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SUMMARY:Post-selection technique for quantum channels with applications to
  quantum cryptography - Matthias Christandl (LMU Munich)
DTSTART:20081128T150000Z
DTEND:20081128T160000Z
UID:TALK15468@talks.cam.ac.uk
CONTACT:Lawrence Ioannou
DESCRIPTION: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 prov
 e that a certain property holds for any arbitrary input\, it is sufficient
  to consider the special case where the input is a particular de Finetti-t
 ype state\, i.e.\, a state which consists of n identical and independent c
 opies of an (unknown) state on a single subsystem. A similar statement hol
 ds for more general channels which are covariant with respect to the actio
 n of an arbitrary finite or locally compact group. \nOur technique can be 
 applied to the analysis of information-theoretic problems. For example\, i
 n quantum cryptography\, we get a simple proof for the fact that security 
 of a discrete-variable quantum key distribution protocol against collectiv
 e attacks implies security of the protocol against the most general attack
 s. The resulting security bounds are tighter than previously known bounds 
 obtained by proofs relying on the exponential de Finetti theorem [Renner\,
  Nature Physics 3\,645(2007)]. \n \nReference: http://arxiv.org/abs/0809.3
 019
LOCATION:MR14\, Centre for Mathematical Sciences
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