Abstract: All the data that flows through optical fibres such as electronics trade, bank transaction, electronics messages etc�. are concerned by this promising technique of the quantum cryptography or the distribution of secret keys and this because it secures the communications making them inviolable. Why does the quantum cryptography allow a secret keys exchange? Thanks to the laws of the quantum physics which usually impose restrictions to the classic physics and permits to offer an absolute protection to the information. In spit of the considerable progress in the quantum encryption (encoding) many questions remain asked and many problems cannot be solved using the present techniques. In order that the quantum cryptology becomes an efficient method with application to large scales, we must introduce techniques for real applications to coding and encoding. This precise point is the aim of our study; we will try knowing the practical limits for the quantum cryptography thus coupling them with techniques borrowed from signal processing with purely quantum theories in order to elaborate correction error methods in quantum cryptography.