Quantum electrodynamics of a communication channel

A simple one-dimensional model is taken to illustrate the quantum effects of a narrowband communication system. The system is quantized, and its dynamics are discussed in terms of the Heisenberg equations for field operators. The detection process of a "coherent joint detector," whose compatibility with quantum physics was justified elsewhere, is discussed. The detector, which is capable of measuring the quadrature-modulating components at Nyquist rate with minimum possible uncertainty allowed by quantum theory and which is most suitable for a continuous channel, is used to determine the channel capacity. It is shown that the noise of the channel, due to both thermal and quantum fluctuations, is additive and Gaussian. The classical concepts and expressions for the average mutual information and channel capacity are still valid, provided that the quantum noise of one quantum is included properly. The results obtained approach straightforwardly Shannon\´s in the classical limit.