  Quantum Key Distribution (QKD) is an innovative quantum communications
protocol which exploits the laws of quantum mechanics to generate
unconditionally secure cryptographic keying material between two geographically
separated parties. The unique nature of QKD shows promise for high-security
applications such as those found in banking, government, and military
environments. However, QKD systems contain implementation non-idealities which
can negatively impact their performance and security.In particular, QKD systems
often employ the decoy state protocol to improve system throughput and mitigate
the threat of Photon Number Splitting (PNS) attacks. In this work, a detailed
analysis of the decoy state protocol is conducted which optimizes both
performance in terms of quantum throughput and security with respect to
detecting PNS attacks. The results of this study uniquely demonstrate that the
decoy state protocol can ensure PNS attacks are detected with high confidence,
while maximizing the secure key generation rate at no additional cost.
Additionally, implementation security guidance is provided for QKD system
developers and users.
