The effect of channel losses on the capacity of noiseless optical communication systems utilizing the number state is studied. Based on these findings, the photon efficiency of these systems is evaluated in the presence of losses. Then, by comparing the photon efficiency of the number-state pulse-position modulation (PPM) and the coherent-state PPM under certain realistic conditions, it is found that the former is always superior to the latter from the perspective of photon efficiency regardless of losses.
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