Michelson interferometry with quantum noise reduction

A Michelson interferometer with noise reduction to sub-shot noise levels is proposed and realized. Multiple measurements of a single signal beam are taken and the quantum property of light plays an essential role in the principle underlying this interferometry. The method makes use of the coherent state of light and requires only a simple modification to the standard Michelson interferometer. The surface fluctuation spectra of liquids are measured using this method down to a few orders of magnitude below the shot noise level. The spectrum derived from hydrodynamical considerations agrees well with the observed results for water. However, for oil, slight deviations are seen at high frequencies ($\gtrsim1\,$MHz), perhaps indicating its more complex underlying physics. The measurement requires a relatively low light power and a short time, so that it has a wide range of applicability.