Information Gain in Quantum Communication Channels
In quantum communications the performances of an amplifier depend on the scheme of the channel in which the device is inserted. For example, both gain and noise figure of the amplifier depend on the kind of coding at the transmitter, and detection at the receiver.  In an optimized ideal channel, detection and coding are ideal, and both are “matched” on the same observable; the alphabet probability is optimized in order to satisfy the physical constraints on the line. In this case, the channel capacity is already achieved, and there is no need of amplification. However, when the channel is nonideal—either because of quantum mismatch between transmitter and receiver, because of losses along the line, or as a result of nonunit quantum efficiency at detectors—an appropriate amplification can improve the transmitted information, ideally achieving the channel capacity for infinite gain.
KeywordsAttenuation Covariance Coherence Convolution Lost
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