QKD with finite resources: secret key rates via R\'enyi entropies
  A realistic Quantum Key Distribution (QKD) protocol necessarily deals with finite resources, such as the number of signals exchanged by the two parties. We derive a bound on the secret key rate which is expressed as an optimization problem over R\'enyi entropies. Under the assumption of collective attacks by an eavesdropper, a computable estimate of our bound for the six-state protocol is provided. This bound leads to improved key rates in comparison to previous results. 
Silvestre Abruzzo, Hermann Kampermann, Markus Mertz, and Dagmar   Bru{\ss}
quant-ph