Fisher information provides a rigorous theoretical benchmark for evaluating quantum sensor sensitivity; however, a comprehensive framework for quantifying the fundamental limits of Rydberg-atom microwave electrometers remains lacking. In this work, we establish such a framework by deriving the Fisher information for slope detection and establishing its connection to sensitivity through the signal-to-noise ratio, leading to an analytical expression jointly determined by photon shot noise and atomic response. Numerical implementation with real parameters in cesium vapor systems reveals a Fisher-optimized sensitivity below the $\mathrm{nV}\mathrm{c}{\mathrm{m}}^{-1}\mathrm{H}{\mathrm{z}}^{-1/2}$ level, highlighting a substantial potential for sensitivity increase in practical experiments through the suppression of technical noise. Importantly, the theory predicts that subnanovolt sensitivity is robust regarding moderate variations in system parameters, thereby delineating both the ultimate sensitivity and the optimal operational regime of Rydberg-atom microwave electrometers.

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• Electric field effects
• Electro-optical spectra
• Metrology
• Quantum Fisher information
• Quantum measurements
• Quantum sensing
• Rydberg atoms & molecules
• Microwave techniques