In practical applications of communication schemes it is often necessary to amplify the transmitted signals. Because of the intrinsic noise due to Heisenberg's uncertainty principle, a quantum signal requires an amplification process which is different than the one used for classical signals.
In theory, quantum key distribution (QKD) provides information-theoretic security based on the laws of physics. Owing to the imperfections of real-life implementations, however, there is a big gap between the theory and practice of QKD. This gap has been recently exploited by several quantum hacking activities.
