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

I need help locating quantum entanglement experiments

  1. Apr 14, 2009 #1
    like a link that describes an entanglement experiment from start to end or something like that. im just learning this quantum stuff, and i understand what happens AFTER theyre entangled but not HOW they get that way. do you just select 2 particles at random, put them in a jar, boom theyre entangled, then theyll have opposite spins? i dont get it!?!
  2. jcsd
  3. Apr 14, 2009 #2


    User Avatar
    Science Advisor
    Gold Member

    Good question. I recommend this link, which is to an undergraduate Bell test using entangled photons:

    Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory.

    Abstract: "We use polarization-entangled photon pairs to demonstrate quantum nonlocality in an experiment suitable for advanced undergraduates. The photons are produced by spontaneous parametric downconversion using a violet diode laser and two nonlinear crystals. The polarization state of the photons is tunable. Using an entangled state analogous to that described in the Einstein-Podolsky-Rosen “paradox,” we demonstrate strong polarization correlations of the entanged photons. Bell’s idea of a hidden variable theory is presented by way of an example and compared to the quantum prediction. A test of the Clauser, Horne, Shimony and Holt version of the Bell inequality finds S = 2.307±0.035, in clear contradiciton of hidden variable theories. The experiments described can be performed in an afternoon."

    Check out the diagram labeled Fig. 2. The secret is PDC crystals, which take a laser source and outputs 2 beams of entangled photons (which can be routed to the detectors). PDC=Parametric Down Conversion.
  4. Apr 14, 2009 #3
    Wikipedia is almost always a good first place to start looking up a popular topic like this : http://en.wikipedia.org/wiki/Quantum_entanglement . In this case, really only the introduction will suit your purposes, but it would be good to read. The external links also have some gems.

    For a walk through of an experiment, there is a good teaching page here : http://www.didaktik.physik.uni-erlangen.de/quantumlab/english/index.html that should give you a good idea on how photons are actually entangled (start with photons before you move to particles with spin - you'll need to build up some quantum intuition first).
  5. Apr 15, 2009 #4
    I have an interest in QM, and a newbie here, so please bear with me while I attempt to (naively perhaps) ask my questions. I didn't think my questions merited a new topic.

    If entanglement occurs, can that happen between more than two particles?

    If two particles get entangled, can one measure the momentum of one and position of the other (simultaneously), thereby violating Heisenberg's uncertainty principle, or are these totally different concepts?


  6. Apr 15, 2009 #5


    User Avatar
    Science Advisor
    Gold Member

    Welcome to PhysicsForums, raagamuffin! You have some good questions, let me take a crack...

    1. Yes, entanglement can occur with more than 2 particles. When more than 2 are entangled, the set of possible particle states does not follow quite the same math as with 2. With 2, usually they are almost like clones of each other. But with 3 or more, the rules are slightly different. However, in all cases of entanglement there are conservation laws in effect and these govern the possible results. Some references:

    Simple scheme for expanding a polarization-entangled W state by adding one photon

    Experimental filtering of two-, four-, and six-photon singlets from single PDC source

    2. No, this is not possible - although it was a good idea! In fact, this exact idea was used by Einstein in a famous 1935 paper called EPR. The concept seems good at first, but it runs afoul of Bell's Theorem. I have referenced PDFs of the originals of these 2 papers, and I highly recommend them if you have not already read them.

    The fact is, even entangled particles - separated by a good distance - will obey the HUP. Observing one somehow alters the other, and the net effect is you don't gain any new knowledge over observing just 1. No one really knows the exact details of the underlying mechanism, but we can make good predictions on the results of experiments.
  7. Apr 16, 2009 #6
    i would guess pdc crystals were discovered by accident? is that kind of how quantum entanglement experiments go, just mess arround with light,electrons, and things and see if any unexplainable patterns pop-up. in which case its called "entanglement" which is just a broad term that explains experimental results which defy logical probability. and so nowone really knows the mechanical reasons for those strange paterns, they just know theyre there.

    are there many types of things which have been found to produce impossible results besides pdc crystals? lemme know if ive got this roughly figured out correctly or not
    Last edited: Apr 16, 2009
  8. Apr 16, 2009 #7
    i guess that theyre using just 2 photons at a time or something there right? that animation thing on entanglement
  9. Apr 16, 2009 #8


    User Avatar
    Science Advisor
    Gold Member

    A lot of things are discovered by accident, and I have wondered about the discovery of PDC myself. However, there is plenty of theory behind PDC. I would mention the work of Glauber. For a book, see L. Mandel and E. Wolf, Optical Coherence and Quantum Optics (Cambridge, New York, 1995). Or try this paper: An Introduction to the Quantum Theory of Nonlinear Optics. Warning: this stuff is notoriously difficult.

    The things called "entanglement" are not just random unexplained phenomena. They are often predicted by theory in advance of experiment.
  10. May 19, 2009 #9

    The Dehlinger and Mitchell description is good but I would like to find a write up of an experiment testing the much simpler entanglement idea which, as I understand it, can be explained by postulating hidden variables and which the Bell Inequality experiments were designed to 'out wit'.

    i.e. Entangled pairs of photons go off in different directions and hit polarizing filters which are parallel to each other. There is a 0.5 probability that any photon will pass the filters (the production process does not constrain the orientation of polarization) however if one photon passes its filter then the entangled pair will collapse to an orientation that guarantees it will also pass. A coincidence meter would give a 100% reading.

    This seems a more immediate and 'spookier' demonstration of quantum entanglement and I have read of it in popular descriptions but no reference to an experiment.
  11. May 19, 2009 #10


    User Avatar
    Science Advisor
    Gold Member

    Welcome to PhysicsForums, paddywwoof!

    This is done as part of all "standard" Bell tests. That is how calibration of the setup is performed. Once you get maximal correlation at identical angles, you can proceed to the main test.

    You can take it for granted that there is near 100% correlation at identical angles. This is a result that is consistent with some local hidden variable theories (but not all).
  12. May 19, 2009 #11
    A very similar 'experiment' is often used to describe the EPR experiment and Bell inequalities.

    Instead of looking at the momentum and position, let's look at the spin of the particles. You can measure the spin in (for example) the z-direction, or in the x-direction. The HUP however does not allow you to know the spin in both directions simultaneously. If you measure the spin of a particle in the z-direction, you get a certain value (up or down). If you then measure it in the x-direction, you again get either up or down. But now, the spin in the z-direction is completely random again! You cannot know both spins at the same time.

    Now let's take two entangled particles and send one to experimenter 1 (often called Alice or A), and the other to experimenter 2 (Bob or B). Suppose that the entangled particles require one of them to be spin up, and the other to be spin down.

    If Alice measures her particle in the z-direction, and finds spin up, then Bob will find his particle to have spin down in the z-direction (and vice-versa).

    You may ask a similar question now:

    If Alice measures her particle's spin in the z-direction, and Bob in the x-direction, wouldn't that imply that both know their particle's spins in both the x and z directions? Since, when Alice measures her spin in the z-direction to be up, Bob knows that his must be down. So he measures the spin in the x-direction: viola, he now knows both the z and x direction! Likewise, Bob can now tell Alice what he measured in the x-direction, and Alice knows her spin in the x-direction as well (while she already knew it in the z-direction).

    If this is true, that would violate the HUP. Luckily, this doesn't work.
    When Alice measures her spin in the z-direction, Bob can indeed know the spin of his particle in the z-direction. However! As soon as he measures his spin in the x-direction, he loses all information about the spin in the z-direction! The spin in his z-direction is again completely random (and the particles are no longer entangled). So he can claim that he now knows both his x and z spin; but if you would ask him to verify his claim (by repeating the measurement of x and z spin) he would find (approximately half the time) that he was wrong!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook