Phase measurements at the theoretical limit - art. no. 013813

It is well known that the result of any phase measurement on an optical mod e made using linear optics has an introduced uncertainty in addition to the intrinsic quantum phase uncertainty of the state of the mode. The best pre viously published technique [H. M. Wiseman and R. B. Killip, Phys. Rev. A 5 7, 2169 (1998)] is an adaptive technique that introduces a phase variance t hat scales as (n) over bar (-15), where it is the mean photon number of the state. This is far above the minimum intrinsic quantum phase variance of t he state, which scales as (n) over bar (-2). It has been shown that a lower limit to the phase variance that is introduced scales as ln((n) over bar)/ (n) over bar (2). Here we introduce an adaptive technique that attains this theoretical lower limit.