Photon Channels with Small Occupation Numbers

We consider a photon channel in which the transmitter sets the average occupation numbers of the field oscillators, while the receiver records a certain portion of the photons emitted by the transmitter. If the average numbers of photons recorded are peak- or average-limited, and are also small compared to unity, then the transmission capacity may be calculated to asymptotic accuracy for quite general conditions based on the transition probabilities, but independent of their specific form. It is demonstrated that, contrary to general opinion, fluctuations in the numbers of photons plays a minor role in the case of small occupation numbers, and the transmission capacity tends toward that of the ideal channel.