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

I Early Quantum Entanglement Experiments basic question?

  1. Jan 21, 2017 #1
    Reading about the early Quantum entanglement experiments by performed by Ernst Bleuler and H.L. Bradt and independently by R.C. Hanna in 1948, they basically used a pair of Geiger counters set around sodium22 and when an electron annihilation event occurred that produced a pair of photons the geiger counters would click when it detected the photons, and the two geiger counters would click at the same time when it detected a pair of so called entangled photons.
    They used Compton polarization where the photons would bounce off of a block of aluminum in random directions except if the photons were entangled they would go off in directions that would signify opposite polarization and the two geiger counters were set up at the correct angles to detect this and would click simultaneously.

    These so called simultaneous clicks were at a higher rate then one would get if no entanglement at all existed.

    My question is as follows
    1) It appears that this experiment only provides evidence for the existence of entangled photons. In an ideal world (thought experiment) one could actually isolate a single pair of photons and continually change a property on one photon and see if the other photon on the opposite side has opposite changes. and demonstrate that the probability of preprogrammed photons would be too low to be a reliable explanation of this phenomenon

    Have Quantum entanglement experiments evolved where they have gone beyond showing the likely existence of entanglement only as described in the above experiments of 1948. I realize that detection methods have improved since the days of geiger counters and particle accelerators are a more robust way of getting electron annihaltion events producing pairs of photons etc but has the basic idea changed since 1948?

    http://www.nature.com/nature/journal/v162/n4113/abs/162332a0.html
     
  2. jcsd
  3. Jan 21, 2017 #2

    zonde

    User Avatar
    Gold Member

    Of course, direct demonstration of faster than light (FTL) influence would be very convincing, but the theory does not predict that anything like this is possible. Actually direct demonstration of FTL influence would indicate the limits of validity for quantum mechanics.

    Yes, entanglement experiments have evolved. Latest loophole free Bell inequality tests conclusively rule out alternative explanations (that do not involve entanglement) for such correlations.
    https://arxiv.org/abs/1508.05949
    https://arxiv.org/abs/1511.03189
    https://arxiv.org/abs/1511.03190
     
  4. Jan 21, 2017 #3

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    Sid, your thought experiment about continually changing photon attributes represents a fundamental misunderstanding of how entanglement works. First, you can measure an entangled property of a photon but you cannot force it to take a specific value. Second, once you measure that property it is no longer entangled.
     
  5. Jan 21, 2017 #4
    Yes I understand that all of these experiments demonstrate Bell inequalities and rule out local realism. Particularly, the separation by larger distances. One idea that Bell originally thought was that the photons are somehow .preprogrammed. So are the coincidence event detectors in these modern experiments so precise that the timing rules out the idea of preprogrammed photons on a probabilistic basis? Is the basic concept the same as the earlier experiment except that the distances are greater and the detectors and generation of photons more robust. What has changed exactly since 1948 to rule out preprogramming as an explanation.
     
  6. Jan 21, 2017 #5
    I appreciate that so far everything has supported the results predicted by Copenhagen Model of Quantum Mechanics. "Thought Experiment" is a misnomer. I simply called it that fully aware that it is a physical impossibility for many reasons.
     
  7. Jan 21, 2017 #6

    Strilanc

    User Avatar
    Science Advisor

    That's the whole point of a Bell test, so yes.

    A "loophole-free" Bell test has to avoid two big historical objections:

    - The signalling objection. What if the two systems are colluding? What if they have time to communicate?
    - The detection objection. What if the chance-of-detection is related to the measurements agreeing? How do we know failures-to-correlate aren't hiding in the photons you didn't detect?

    The signalling objection is countered by synchronizing the measurement events precisely enough that light doesn't have time to travel from one to the other. You get the two measurement sites as far apart as you can, and use fast electronics to get the measurements done quickly.

    The detection objection is countered by getting the detection rate really high, or by using some third system to herald which detections you will use

    Historically it was difficult to address both of the objections in the same experiment. But recently we have done experiments that addressed both.
     
  8. Jan 21, 2017 #7
    So in the example of the original experiment I reference the coincidence of two geiger counters clicking at the same time , the definition of simultaneous detection was somewhat loose, i.e. did the two Geiger counters click within a 1/10 th of a second of each other or 1/1,000,000 th of a second of each other? With the modern equipment do the detectors involve atomic clocks to define simultaneous detection? If yes how exact is the simultaneous detection?
     
  9. Jan 22, 2017 #8
    Let me ask the question about a "loophole free" Bell Test in a different way. How does overcoming the signaling detection and detection objection as described eliminate the possibility that somehow the entangled photons are preprogrammed depending on how the electron annihilation event itself takes place? An analogy could be that most twin birthing events result in a fraternal twins but once in a while we get identical twins. Unlike the photons the identical twins develop identically instead of opposite but still identical twins are a function of prenatal development...could the photons preprogrammed characteristics not be a function of the electron annihilation event itself? So far it appears the one thing we have evidence of is that local variables can not be the explanation as this involves actions on the photons that go against special relativity.
     
  10. Jan 22, 2017 #9

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    The first issue you need to look at is your understanding of Bell's Theorem. It says:

    No physical theory of local Hidden Variables can ever reproduce all of the predictions of Quantum Mechanics.

    There are no pre-programmed value sets that work as you describe except in a few very special measurement settings. It just happens that one of those special settings is when the same measurement is performed on both particles. But for most settings, that is not the case. So generally the pre-programmed idea fails, even though it works for your analogy. It's like saying that all people in the world are British when you yourself are in Britain. You need to look a bit further, as Bell did in 1964.

    The easiest way to see this is with PDC Type I entangled photon pairs measured at various combinations of 0, 120 and 240 degrees. If you attempt to hand pick a data set of values for those, you will quickly learn that your hand picked values will NOT match the predictions of Quantum Mechanics. The QM match rate for when the angles are the same is 100%, and when they are different is 25% (for the angles I gave you). Good luck on making the pre-programmed concept work, you can't.

    Now the norm is for the person who is not familiar with Bell to ignore everything I am saying because it is too much trouble to read and understand. So to help, I have provided a page that explains this that I think you might benefit from:
    Bell's Theorem with Easy Math
     
  11. Jan 23, 2017 #10
    Dr Chinese, first of all, Kudos to you, for your painstaking effort to present a very lucid presentation of the EPR and Bells Theorem. My question is in effect why not a hidden variable theory that would explain some of the predictions of Quantum Mechanics , not All of them. One of the predictions of QM is how Einstein in his 1935 paper (EPR) facetiously put it "spooky action at a distance". Possibly, this is the prediction that irked him the most as this is what would appear to violate special relativity if local realism is to be believed. . The very exception you refer to is when the same measurement is performed on both particles. As I understand it the reason the experiment I referred to above deliberately uses the same measurement on both particles was to isolate entangled pairs. Later evolution of these experiments removed some of the loopholes such as distance that would allow the detectors themselves to somehow influence each other.
    I appreciate that if everything was preprogrammed to behave in the way we would expect in the non QM world that the experimental results from Bell tests would have pleased him. Instead, we are left with experimental results that support the notion that as Einstein implied if the moon is not being observed it does not exist. My question is focused on the case of entanglement that appears to violate special relativity. Would it not be less far fetched that we get rare cases of preprogrammed photons arising from perhaps how the electron annihilation event occurs rather then theories that try to circumvent general relativity i.e. warped space etc. ?
     
  12. Jan 23, 2017 #11

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    Just to be clear, and make sure we are on the same page:

    - The EPR paper used the idea of spatially separated particles measured in the same way. QM correctly predicts this, as do some local realistic theories (but not all). It does not isolate the entangled pairs though. Other things accomplish that.

    - The Bell paper used the idea of spatially separated particles measured in (almost any) different ways. QM correctly predicts this, but NO local realistic theories do.

    It is true that the spatial separation was ultimately increased (and timing shortened) to exceed light speed, which makes no difference for the QM side.
     
  13. Jan 23, 2017 #12
    Yes we are on the same page.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted