Thursday, July 28, 2016

quantum information - Bell polytopes with nontrivial symmetries


Take N parties, each of which receives an input si1,,mi and produces an output ri1,,vi, possibly in a nondeterministic manner. We are interested in joint conditional probabilities of the form p(r1r2rN|s1s2sN). Bell polytope is the polytope spanned by the probability distributions of the form p(r1r2rN|s1s2sN)=δr1,r1,s1δrN,rN,sN for all possible choices of numbers ri,si (in other words, each input si produces a result ri,si either with probability 0 or 1, regardless of other players' inputs). Polytopes of this kind are of interest in quantum information theory.


Every Bell polytope has a certain amount of trivial symmetries, like permutation of parties or relabelling of inputs or outputs. Is it possible to give an explicit Bell polytope with nontrivial symmetries? (e.g. transformations of the polytope into itself that takes faces to faces and is not trivial in the above sense) In other words, I'm interested whether a specific Bell scenario can possess any "hidden" symmetries


Bell polytopes in literature are usually characterized by their faces, given by sets of inequalities (Bell inequalities), which, however, usually do not have any manifest symmetry group.




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