A general Mach-Zehnder interferometer with coherent
and squeezed vacuum
states as input. Beam splitters (BS) mix the two spatial modes, while mirrors (M) impart a phase shift, which can be safely neglected since it is common to both modes. A phase shift ϕ represents the phase difference between the two arms of the MZI, which can be due to a path length difference. Our goal is to estimate the unknown parameter ϕ, which corresponds to the interaction of the quantum state with some process of interest.