In order to make quantum information science applicable to the real world, it is necessary to change direction to focus on engineering technology based on quantum communication theory. Consequently, we can expect the following abilities: the possibilities of ultrahigh-speed optical communication, the feasibility of all-weather sensors, and enhancing the security of the physical layer of current optical networks. In addition, clarifying decoherence properties of quantum processors in the sense of noise analysis is important for real applications of quantum technology. The aim of Special Issue was to consolidate and publish the latest research trends by researchers who are conducting research toward the above goals.