Postselection technique for quantum channels with applications to quantum cryptography.

@article{Christandl2008PostselectionTF,
  title={Postselection technique for quantum channels with applications to quantum cryptography.},
  author={Matthias Christandl and Robert K{\"o}nig and Renato Renner},
  journal={Physical review letters},
  year={2008},
  volume={102 2},
  pages={
          020504
        }
}
We propose a general method for studying properties of quantum channels acting on an n-partite system, whose action is invariant under permutations of the subsystems. Our main result is that, in order to prove that a certain property holds for an arbitrary input, it is sufficient to consider the case where the input is a particular de Finetti-type state, i.e., a state which consists of n identical and independent copies of an (unknown) state on a single subsystem. Our technique can be applied… 
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