  We quantitatively investigate phase measurement in a Mach-Zehnder
interferometer (MZI), which is injected with a weak coherent and a squeezed
vacuum generated from a spontaneous parametric down-conversion. The measured
three-photon coincidence in this system is quantified as a function of a ratio
between the amplitude of the coherent state and the squeezed parameter of
squeezed vacuum when the photon number is detected at one output of MZI. It
shows that the phase sensitivity reaches the Heisenberg limit when an optimal
ratio is chosen. This may introduce one approach to quantify phase sensitivity
in precision phase measurement.
