  The applications of the general formulae of channel capacity developed in the
quantum information theory to evaluation of information transmission capacity
of optical channel are interesting subjects. In this review paper, we will
point out that the formulation based on only classical-quantum channel mapping
model may be inadequate when one takes into account a power constraint for
noisy channel. To define the power constraint well, we should explicitly
consider how quantum states are conveyed through a transmission channel. Basing
on such consideration, we calculate a capacity formula for an attenuated noisy
optical channel with genreral Gaussian state input; this gives certain progress
beyond the example in our former paper.
