Interferometric detection allows us to measure tiny variations of the optical path of a light beam, corresponding to very small phase shifts of the radiation field. Even with conventional interferometers, such measurement is generally considered very accurate. However, in special situations, such as for detecting gravitational waves, a much more accurate measurement is needed @1#. The limit to the precision of the measurement is posed by the limited available amount of field energy, which, in the form of radiation pressure, perturbs the parameter that is monitored by the phase shift. In conventional two-path Mach-Zehnder ~or Michelson! interferometers that use coherent light, phase sensitivity is bounded by the so-called shot-noise limit