Quantum Fisher-information limits of resonant nanophotonic sensors: why high-Q is not optimal even at the quantum limit

TL;DR

This paper develops a quantum metrological framework for nanophotonic sensors, revealing that optimal sensing precision depends on phase shift generators rather than cavity Q-factor, challenging the assumption that high-Q is always best.

Contribution

It introduces a quantum Fisher information analysis for resonant nanophotonic sensors, showing that optimal sensitivity is governed by phase shift generators, not cavity quality factor.

Findings

Optimal estimation precision depends on phase shift generators.

Maximum-Q resonance does not always yield best sensitivity.

Quantum resources improve sensitivity without changing optimal geometry.

Abstract

We develop a quantum metrological framework for resonant nanophotonic sensors based on subwavelength Fabry--Perot slit cavities. Building on classical Fisher-information analyses of resonant transmission sensors, we model parameter encoding as a phase-and-loss quantum channel embedded in one arm of a Mach-Zehnder interferometer. We derive the quantum Fisher information (QFI) for coherent and Gaussian probe states under linear loss and show that, even at the quantum limit, optimal estimation precision is governed by the generator of parameter-dependent phase shifts rather than by the cavity quality factor. Consequently, the operating point that maximizes the QFI does not generally coincide with the maximum-Q resonance. Quantum resources enhance sensitivity but do not redefine the optimal geometry. Our results provide physically transparent design principles for quantum-enhanced nanophotonic sensing.