Quantum Sensors Achieve Optimal Precision Beyond High-Q Limits

Resonant nanophotonic sensors offer promising avenues for precision measurement, but designing these devices for ultimate sensitivity presents a surprising challenge, as conventional wisdom prioritises maximising the cavity quality factor. J. Sumaya-Martinez investigates this issue, demonstrating that achieving the highest possible precision does not necessarily require the highest-Q resonance.

This research develops a new theoretical framework for analysing these sensors, revealing that the key to optimal performance lies in the generator of parameter-dependent phase shifts, rather than simply increasing the cavity’s ability to store light. These findings provide a crucial step forward in designing nanophotonic sensors, offering physically transparent principles that move beyond the limitations of traditional approaches and paving the way for more sensitive and accurate measurements. Model parameter encoding functions as a phase-and-loss quantum channel embedded within one arm of a Mach-Zehnder interferometer.

The team derives the quantum Fisher information (QFI) for coherent and Gaussian probe states under linear loss, demonstrating that, even at the quantum limit, optimal estimation precision is governed by the generator of parameter-dependent phase shifts, dominated by interface contributions, rather than by the cavity quality factor. Consequently, the operating point that maximizes QFI does not generally coincide with the maximum-Q resonance. Quantum resources enhance sensitivity, but do not redefine the optimal geometry, and the results provide physically transparent design principles. Quantum Precision via Nanophotonic Resonance Enhancement Quantum metrology establishes fundamental limits on the precision with which physical parameters can be estimated using quantum systems. For a parameter encoded in a family of quantum states, the quantum Cramér-Rao bound dictates that the variance of any estimator is bounded by the inverse of the quantum Fisher information (QFI). The QFI represents the maximum sensitivity permitted by quantum mechanics, quantifying the ultimate precision achievable with a given measurement. Resonant nanophotonic structures, including plasmonic cavities and photonic crystal resonators, are widely used as optical sensors, and their performance is often assessed using figures of merit such as resonance shift, linewidth, or quality factor Q. A common intuition suggests that higher Q implies superior sensitivity due to sharper resonances, however, linewidth-based arguments alone do not provide a bound on estimation precision, which depends on how the parameter is encoded and the statistics of the measurement. Recent classical Fisher-information analyses of metallic subwavelength slit resonators demonstrated that the operating point maximizing estimation precision does not necessarily coincide with the maximum-Q configuration, instead being governed by the phase sensitivity of the structure to the parameter of interest, with interface-induced phase contributions playing a dominant role.

This research investigates whether access to quantum resources and optimal measurements restore the primacy of high-Q resonances, or if the conclusion that Q is not optimal persists at the quantum limit.

The team embedded a slit sensor in one arm of a Mach-Zehnder interferometer, so that the parameter is estimated through a physically meaningful interferometric phase readout. The results demonstrate that even at the quantum limit, the optimum is governed by the generator through which the parameter is encoded, a phase derivative dominated by boundary contributions, not by resonance sharpness. Quantum resources such as squeezing enhance the achievable precision but do not generally shift the optimal structural point.

The team considered a single subwavelength metallic slit, where the total round-trip phase is determined by the propagation constant and interface-induced phase shifts. The metrologically relevant quantity is the phase at an operating wavelength, and the geometry maximizing Fisher information does not necessarily coincide with the maximum-Q condition. In the quantum extension, the phase derivative becomes the central generator of distinguishability. The slit sensor was modeled as a linear, passive phase shift combined with loss, representing absorption, radiation leakage, and imperfect mode matching. This channel was embedded in one arm of a Mach-Zehnder interferometer to avoid ambiguities of absolute phase measurement. For coherent-state probing and Gaussian detection noise, the output statistics are fully characterized by the mean field and its variance. The QFI can be computed from the covariance matrix and displacement, and the geometry maximizing QFI is determined by the Fabry-Perot phase derivative. Consequently, the maximum of Q does not coincide with the maximum of the generator strength, and quantum resources amplify sensitivity without shifting the optimal geometry. The research clarifies that high-Q is not a reliable proxy for quantum-optimal performance.

Phase Sensitivity Governs Nanophotonic Sensor Precision Scientists have developed a new metrological framework for resonant nanophotonic sensors, focusing on subwavelength Fabry-Perot slit cavities and establishing fundamental limits on estimation precision. The research team modeled parameter encoding as a phase-and-loss channel within a Mach-Zehnder interferometer, deriving expressions for the quantum Fisher information (QFI) using both coherent and Gaussian probe states under linear loss conditions.

Results demonstrate that optimal estimation precision is governed by the generator of parameter-dependent phase shifts, specifically contributions from interface effects, rather than the cavity quality factor, even at the quantum limit. Experiments revealed that the operating point maximizing the QFI does not generally coincide with the maximum-Q resonance, challenging the prevailing intuition that sharper resonances automatically imply superior sensitivity.

The team showed that quantum resources, such as squeezing, enhance sensitivity but do not redefine the optimal structural geometry, confirming that the “Q is not optimal” conclusion persists even with access to quantum resources. Data shows that the structural dependence enters through the square of the phase derivative, rather than the linewidth or quality factor, indicating that interface-induced phase contributions dominate estimation performance. The research team modeled the slit sensor as a linear, passive phase shift combined with loss, embedding it within one arm of a Mach-Zehnder interferometer to avoid ambiguities of absolute phase. For coherent probing, the attainable precision is shot-noise limited, with the QFI scaling with the square of the phase derivative and a loss-dependent factor. Measurements confirm that squeezing enhances precision, but the optimal structural point remains governed by the phase derivative, not resonance sharpness. Specifically, the team achieved results demonstrating that the maximum of the normalized QFI and the generator strength do not necessarily coincide, highlighting the importance of phase sensitivity over resonance quality.

Phase Sensitivity Governs Nanophotonic Sensor Performance Researchers have developed a refined understanding of how to maximize the sensitivity of nanophotonic sensors, specifically those based on subwavelength slit cavities. Building upon established principles of Fisher information analysis, the team modeled these sensors as a phase-and-loss channel within a Mach-Zehnder interferometer, allowing for precise estimation of parameters through interferometric phase readout. Their work demonstrates that optimal sensor performance is governed by the rate at which the structure’s phase shifts with changes in the parameter being measured, rather than simply by maximizing the cavity’s quality factor, a finding that challenges conventional design approaches. The investigation extends these principles to the quantum limit, exploring whether utilizing quantum resources like squeezed light could restore the importance of high-quality factor resonances. However, the results confirm that even with these advanced resources, the optimal sensor geometry remains dictated by phase sensitivity, not resonance sharpness, and that quantum enhancements improve precision without altering this fundamental principle. This suggests that focusing on designs that maximize phase shifts offers the most effective path towards creating highly sensitive nanophotonic sensors. 👉 More information 🗞 Quantum Fisher-information limits of resonant nanophotonic sensors: why high-Q is not optimal even at the quantum limit 🧠 ArXiv: https://arxiv.org/abs/2512.14899 Tags: