Quantum estimation in an expanding spacetime

TL;DR

This paper explores quantum estimation of the Hubble parameter in expanding de Sitter space using quantum metrology, showing how different vacuum states and scalar field couplings affect measurement precision.

Contribution

It introduces a detailed analysis of quantum Fisher information in de Sitter space, highlighting how scalar field coupling and vacuum choice improve estimation accuracy.

Findings

Maxima of FI/QFI depend on initial probe state and evolving time.

Proper scalar field coupling enhances estimation precision.

α-vacua improve measurement accuracy due to their squeezed nature.

Abstract

We investigate the quantum estimation on the Hubble parameter of an expanding de Sitter space by quantum metrological techniques. By exploring the dynamics of a freely falling Unruh-DeWitt detector, which interacts with a scalar field coupling to curvature, we calculate the Fisher information (FI) and quantum Fisher information (QFI) for the detector, which bound the highest precision of the estimation on Hubble parameter. In standard Bunch-Davies vacuum, we show that the maxima of FI/QFI are located for particular initial state of probe. Beside its dependence on the evolving time of detector and the energy spacing of atom $\omega$, we show that the maxima of FI/QFI can be significantly enhanced once a proper coupling of scalar field to curvature is chosen. For instance, we show numerically that the estimation in the scenario with minimally/nearly minimally coupling scalar field can always outperform that with conformally coupling scalar field, corresponding to a higher FI/QFI in estimation. Moreover, we find that for general $\alpha-$vacua of de Sitter space, a further improvement of estimation can be achieved, attributed to the squeezed nature of $\alpha-$vacua that heavily constrains the measurement uncertainty. Some implications of our results are also discussed.