI, so who is questioning this? And what does this have to do with the experiment you are referencing? Shouldn't we be talking about an Aspect-like experiment?
  15. May 25, 2005 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Can you point out exactly which set up you are talking about ? Usually, in order to see "signal beam interference", you need to subselect a signal beam image by an idler detection in one way or another.

  16. May 26, 2005 #15
    Interesting analogy. But when blocking one of two slits, we understand that light already through the blocked slit continues to propagate to the detector, and interference-effects only disappear once the time taken for those waves to arrive have been reached. We have a well defined and well understood model for the disappearance of the interference pattern.
    Classical !? Whilst we know the blocking of the idler beam propagates to the Signal detector, since we see the interference pattern disappear, we have no model to discover along which route or how soon the cause propagates to the effect ! And you are just guessing by your answers first of 2), and now of 3).

    Can you come up with a model justifying answer 1, 2, 3 or 4 ?

  17. May 26, 2005 #16
    Vanesch, the experiment is pretty fully detailed at:

    www.google.com/url?sa=U&start=1&q=[PLAIN]http://student.science.nus.edu.sg/~g0203645/Atomic%2520Molecular%2520and%2520Optical%2520Physi [Broken] cs/Quantum%2520effects%2520in%2520one-photon%2520and%2520two-photon%2520interference.pdf&e=9707[/URL]
    IFABOVEDOESN'TWORKTHENUSEwww.google.com/url?sa=U&start=1&q=http://student.science.nus.edu.sg/~g0203645/Atomic%2520Molecular%2520and%2520Optical%2520Physi [Broken] cs/Quantum%2520effects%2520in%2520one-photon%2520and%2520two-photon%2520interference.pdf&e=9707

    See Fig 6 and the text there.

    Last edited by a moderator: May 2, 2017
  18. May 26, 2005 #17
    I don't know what the time gap is, maybe we can say that detection of the path of the idler photon causes immediate collapse and so destroys interference, however I don't like to do that since I feel that it implies that the particles are little balls with FTL intercom. I don't know what they are and I don't think anyone do. We only know how they behave, what will happen in experiments (roughly). There is also the problem of determining when interference have disappeared, you cant do it with only one detection, can you do it with two !? I think that any answer to how long it takes for the interference to disappear has to do with ones idea of what is going on, so it hard to use these answers to figure out what is going on :uhh: Eg. if you use somekind of wave model than the interference occurs only at the detector and thus the interference is destroyed when one of the waves fails to show up. If you use the particle model, than the interference can be said to be destroyed when the path of the idler photon is detected, collapse is immediate. :smile:
  19. May 26, 2005 #18
    One can also say that interference is destroyed at the moment you decide what experiment to do. This as DrChinese said where similar to the standard two slit experiment. People have tried every imaginable thing to try to fool the photon into giving up what path they took and still have interference. It seems the photon are smarter than us.
  20. May 26, 2005 #19


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Ah, I didn't know that one. I have to say that I'm surprised about the result, and I have some difficulties with the explanation given in the paper, but you are right, this is one of the few quantum erasure experiments where there is NO subset selection.

    I think that this is a very interesting experiment, and I don't understand it completely. However, I disagree with the explanation given in the paper as such; indeed, according to the explanation, it wouldn't make any difference if i1 and i2 were combined with i1 going AROUND NL2 or going through NL2. And if that were true, we could obtain paradoxial situations: we could in principle send i1 and i2 on long, long long optical fibers (say, a light minute long), register the interference pattern or not at Ds during a minute, and only decide then whether or not we mix i1 and i2. And according to whether we decide to mix them or not, we would see our results of the past minute CHANGE !
    We could even make a paradoxial machine in the following way: if the computer controlling the experiment has statistical evidence for interference during the minute of Ds measuring time, he decides to BLOCK the output of the fiber of i1, in which case we shouldn't see an interference pattern (because we can find out which photons came from NL2, knowing the delay of 1 minute and what comes out of fiber 2). But we SAW an interference pattern. On the other hand, if the computer has statistical evidence for no interference, he can decide to mix the two fiber outputs in a beam splitter, and according to the paper, we now have erasure of the information which is at the origin of the interference pattern, so we should observe interference, but we didn't !

    I will try to write down of what I can make up of the setup:

    The fact that an interference pattern arises can be understood in a certain way because when a single photon of the argon laser is split at BS(pump), you have that the state of this pump photon is 1/sqrt(2)(|pump1> + |pump2>), and hence the two photon pairs from the NL1 and NL2 are entangled:

    1/sqrt(2) Integral ( |pair1> + e^(i theta) |pair2> ) rho(theta) dtheta

    Here, theta is the difference in optical path length between the point in NL1 where the conversion |pump1> -> |pair1> took place, and the point in NL2 where the conversion |pump2> -> |pair2> took place. But of course we have to (Feynman path integral) integrate over ALL possible thetas because that conversion point is "floating", within the xtal, and it is this smearing out over different positions which makes that there is no fixed phase relationship between pair1 and pair2 (and hence between a single component of each).

    It is somehow my feeling that the fact that the idler photon of pair 1 goes through NL2 changes rho(theta) (gives preferred positions for conversion of |pump2> -> |pair2>. In a non-linear xtal, this is not unexpected. From the moment that rho(theta) is somehow peaked, there is a phase relationship between pair1 and pair2 and you can expect interference.
    But I'd be highly surprised that this same experiment works if the combination of i1 and i2 is done without i1 going through NL2 ! A way to find out would also be to change the path length of i1 before it gets to NL2. I'm pretty sure that this SHIFTS the interference pattern at Ds, which means that i1 DID SOMETHING to NL2.

    Do you know of more experiments of this kind ? It looks interesting in any case.

    Last edited: May 26, 2005
  21. May 26, 2005 #20
    No you cant do this because you have already made it possible to in principle decide what path the photon took. This is no different from throwing meassurering results in the trash, its not a quantum erasure.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook