Achieving the Quantum Fisher Information Bound in Pseudo-Hermitian Sensors

TL;DR

This paper develops a covariant quantum Fisher information framework for pseudo-Hermitian systems, revealing their potential for enhanced parameter sensitivity and establishing bounds and optimal measurement strategies.

Contribution

It introduces a covariant QFI formulation for pseudo-Hermitian Hamiltonians, ensuring consistent sensitivity analysis and identifying conditions for quantum sensor advantage.

Findings

Covariant QFI preserves state norm in pseudo-Hermitian systems.

Duality between covariant QFI and Hermitian QFI established.

Upper bounds and optimal measurements for pseudo-Hermitian sensors identified.

Abstract

Non-Hermitian systems have attracted considerable interest over the last few decades due to their unique spectral and dynamical properties not encountered in Hermitian counterparts. An intensely debated question is whether non-Hermitian systems, described by pseudo-Hermitian Hamiltonians with real spectra, can offer enhanced sensitivity for parameter estimation when they are operated at or close to exceptional points. However, much of the current analysis and conclusions are based on mathematical formalism developed for Hermitian quantum systems, which is questionable when applied to pseudo-Hermitian Hamiltonians, whose Hilbert space metric is intrinsically parameter dependent. Here, we develop a covariant formulation of quantum Fisher information (QFI) defined on the deformed Hilbert space of pseudo-Hermitian Hamiltonians. This covariant framework ensures the preservation of the state norm and enables a consistent treatment of parameter sensitivity. We further show that the covariant QFI of pseudo-Hermitian systems is dual to the ordinary QFI of corresponding Hermitian systems. Importantly, this correspondence naturally imposes an upper bound on the covariant QFI and allows one to identify optimal projections which saturate the corresponding classical Fisher information to this ultimate limit. The developed framework also enables to set the criteria under which pseudo-Hermitian sensors can exhibit an advantage over their Hermitian counterparts of the same dimensionality.