# Information Security and Cryptography Research Group

## Unifying classical and quantum key distillation

### Matthias Christandl, Artur Ekert, Michal Horodecki, Pawel Horodecki, Jonathan Oppenheim, and Renato Renner

Theory of Cryptography Conference — TCC 2007, Lecture Notes in Computer Science, Springer-Verlag, Aug 2006, Available at http://arxiv.org/abs/quant-ph/0608199.

Assume that two distant parties, Alice and Bob, as well as an adversary, Eve, have access to (quantum) systems prepared jointly according to a tripartite state. In addition, Alice and Bob can use local operations and authenticated public classical communication. Their goal is to establish a key which is unknown to Eve. We initiate the study of this scenario as a unification of two standard scenarios: (i) key distillation (agreement) from classical correlations and (ii) key distillation from pure tripartite quantum states. Firstly, we obtain generalisations of fundamental results related to scenarios (i) and (ii), including upper bounds on the key rate, i.e., the number of key bits that can be extracted per copy of the initial state. Moreover, based on an embedding of classical distributions into quantum states, we are able to find new connections between protocols and quantities in the standard scenarios (i) and (ii). Secondly, we study specific properties of key distillation protocols. In particular, we show that every protocol that makes use of pre-shared key can be transformed into an equally efficient protocol which needs no pre-shared key. This result is of practical significance as it applies to quantum key distribution (QKD) protocols, but it also implies that the key rate cannot be locked with information on Eve's side. Finally, we exhibit an arbitrarily large separation between the key rate in the standard setting where Eve is equipped with quantum memory and the key rate in a setting where Eve is only given classical memory. This shows that assumptions on the nature of Eve's memory are important in order to determine the correct security threshold in QKD.

## BibTeX Citation

@inproceedings{CEHHOR06,
author       = {Matthias Christandl and Artur Ekert and Michal Horodecki and Pawel Horodecki and Jonathan Oppenheim and Renato Renner},
title        = {Unifying classical and quantum key distillation},
booktitle    = {Theory of Cryptography Conference --- TCC 2007},
series       = {Lecture Notes in Computer Science},
year         = 2006,
month        = 8,
note         = {Available at http://arxiv.org/abs/quant-ph/0608199},
publisher    = {Springer-Verlag},
}