Information leakage through a passive optical channel

From the basic entropy of information, the information leakage through a passive coherent channel is calculated. In the course of the derivation, the optical channel is decomposed into a series of cascaded subchannels. It is shown that the amount of information decreases as the information flows through the cascaded channels. The best a passive channel can achieve, in principle, is to preserve the entire input information. It is possible to obtain at the output an amount of information greater than the input, but only through the use of active channels. However, an active channel requires an external source of energy which in some cases could lead to an enormous amount of energy.