Quantum Communications Systems with Thermal Noise

Abstract

The chapter deals with the analysis of Quantum Communications systems in the presence of thermal noise, sometimes called background noise, neglected in the previous chapter. This is achieved by passing from pure states to mixed states, described by density operators. Consequently, the analysis and especially the optimization become very difficult. For binary Quantum Communications systems the optimization is based again on Helstrom’s theory, whereas for multilevel systems the numerical optimization is used, and especially the square root measurement (SRM), which is suboptimal but gives a good approximation of the performance. The SRM technique is applied to the most popular Quantum Communications systems (QAM, PSK, and PPM) seen in the previous chapter in the absence of thermal noise. We will find that, also in the presence of thermal noise, Quantum Communications systems perform better than the classical counterparts.