The black paper of quantum cryptography: real implementation problems

Scarani, Valerio
Kurtsiefer, Christian
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The laws of physics play a crucial role in the security of quantum key distribution (QKD). This fact has often been misunderstood as if the security of QKD would be based only on the laws of physics. As the experts know well, things are more subtle. We review the progresses in practical QKD focusing on (I) the elements of trust that are common to classical and quantum implementations of key distribution; and (II) some threats to security that have been highlighted recently, none of which is unredeemable (i.e., in principle QKD can be made secure). This leads us to guess that the field, similar to non-quantum modern cryptography, is going to split in two directions: those who pursue practical devices may have to moderate their security claims; those who pursue ultimate security may have to suspend their claims of usefulness.
Comment: Small changes and clarifications. This paper still describes the situation of QKD in June 2009, when its first version was sent out; we have mentioned a few developments in a Note added in proof. Accepted in a special issue of "Theoretical Computer Science" (T. Mor and R. Renner, editors)
Quantum Physics