Entropic uncertainty relations and the extremely allowable critical error in quantum cryptography

The main parameter of any quantum cryptography protocol is the critical error $Q_c$ to which secret key distribution is possible. The critical error of all known quantum key distribution protocols does not exceed $20\%$. In this work, a protocol with which secret key distribution is possible to $Q_c\rightarrow 50\%$ is described and a simple proof of the security of the protocol is presented using fundamental entropic uncertainty relations. This critical error is a theoretical, but achievable limit, which cannot be improved.