Limiting rate of secret-key generation in quantum cryptography in spacetime

The fundamental restrictions on the maximum admissible rate of secret-key commitment in quantum cryptography in real time are discussed. It is shown that the maximum rate in a quantum channel with limited transmission band is achieved in a cryptosystem on orthogonal states. The dimensionless rate (the number of bits per unit time frequency band through unit of the channel) is determined by the universal function $C(\lambda_0(\Delta k\cdot T))/\Delta k\cdot T$ [where $C(\lambda_0(\Delta k\cdot T))$ is the transmission capacity of a classical binary channel, $\Delta k$ is the transmission band width, $1/T$ is the transmission frequency of quantum states, and $\lambda_0$ is the maximum eigenvalue of a certain integral equation].