Light was one of the earliest scientific research subjects of mankind. A photon is the smallest unit of light energy and has basic quantum characteristics. With the development of science and technology, human beings have been able to reach the limit of detecting a single photon. The conventional detection of light has been widely used in ground level optical fiber communication, but the detection of light at the single-photon limit has important application in free space quantum communication and ultralong distance deep space optical communication. This paper introduces the typical applications and core technologies of photon detection technology in quantum science experiments and photonic communication in space.