  We report a theoretical and experimental study on the role of
indistinguishability in the estimation of an interferometric phase. In
particular, we show that the quantum Fisher information, which limits the
maximum precision achievable in the parameter estimation, increases linearly
with respect to the degree of indistinguishability between the input photons in
a two-port interferometer, in the ideal case of a pure probe state. We
experimentally address the role played by the indistinguishability for the case
of two photons entering a polarization-based interferometer, where the degree
of indistinguishability is characterized by the overlap between two spatial
modes. The experimental results support the fact that, even in the presence of
white noise, a quantum enhancement in the interferometric phase estimation can
be obtained from a minimum degree of indistinguishability.

Quantum Physics