A quantum-metrological scheme is proposed for resonant nanophotonic sensors based on subwavelength slot Fabry-Perot resonators. The parameter encoding is modeled as a phase-amplitude quantum channel in one arm of a Mach-Zehnder interferometer. For coherent and Gaussian probe states under linear loss, the quantum Fisher information (QFI) is computed. It is shown that even at the quantum limit, the optimal estimation precision is determined by the generator of parameter-dependent phase shifts, not by the cavity's quality factor. Consequently, the operating point that maximizes the QFI generally does not coincide with the resonance maximum. Quantum resources increase sensitivity but do not redefine the optimal geometry. These results establish physically transparent design principles for quantum-enhanced nanophotonic sensors.
Building ultrasensitive sensors for light measurements and spectroscopy starts with tiny chambers where light ricochets like an echo in a canyon. Previously, engineers chased the quality factor—the duration of this echo: the longer it rings, the more noticeable the slightest changes.
But the laws of the quantum world, derived from the uncertainty principle, point to a different path. Precision isn't limited by the echo's duration, but by how sharply the wave's phase shifts—that is, the moment when the crest hits the chamber wall. In other words, we should listen not to the volume, but to the shift in rhythm.
This was confirmed by an experiment with a Michelson interferometer—a device that splits light into two beams traveling at the speed of light, then recombines them. It turned out that the optimal tuning lies where the echo has almost faded, but the phase responds most sharply to changes.
🎯 A phase-shift-based sensor can detect a single bacterium landing on it—it's that sensitive to the tiniest changes.