Abstract
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.
Highlights
I T HAS been recognized for a long time that quantum effects set limits on the information capacity of optical communication links [1]
The purpose of this paper is to provide an introduction to the Holevo capacity limit and to relate it to the standard Shannon capacity limit for linear additive white Gaussian noise (AWGN) channels used as a benchmark when evaluating the performance of optical communication systems [24]–[27]
When discussing quantum capacity limits it is essential to distinguish between noise contributed by the propagation of the optical signal and that introduced by the detection process
Summary
I T HAS been recognized for a long time that quantum effects set limits on the information capacity of optical communication links [1]. The receivers can implement unconventional detection strategies that exhibit sensitivity beyond shot-noise-level direct detection or coherent detection [8]– [10] Another possibility is to use non-classical states of light for communication, such as Fock states, that carry a well-defined number of photons [1], [11], [12], or squeezed states, that exhibit quadrature fluctuations below the shot noise level [13], [14]. When discussing quantum capacity limits it is essential to distinguish between noise contributed by the propagation of the optical signal and that introduced by the detection process. As this tutorial will emphasize, there is no single universal figure for the detection noise, which needs to be characterized for a given detection scheme.
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