Fragility of Optimal Measurements due to Noise in Probe States for Quantum Sensing

TL;DR

This paper investigates how noise affects the stability of optimal measurement strategies in quantum sensing, revealing that discontinuities in Fisher information lead to fragility and proposing methods to enhance robustness.

Contribution

It introduces a framework linking Fisher information discontinuities to measurement fragility and suggests ways to design more noise-resilient POVMs for quantum metrology.

Findings

Discontinuities in Fisher information quantify measurement fragility under noise.

A Jensen's inequality-based framework explains how discontinuities increase fragility.

Proposed methods improve robustness of quantum measurements against noise.

Abstract

For a given quantum state used in sensing, the quantum Cram\'er-Rao bound (QCRB) sets a fundamental limit on the precision achievable by an unbiased estimator of an unknown parameter, determined by the inverse of the quantum Fisher information (QFI). The QFI serves as an upper bound on the classical Fisher information (CFI), representing the maximum extractable information about the unknown parameter from measurements on a physical system. Thus, a central goal in quantum parameter estimation is to find a measurement, described by a POVM, that saturates the QFI (achieves maximum CFI), and thereby achieves the QCRB. In the idealization that one uses pure states and unitary encodings for sensing, discontinuities can appear in the CFI but not the QFI. In this article, we demonstrate that these discontinuities are important features, quantifying how much Fisher information is lost in the presence of noise. We refer to this as the Fisher information "fragility". We present a simple framework for understanding how discontinuities increase fragility through Jensen's inequality, and demonstrate how one can use this framework to design more robust POVMs for quantum advantage in metrology.