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Quantum Cramer–Rao Bound for a Massless Scalar Field in de Sitter Space

Department of Physics, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
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Universe 2017, 3(4), 71; https://doi.org/10.3390/universe3040071
Received: 31 July 2017 / Revised: 6 October 2017 / Accepted: 10 October 2017 / Published: 13 October 2017
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Abstract

How precisely can we estimate cosmological parameters by performing a quantum measurement on a cosmological quantum state? In quantum estimation theory, the variance of an unbiased parameter estimator is bounded from below by the inverse of measurement-dependent Fisher information and ultimately by quantum Fisher information, which is the maximization of the former over all positive operator-valued measurements. Such bound is known as the quantum Cramer –Rao bound. We consider the evolution of a massless scalar field with Bunch–Davies vacuum in a spatially flat FLRW spacetime, which results in a two-mode squeezed vacuum out-state for each field wave number mode. We obtain the expressions of the quantum Fisher information as well as the Fisher informations associated to occupation number measurement and power spectrum measurement, and show the specific results of their evolution for pure de Sitter expansion and de Sitter expansion followed by a radiation-dominated phase as examples. We will discuss these results from the point of view of the quantum-to-classical transition of cosmological perturbations and show quantitatively how this transition and the residual quantum correlations affect the bound on the precision. View Full-Text
Keywords: Fisher information; entanglement; de Sitter space Fisher information; entanglement; de Sitter space
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Rotondo, M.; Nambu, Y. Quantum Cramer–Rao Bound for a Massless Scalar Field in de Sitter Space. Universe 2017, 3, 71.

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