Quantum cryptography over the FSO channel with PPM and FSK modulations

Data encryption has become crucial in modern communication. Complex encryption algorithms are commonly used with the disadvantage that, with fast enough classical computers or not yet usable quantum computers, decryption is possible. Quantum key distribution (QKD) offers however unconditional security. This paper describes an innovative QKD protocol, the time-frequency (TF) BB84-like protocol, and discusses its ability to distribute an unconditionally secure key at a high data rate, particularly with regard to free-space links.