It was more than ten years ago that an original version of this monograph was published with the title Quantum Optics in Japanese from Iwanami Shoten in Tokyo. Therefore, making the best use of this chance to translate the book into an English version, we have tried to include the exciting developments of the relevant subjects in these ten years, especially novel nonlinear optical responses of materials. The ?rst example of these nonlinear optical phen- ena is laser cooling and subsequent observation of Bose-Einstein and Fermi condensation of neutral atoms. Second, it is now possible to...