ABSTRACT
In the Maxwell continuum wave theory of light, a photodetector measures the intensity of the Electro Magnetic wave. The output current of the photodetector is proportional to the intensity of input light. Since the introduction of the concept of photon, or the concept of field quantization, physicists started to premeditate single-photon detection, theoretically and experimentally. Due to slight differences in their optical paths, at certain locations, the radiations superposed constructively, at other locations the field superposed destructively. A sinusoidal modulation is thus observable. However, people found this interpretation may not be appropriate when the experimental physicists improved the sensitivity of their photodetector to the single-photon level. Almost everyone was surprised that the interference was still observable in the observation plane, and the sinusoidal modulation observed from single-photon was exactly the same as before. A single-photon counting detector is able to count photons one by one.
