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  • Open Access

Quantum Metrology through Spectral Measurements in Quantum Optics

Alejandro Vivas-Viaña1,2,* and Carlos Sánchez Muñoz2,†

  • *Contact author: vivasa@chalmers.se
  • †Contact author: carlos.sanchez@iff.csic.es
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PRX Quantum 7, 010346 – Published 6 March, 2026

DOI: https://doi.org/10.1103/xrgc-lrj8

Abstract

Continuously monitored quantum systems are emerging as promising platforms for quantum metrology, where a central challenge is to identify measurement strategies that optimally extract information about unknown parameters encoded in the complex quantum state of emitted radiation. Different measurement strategies effectively access distinct temporal modes of the emitted field, and the resulting choice of mode can strongly impact the information available for parameter estimation. While a ubiquitous approach in quantum optics is to select frequency modes through spectral filtering, the metrological potential of this technique has not yet been systematically quantified. We develop a theoretical framework to assess this potential by modeling spectral detection as a cascaded quantum system, allowing us to reconstruct the full density matrix of frequency-filtered photonic modes and to compute their associated Fisher information. This framework provides a minimal yet general method to benchmark the performance of spectral measurements in quantum optics, allowing to identify optimal filtering strategies in terms of frequency selection, detector linewidth, and metrological gain accessible through higher-order frequency-resolved correlations and mean-field engineering. These results lay the groundwork for identifying and designing optimal sensing strategies in practical quantum-optical platforms.

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Popular Summary

Quantum technologies promise to revolutionize how we measure the world, from detecting tiny magnetic fields to searching for gravitational waves. At the core of this effort lies quantum metrology, which aims to push measurement precision to the fundamental limits allowed by quantum mechanics.

Most quantum systems are open: they constantly interact with their surroundings and release light (radiation). This poses a major challenge for metrology: this emitted light contains valuable clues about the system, but much of that information is hidden in ways that are not easy to detect. Traditional detection methods, such as measuring intensity, miss much of the hidden information carried by this emitted light, leaving much of its potential untapped.

In our work, we show that two well-known tools—measuring the color (frequencies) of photons and studying when different colors are detected together (correlations)—can be transformed into powerful methods for quantum metrology. Our study reveals that these measurements unlock hidden information, approaching the ultimate limits of what is possible in quantum mechanics. Crucially, these strategies rely on standard techniques already available in many laboratories, making them directly relevant to ongoing experiments.

Our results demonstrate that something as familiar as analyzing the color of light can become a powerful new resource for quantum sensing. This approach bridges quantum optics and quantum metrology, paving the way for practical sensors with unprecedented precision. Looking ahead, our work opens opportunities for better tools in areas ranging from fundamental physics experiments to emerging quantum technologies.

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PRX Quantum 7, 010346– Published 6 March, 2026
  • Received 19 September 2025
  • Revised 30 November 2025
  • Accepted 22 January 2026
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