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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. © 2013 American Physical Society.

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Physical Review A - Atomic, Molecular, and Optical Physics