Talk:GHZ experiment

message: A couple of questions:  
1. Why should there necessarily be only the one hidden variable λ?  In a paper I've just glanced through (Greenberger,  Daniel M, “Two-particle versus three-particle EPR experiments”, Annals of the New York Academy of Sciences 755, 585-599 (1995)) they allow for three hidden variables, though (for some reason I have not yet fathomed) restricted so that the sum is zero.
2.  Has the QM prediction been experimentally verified?
Caroline Thompson 18:02, 20 Aug 2004 (UTC)

message: This experiment has taken place (at least once!).  Here is one: http://arxiv.org/abs/quant-ph/9810035
Here is a local hidden variable theory that creeps in through error in a real experiment (where the simultaneous eigenstate cannot be obtained, only an approximate eigenstate obtained) http://arxiv.org/pdf/quant-ph/0007102
And here is an analysis of the results of the experiment, that claims to eliminate the possibility of a LHVT explaining the results of that experiment http://arxiv.org/pdf/quant-ph/9811013
In addition I have my five pennies:
Now let us suppose there are LHVs situated in the respective measuring devices. Let us denote them with: Sa for A, Sb for B and Sc for C. As an example we may take Sa to be an array of +1/-1 that are out of control of the experimenter (here in the example below A related Alice).
We have
Sa=(-1,+1,+1,-1, ... ,-1, ... ,-1 ...)
in an N-dimensional space: {-1,1}x{{-1,1}x...x{-1,1}x.... (x cartesian product here).
Furthermore, let us suppose that Alice, Bob and Claudia cannot change the setting of their measuring device without disturbing in an uncontrollable manner the Sa, Sb, and, Sc. This would lead to a different set of equations 1 ...4. We see:
1.  A( a2 , λ, Sa ) B( b2 , λ, Sb ) C( c2 , λ, Sc ) = -1, 
2.  A( a2 , λ, Sa ) B( b1 , λ, Sb' ) C( c1 , λ, Sc' ) = 1, 
3.  A( a1 , λ, Sa' ) B( b2 , λ, Sb ) C( c1 , λ, Sc' ) = 1, and 
4.  A( a1 , λ ,Sa') B( b1 , λ, Sb' ) C( c2 , λ,Sc ) = 1.
 
That is, if, e.g., A changes the parameter setting from a2 to a1, the hidden state denoted with Sa will transform from its original state to an unkown state Sa' that cannot be controlled by the experimenter (here Alice). For instance, if we start from
Sa=(-1,+1,+1,-1, ... ,-1, ... ,-1 ...),
after the change of a2 to a1, we could see (e.g.)
Sa'=(+1,+1,+1,+1, ... ,-1, ... ,+1 ...)
and note that when a1 is changed back to a2, Sa, can arise that most likely is different from Sa.
If this scheme of uncontrollable changes in spin-like local (namely hidden in the measurement apparatus) variables, then GHZ will not reach a contradiction. In fact, I would like to ask GHZ how they plan to have for Alice, Bob and Claudia exactly the same value for the local hiden variables in order to arrive at the contradiction. What should the experimenters do to get that?
Yours
Han Geurdes  —Preceding unsigned comment added by 212.123.206.71 (talk) 13:32, 19 February 2009 (UTC)
thread_title:  Actual experiments 

message: The first source you mentioned is only talking about three-photon entanglement. Though some of the authors did a GHZ-experiment ("Experimental Test of Quantum Nonlocality in Three-photon GHZ Entanglement", Nature 403, 515 (2000)), accessible via D. Bouwmeester's website
It would be very nice to add some information about the real experiments.
--QBenni (talk) 17:11, 17 January 2011 (UTC)
thread_title:  Actual Experiments II 

message: Without orientation, 45° from horizontal and vertical is the same. Does that sentence ("...+45° from horizontal. However, quantum mechanical theory predicts that it will be +45° from vertical...") mean that the predictions differ by 90°? It would be helpful to say so explicitly. --User:Haraldmmueller 19:19, 30 April 2018 (UTC)
thread_title:  +45° from horizontal, +45° from vertical