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Another entanglement question

  1. Sep 12, 2012 #1
    Say you have two cards: an ace of spade and a king of spades. You put one in your hand without looking at it, then you travel 10 km away. You look at the card, it's an ace of spades and you know instantaneously that the other card 10 km away is a king of spades. Information has just traveled at an instantaneous rate. When I've read of this phenomenon in physics texts it is presented as a paradox but I don't see why. The information first traveled slower than the speed of light when you went 10km away you just covered up the information while you traveled. I don't understand how this violates the no information can travel faster than light maxim.
     
  2. jcsd
  3. Sep 12, 2012 #2
    Because you know both the states of the cards before travelling, they aren't entangled.
     
  4. Sep 12, 2012 #3

    phinds

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    No information was transmitted, so there is no paradox. You cannot communicate ANYTHING, even a single binary bit, of actual information using this method. Actual entanglement can't transmit any information either, but it's still "spooky action at a distance" which, as StevieTNZ pointed out, your card example is not and despite your title, your question was about a situation that has nothing to do with entanglement.
     
  5. Sep 12, 2012 #4
    Perhaps we can look at the situation this way, which may help you in your original query.

    You have two cards, entangled, and described as |AceSpades>|KingSpades> + (or even -) |KingSpades>|AceSpades>. Each card is in a superposition of being Ace of Spades and King of Spades; neither card is in either of those states (unless Bohm Mechanics is correct - someone may need to clarify this for me, as even in Bohm Mechanics the card may be in no definite state as well).

    You measure the card you've taken with you some distance from the other. You find it is Ace of Spades, which implies the other card is King of Spades.
    (a) What information are you thinking is being transferred faster than light? Information between the cards?
    (b) Does Nature even need to have information transferred faster than light for predicted Quantum Mechanical entanglement results?
     
  6. Sep 12, 2012 #5
    supposedly do not have a defined value (a property), i.e. counterfactual definiteness (CFD),
    is thought that quantum states has no CFD.
     
  7. Sep 13, 2012 #6

    DrChinese

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    This is the Bertlmann's Socks analogy. It is not an accurate picture of entanglement and does not lead to the Bell contradiction.
     
  8. Sep 22, 2012 #7
    Sorry this is so long and a diversion from the original question! Ages ago I asked a question here
    https://www.physicsforums.com/showthread.php?t=307449
    and DrChinese answered very helpfully.

    I wanted to find some actual experimental data for what DrC explained was basically calibrating the apparatus. I had thought that if two entangled photons (with the same polarisation) were measured at two distant filters which were orientated the same way then the fact that the first had passed through would mean that its polarisation had been measured and the distant photon would collapse to the same orientation and hence always pass the distant filter. To my mind this would be a dramatic display of action at a distance and would obviate the statistical cunning of John Bell's suggested experiment.

    DrC said that it is taken as read that there is a 100% correspondence. Eventually, however, I did find some records of data from a similar experiment (can't remember where) and in fact there isn't 100% correspondence, it's much less. So I then thought a bit more about the experiment and resorted to a larger scale photon (to help my brain). Imagined the entangled photons as radio waves of several metres wavelength and the polarising filters are large grids of aluminium tubes. This is all stuff used in the design of aerials etc. When the electrical field is in line with the tubes then an electrical current is induced which is just enough to create a photon in the opposite direction to the original and destructively interfere with it. i.e. it's reflected (Occasionally the photon is absorbed as heat and the original photon still destructively interfered out of existence). When the electrical field is at right angles to the tubes then no current (too small a current) is produced and the photon passes through the grid unimpeded. At intermediate angles there is a probable outcome depending on Malus' law. All the photons that have passed through the grid have an electrical field orientated at right angles to the tubes even though a vanishingly small percentage (a.k.a. None) of the incident ones did.

    So, my thinking went: When the first entangled photon is 'measured' and its wave-function collapses then it must have collapsed before the filter. All we really know is that it had a probable direction of polarisation after collapse that is consistent with Malus' law (we know that a polarised photon has a 50% probability of passing a filter set at 45° etc.) And the same goes for the distant, entangled partner. I could then see that the calibration stage of this experiment was always going to give the same results at photons that just had the same polarisation rather than having quantum properties and this explained the data that I had been able to find.

    So I then wondered my way through the following stages:
    1. What if the whole Bell's experiment was done with a photon source that guaranteed polarisation of the source photons in a certain direction? Presumably the inequality wouldn't be exceeded. This is the 'null' hypothesis that the experiment sets out to disprove so must be done as a matter of course each time the experiment is performed.
    2. What if the experiment was done with a large number of photon sources each set at a different angle. If the timing of each source was know then this would be identical with question 1. So what if the timing was not known but controlled by a computer that randomly switched just one source on at a time. Would it matter how the randomness was generated, i.e. an external 'quantum' random number generator or a pseudo random computer generator? Again I can't believe that the experiment could show a positive result if there was any way of retrospectively finding when each photon source had been running.
    3. What if the experimental set-up is the same as in 2. with a large number of photon sources producing polarised photons at different angles but they are all running at the same time with the beams combined so there is no way of telling when a photon leaves each source? So when a photon is counted by either detector there is no way to tell which source it came from and what its polarisation was. But it definitely had some real angle of polarisation, we just don't know which
    4. Now we use natural light from the window as a source for our entangle photon generator. We know that the the light has been scattered by dust, clouds, neighbouring buildings etc. and each photon has been polarised in a real direction by a deterministic processes exactly the same as happens in our polarising filters. It has happened as part of a huge complex system that we cannot begin to analyse so we don't know what the direction of polarisation is for a given photon but we know that if a polarising filter is sufficient to collapse a wave-function then all the light that comes in through the window has been collapsed.

    I am quite familiar with the ideas of QM (though a unhappy about some of the 'theological, consciousness' interpretations) so I'm not really seeking a theoretical explanation of these ramblings, but is anyone familiar with actual experimental results testing these four stages?
     
  9. Sep 24, 2012 #8

    DrChinese

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    Of course the 100% is the ideal case. In practical experiments, there are a number of issues in determining the set of events which are to be considered "entangled particle pairs". Usually they simply graph the qualifying events at different angles (or alternately theta, the angle difference). Then that graph is examined to see if it is near the QM predicted shape. That supports QM.

    Usually, it can also be determined that a specific Bell Inequality is violated as well. That violation then rules out local realistic theories.
     
  10. Sep 24, 2012 #9

    DrChinese

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    1. This does not need to be performed. Such pairs are not entangled. Bell tests apply only to entangled pairs. So the object is to produce a set of candidate events showing entanglement.

    3. This has been actually performed. The result IS entanglement, as you hypothesize. If you will study what is called Type I PDC production, you will see that there are 2 crystals producing pairs of known polarization. However, by combining their output streams in a way in which the source is not knowable, even in principle, the result is an entangled stream.

    http://arxiv.org/abs/quant-ph/0205171

    See figure 2.
     
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