Sub-Heisenberg phase uncertainties

Phase shift estimation with uncertainty below the Heisenberg limit, Delta phi(HL) alpha 1/(N) over bar (T), where (N) over bar (T) is the total average number of particles employed, is a mirage of linear quantum interferometry. Recently, Rivas and Luis, [New J. Phys. 14, 093052 (2012)] proposed a scheme to achieve a phase uncertainty Delta phi(HL) alpha 1/(N) over bar (k)(T), with k an arbitrary exponent. This sparked an intense debate in the literature which, ultimately, does not exclude the possibility to overcome Delta phi(HL) at specific phase values. Our numerical analysis of the Rivas and Luis proposal shows that sub-Heisenberg uncertainties are obtained only when the estimator is strongly biased. No violation of the Heisenberg limit is found after bias correction or when using a bias-free Bayesian analysis.