An interference pattern is obtained with a Michelson interferometer; the intensity distribution in the pattern is determined by counting the tate of photons by means of a photomultiplier. It is shown that the pattern obtained for very low intensities of light does not differ outside the margin of experimental error from the pattern obtained for normal intensities. At low intensities about 106 photons enter the interferometer per sec ; thus for these low intensities in average at any time much less than one photon is contained inside the interferometer.
By making use of the coincidence technique, the intensity correlation between two coherent light beams produced by splitting a light beam by means of a semitransparent mirror could be verified. The data obt~ined in this way are in good agreement with the result predicted by a semi-classical theory.
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