We show that, in two-path optical interferometry, the measurement of photon number parity achieves maximal phase sensitivity at the quantum Cramér-Rao bound for a certain type of path-symmetric states. All pure states of light proposed for sub–shot-noise phase sensitivity thus far in the literature are of the form of these path-symmetric states. Thus, parity measurement presents an optimal detection strategy for all of them. Our scheme is applicable for local phase estimation.

• Received 29 June 2012

DOI:https://doi.org/10.1103/PhysRevA.87.043833