Coherent eavesdropping attacks in tomographic quantum cryptography: Nonequivalence of quantum and classical key distillation

Abstract

The security of a cryptographic key that is generated by communication through a noisy quantum channel relies on the ability to distill a shorter secure key sequence from a longer insecure one. We show that-for protocols that use quantum channels of any dimension and completely characterize them by state tomography-the noise threshold for classical advantage distillation of a specific kind is substantially lower than the threshold for quantum entanglement distillation if the eavesdropper can perform powerful coherent attacks. In marked contrast, earlier investigations had shown that the thresholds are identical for incoherent attacks on the same classical distillation scheme. It remains an open question whether other schemes for classical advantage distillation have higher thresholds for coherent eavesdropping attacks. © 2005 The American Physical Society.