Sub shot-noise interferometric phase sensitivity with beryllium ions Schrodinger cat states

Interferometry with NOON quantum states can provide unbiased phase estimation with a sensitivity scaling as Delta theta similar to 1/ N-T given a prior knowledge that the true phase shift theta lies in the interval -pi <= theta <= pi. The protocol requires a total of N-T = 2(p) - 1 particles ( unequally) distributed among p independent measurements and overcomes basic difficulties present in previously proposed approaches. We demonstrate the possibility to obtain a phase sensitivity beating the classical shot-noise limit using published probabilities retrieved experimentally for the creation of Schrodinger cat quantum states containing up to N = 6 beryllium ions. Copyright (C) EPLA, 2007.