7. Quantum Communications Systems

In this chapter the quantum decision theory, developed in the previous chapters, is applied to quantum communications systems by specifying the physical nature of the quantum states that carry the information. In practice, the states of a quantum communications system refer always to optical frequencies because at radio frequencies quantum phenomena are not appreciable. Specifically, quantum states represent coherent monochromatic radiations emitted by a laser and are called coherent states. Therefore the theory of coherent states, which is presented according to the formulation developed by Glauber in the 1980s of the last century, will play a fundamental role. Also squeezed states which represent an efficient form of optical radiation are considered. In the second part of the chapter, the most popular quantum communication systems are developed, starting from the simplest ones: The binary systems whose format may be the OOK (on–off keying) modulation and the 2-PSK (phase-shift keying) modulation. Then multilevel systems are developed with the formats: QAM (quadrature amplitude modulation), multilevel PSK modulation, and PPM (pulse position modulation). All these quantum systems will be compared with the classical counterparts, and the superiority in performance of quantum systems with respect to classical systems will be clearly stated.