According to relevant basic concepts, the origin of the speed of light is presented from the point of dimensional analysis and thought experiment. The impossibility of faster-than-light is pointed out. The connotation of the concepts such as light quantum, quantum state, etc. is discussed in depth. On the basis of the view of photon splitting, the physical essence of quantum state representation of binary data is analyzed and the concept of controllable photon splitting is proposed. Controllable photon splitting is the precondition for realizing quantum state representation of binary data. The problems which the existing theoretical basis of quantum communication may face are discussed.