Finite Key Underwater Quantum Key Distribution: Performance Analysis and Improvements

Quantum Key Distribution (QKD) provides a method for two parties to establish a secure secret key based on the laws of physics rather than computational complexity. While extensively studied for fiber and free-space channels, its application in underwater environments is a newer area of research. This paper provides the first simulation of the decoy-state BB84 protocol under realistic finite-key constraints, moving beyond the optimistic asymptotic limits found in previous literature.

The authors employ standard underwater communication models to simulate the performance of QKD systems. They evaluate two different decoy-state protocols—a standard version and a simpler variant requiring fewer decoy settings—across various water conditions (e.g., clear vs. turbid water). The study also incorporates Classical Advantage Distillation (CAD), a post-processing technique, to determine if it can improve key generation rates in high-loss or high-noise underwater environments.

• Finite-Key Reality: Practical finite-key performance is notably lower than theoretical asymptotic bounds, particularly in the presence of ambient light at sea level.
• Protocol Optimization: The "simpler" decoy-state protocol can outperform the standard protocol depending on the specific underwater channel conditions, suggesting that protocol selection should be dynamic based on depth, distance, and water quality.
• Performance Enhancement: Classical Advantage Distillation (CAD) is a powerful tool for underwater QKD, enabling secret key generation in scenarios where standard protocols would otherwise fail, especially when background noise is high.