Optimal estimation of quantum boundary effect in cosmic string space-time

Abstract

The presence of a cosmic string modifies vacuum fluctuations, making the evolution of a two-level polarizable atom position-dependent. Such modifications produce effects on the atomic dynamics analogous to those induced by a reflecting boundary. We show that these quantum boundary effects can be estimated by performing a sequence of $N$ measurements on a single probe atom. For a fixed total probe time, the precision limit is attained by preparing each probe in its optimal initial state, performing the corresponding optimal measurement, and shortening the probe time of each probe. The optimal measurement is uniquely determined by the probe's initial state, and the precision limit obtained with the atom initially in the excited state is four times higher than that for an equal-weight superposition state. The estimation precision displays damped oscillatory behavior as the atom-boundary or atom-string separation increases. In the boundary case, the optimal polarization is always parallel to that boundary. In cosmic-string spacetime, by contrast, the optimal polarization is along the azimuthal direction around the string. The relative advantage of the other polarization directions depends on both the atom-string separation and the deficit angle. For small deficit angles and sufficiently large separations, radial polarization perpendicular to the string becomes less favorable than the other directions.