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  • Featured in Physics
  • Open Access

Entangled Dual-Comb Spectroscopy

Abdulkarim Hariri1, Shuai Liu1, Haowei Shi2, Quntao Zhuang2,3, Xudong Fan4, and Zheshen Zhang1,*

  • *Contact author: zszh@umich.edu
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Phys. Rev. X 15, 041009 – Published 15 October, 2025

DOI: https://doi.org/10.1103/plh2-cr8s

Abstract

Optical frequency combs have emerged as a cornerstone for a wide range of areas, including spectroscopy, ranging, optical clocks, time and frequency transfer, waveform synthesis, and communications. However, quantum-mechanical fluctuations of the optical carrier impose fundamental performance limits on the precision of classical optical frequency combs, particularly in their use for interferometry and spectroscopy. Entanglement, as a quintessential quantum resource, allows for surpassing the fundamental limits of classical systems. Here, we introduce entanglement into the realm of optical frequency combs, formulating entangled dual-comb spectroscopy (EDCS) that surmounts the fundamental limits of classical DCS. EDCS capitalizes on tailored entangled structures across the frequency comb, enabling simultaneous detection of all comb lines below the standard quantum limit of classical DCS. Applying EDCS in gas detection, we achieve a 2.6-dB enhancement in signal-to-noise ratio and a 1.7-fold reduction in integration time over classical DCS, rendering EDCS particularly suited for dynamic chemical and biological sensing, where fast, precise measurements subject to power constraints are required. EDCS opens a new avenue for exploiting quantum frequency combs, underscoring their prospects in a plethora of applications in precision metrology, spectroscopy, and timekeeping.

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synopsis

Entanglement Boosts Spectroscopy

Published 15 October, 2025

A new spectroscopy method outperforms its rivals by pairing an ordinary frequency comb with an entangled one.

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Popular Summary

Optical frequency combs are essential tools in precision measurement, powering technologies such as optical clocks and gas detectors. However, classical combs face a fundamental limitation: Quantum fluctuations of photons introduce noise that can only be reduced with more laser power or long averaging times. In this work, we show that quantum entanglement can overcome this limitation. We introduce entangled dual-comb spectroscopy (EDCS), the first quantum dual-comb approach that outperforms the best classical methods under the same optical power, boosting the signal-to-noise ratio and reducing measurement time.

To achieve this, we combine a bright classical comb with a specially engineered quantum comb containing many entangled frequency pairs. This generates an entangled signal comb with power comparable to that of traditional spectroscopy sources. When this entangled signal comb probes a sample, its correlations are read out by a reference comb at the detector. The entanglement suppresses quantum noise across the spectrum, as demonstrated in experiments with hydrogen cyanide vapor. All absorption lines are resolved simultaneously with precision beyond the classical limit, showing that EDCS enables faster and more sensitive spectroscopy.

EDCS could be implemented in compact, chip-based sensors for environmental monitoring, biomedical diagnostics, chemical detection, and materials testing. Beyond sensing, entangled frequency combs offer opportunities for faster optical clock readout, better calibration of astronomical spectrographs, and entirely new approaches to quantum metrology. Improvements in squeezing levels, spectral range, and source-detector integration will bring us closer to practical, field-ready entangled frequency-comb instruments.

Article Text

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Outline

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Phys. Rev. X 15, 041009– Published 15 October, 2025
  • Received 13 March 2025
  • Revised 28 June 2025
  • Accepted 28 August 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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