In our field of applied superconductivity for particle accelerators, theories have been mainly understood with classical models. This is not sufficient to describe the state-of-the-art progress of our experimental research toward high gradient and high quality factors. The most elementary quantum description was in fact established in the 1960s but its application for realistic materials has not been well considered because the main interest of the community went to high-Tc superconductors after its discovery. Although people might believe that they fully understood conventional BCS superconductors, there remain some fundamental mysteries when the superconductors are irradiated to strong RF for practical use, such as particle accelerators. This seminar is a kick-off for future collaboration between experimentalists and theorists for superconducting RF in Europe.