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How do you entangle particles?

  1. May 7, 2006 #1
    Okay, so whatever happens to one "entangled" quantum particle happens to its twin, simultaneously.
    What I don't get is, how do you entangle them in the first place?
  2. jcsd
  3. May 7, 2006 #2


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    that is one of those "over reaching" statements that tend to lead to false conclusions
    Let's see if a simple example will do the trick. Say you have a molecule of hydrogen gas, it consists of two hydrogen atoms with one electron each. For reasons we won't go into here, these electrons have a property called "spin" which can be characterized as either "up" or "down". In such a molecule, if one of the electrons is up, the other has to be down. Now, in the molecule state, the spin of any one of the electrons has not yet been determined. (IOW, it hasn't really "decided" which spin it has.)

    These two electrons are entangled, since whatever spin one has, the other has to have the other.(even though neither has actually settled on a state)

    Now let's pull these atoms apart and separate them by some distance. The electrons remained entangled (they maintain opposite spins), but the the spin of either have not yet been determined.

    If use a method to measure the spin of one of the electrons, we force it to settle on one state or the other. The instant we do this, the other electron is forced to take on the opposite state. The action we take on one of the electrons (measuring its spin) is reflected in the other electron( it takes on the opposite state).
  4. May 7, 2006 #3


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    The most common experimental method is called spontaneous parametric down conversion (PDC). You can google that to learn more about it. Basicly, you start with a pair of newly created particles in which there are conserved quantities in total (such as spin, energy, etc.), but you don't know which particles have which specific values. They will have a combined wave function, and that is the entanglement.
  5. May 7, 2006 #4
    Talking about "over reaching" statements, sometimes I read everything is entangled with everything else and the like. But am I rigth to assume that to get an entangled state you have to make very careful designed experiments, i.e. entangled states do not occur often in nature?
  6. May 8, 2006 #5

    But if they don't yet have defined spins, how could their molecules have been able to join up in the first place? Don't they need to be defined as opposites first?
  7. May 8, 2006 #6
    Your not forcing things to be entangled you must find testable entangements. In the same manner you don’t need to force match spin H atoms to make an H molecule. The do that work by themselves. You just want to take advantage of the molecule so that when you take them apart the original “spin” must be conserved.

    An analogy can help BUT remember this is an analogy, NOT A DESCRIPTION of how an election “spins” in an atom.

    View the two H atom electrons like gears that turn - clockwise or counter-clockwise. By picking some reference side and view you can call that up or down. When they come together they need no help in becoming aligned it is there combining that aligns them such that they fit together on their own what ever it takes.
    Now that they are together what is the total spin?
    Based on how gears fit together one turning clockwise means the other is turning the other way i.e. the net spin is Zero!
    Now although we might not now the details of how they came together we do know where we carefully take them apart if one is up the other will have to be down to add up to the original zero we started with.
    NOW COMES the problem of detailing when and where you can “define” what the spin is for each of the two electrons as they depart – that is when you have something testable you can build data on to attempt to understand (or deny as some still try) entanglement.

    Working with particles it can be very hard to visualize the spin measurements.
    Most find looking DrC’s example of taking one photon though a crystal easier to follow than particle spins. The PDC crystal reduces each photon frequency in half, since energy etc. must be conserved the only way a crystal can do that is to spit out two photons for every one that comes in. Here the “spin” entanglement shows up in the alignment of polarization.
  8. May 18, 2006 #7
    Last edited by a moderator: Apr 22, 2017
  9. May 22, 2006 #8
    Hartle and Hawking's famous wave-function of the universe has universal entanglement and all the particles within it have different levels of entanglement.
  10. May 25, 2006 #9
    Hi, this is my first post in these forums. :)

    Entanglement, as I understand it, arises due to lack of information about a system. Somebody mentioned SPDC (Spontaneous parametric down-conversion), which is a good example. What happens here is, a photon of higher energy (say blue light) after interaction ith a non-linear medium (non-linear as in the susceptibility has higher order terms) gets converted into two photons of lower energy (infrared light, say).

    Now, these two photons are "entangled in frequency), because energy is always conserved. So if you measure a freq of f1 for IR photon1, then you automatically know that the freq of photon2 is f-f1, where f is the freq of the input Blue photon.

    Entanglement can be of several types and basically arises due to heisenberg's uncertainity. In an interference experiment for example, if the "which path" information is hidden, you can see photon entanglement (very loosely speaking).

    In slow-light experiments, atoms are entangled with each other (mediated by photons) and you have an entity called a polariton, which is both light and matter.

    Entaglement is a fascinating topic!
  11. May 26, 2006 #10
    But how 'common' is entanglement? Do entangle states occur all the time in nature? Are Super Nade, Caribou and I entangled? (Sort of?)
    Last edited: May 26, 2006
  12. May 26, 2006 #11
    The only macroscopic entanglement I could wish for is with the girl next door :p

    Entangled states are not very common. Macroscopic entanglement is very very rare if not impossible to find in nature, simply because of decoherence. The only large-scale macroscopic entanglement we see is in a BEC (Bose Einstein Condensate). Heck, creating anything beyond a 5-photon state itself is so hard, that people don't even try to evaluate it theoretically. Also, the probabilities of obtaining a perfect n photon state are miniscule, given that one proceeds via Franson-type interference.

    One more thing, it has become very fashionable for quacks, charlatans and other idiots to misuse the concept of entanglement to sell their books. These guys should be in jail for ripping off people.
  13. May 26, 2006 #12
    Thanks, Super Nade.

    And a very warm welcome to Physics Forums! You look quite knowledgeable. Hope you will stick around for a long while at this nice site and contribute with more informative posts.
  14. May 26, 2006 #13
    Thanks for the welcome mate :)

    I'll do my best to sound coherent.

    Speaking of coherent, reminds me of the "coherent state". So here is a short quiz:

    If anybody can give me a proper physical picture of a coherent state, they win a cookie!
  15. May 26, 2006 #14
    That is not entanglement; this is easily understood classically with f2 = f-f1 being set locally. Entanglement is used to explain collections of observations with weird action at a distance requiring Non-Local explanations like QM-HUP etc.

    Trying to lose weight so I don’t need a cookie, but I’ll listen in....
    Last edited: May 27, 2006
  16. May 26, 2006 #15
    It is entanglement, as the frequencies of both photons are not determined prior to making a measurement!
  17. May 27, 2006 #16
    Not true; if you look at the details of a real PDC experiment the frequency of f1 or f2 will be known in advance by where the observation will be taken. With one being “red” and the other “blue” when compared to each other they come out of the PDC in different size rings of light due to the different energy. Only in a sloppy experiment where the detector overlaps both rings of light would you not know in advance what the frequency would always be. That’s classical and local, unlike the polarization issues in a correctly preformed entanglement experiment.
  18. May 27, 2006 #17
    PDC can be of many types, degenerate, non-degenerate Type I or Type II. In type I SPDC, the photons are spatially seperated (I suppose this is what you are referring to as "rings"). However, this has no bearing on their individual frequencies, which have an intrinsic uncertainity associated with them. There is no a-priori way of determining the frequencies of the photons without making a measurement. This is commonly called the SPDC bandwidth. SPDC is a wide-bandwith process. Only recently, have techniques been implimented which produce narrow-band two photon states (by placing the SPDC source in a cavity). Search for mode-locked two photon states and you will find many papers which describe it.

    Edit* You might want to look up frequency entangled states as well (for example):-
  19. May 27, 2006 #18
    I have no clue what your saying here:
    Are you claiming a major revision QM theory and entanglement?
    Since I only need one and only one photon to determination it's frequency and (therefore the other) I no longer need the statistical results of many observations to see entanglement!
    Therefore HUP no longer applies!
    Has this eliminated the collapsing wave function then and you have a specific description of how these entanglement worked and is new proposed theory to detail it’s mechanism that is not QM-HUP or Classical?

    Does it have some new proof that shows how the classical cannot explain this individual measurement. That is, if I assume the f1 reading had been f1 and only f1 since leaving the PDC there is something that shows this to be impossible classically, in a similarly way that the Bell Test does statistically? More than your saying ‘gee I don’t know what it is till I test it’ as that is miles short of the Bell Standard.
  20. May 27, 2006 #19
    No. Where did I say that? I don't think we understand each other clearly.
    Entanglement is determined by performing a Franson Interference experiment on these photon pairs and registering coincidence counts.

    Did you get the chance to read the paper I linked to?

    Sorry, I can't parse that either.
  21. May 31, 2006 #20
    1) Did not say you said anything – that’s what I was trying to figure- just what you were saying if not non-sense.

    2) "Coincidence counts" what is that needed for that. Are the writers of your paper not able to detect a single photon filtered at a detected Hz (f1 vs. f2)?
    It is clear to me your stated f1=f-f2 will follow with classic and local logic when following that variable and 100% correlation all the time. No non-local issues or HUP involved; as are found when you carefully watch polarity, where following polarizations through with local logic seems to give some negative probabilities and some greater than 1.

    3) No, I don’t care to pay for an article on such a trivial matter.
    Have you looked closely at any of the easy to find drawings describing the view into a PDC showing how different energy light comings out in different size cones of light? And how and why experimentalist must carefully select which overlapping points of those various cones are used for an effective entanglement experiment.
    Last edited: May 31, 2006
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