  This tutorial reviews the Holevo capacity limit as a universal tool to
analyze the ultimate transmission rates in a variety of optical communication
scenarios, ranging from conventional optically amplified fiber links to
free-space communication with power-limited optical signals. The canonical
additive white Gaussian noise model is used to describe the propagation of the
optical signal. The Holevo limit exceeds substantially the standard Shannon
limit when the power spectral density of noise acquired in the course of
propagation is small compared to the energy of a single photon at the carrier
frequency per unit time-bandwidth area. General results are illustrated with a
discussion of efficient communication strategies in the photon-starved regime.
