USTC, UCL and PTB published a full article in the journal Science Advances describing significant progress in a spectroscopic gas thermometer using CO molecules.

PriSpecTemp Project Management Board member Dr. Rainer Stosch (PTB) said: “Classical, rotationally resolved absorption spectroscopy has long underpinned SI-traceable gas measurements. Our new Science Advances study shows how double-wavelength coherent multidimensional spectroscopy (DW-CMDS) lifts this foundation: by resolving molecular line-intensity ratios at the 0.003% level and enabling line-intensity ratio thermometry (LRT) with sub-millikelvin precision, we overcome long-standing limits from pressure broadening and thermal population effects. For PriSpecTemp, this delivers practical metrology—tighter inputs for temperature determination, stronger traceability and uncertainty evaluation, and credible pathways to future CMC claims in gas composition and isotopic analysis—while reducing reliance on physical reference materials.” 

Corresponding authors Prof. Shuiming Hu (USTC) and Dr. Yan Tan (USTC) said: "In this work, our team pioneered the development of dual-wavelength cavity mode dispersion spectroscopy technology, which reduced the measurement uncertainty of molecular line intensity ratios to 0.003%. For the first time, we achieved primary gas thermometry based on an optical thermometry that can be directly traced to the Boltzmann constant. The statistical uncertainty was as low as 0.5 mK, improving the accuracy by more than tenfold compared with similar work reported in previous literature."

The link to the Science Advances:

  https://www.science.org/doi/10.1126/sciadv.adz6560

🔬 Quantum Chemistry Enables High Accuracy in CO Spectroscopy !

PriSpecTemp partner UCL published in the Journal of Quantitative Spectroscopy and Radiative Transfer (https://doi.org/10.1016/j.jqsrt.2025.109510) where researchers have achieved subpercent accuracy across all rovibrational bands in calculating carbon monoxide (CO) line intensities using purely ab initio methods. This represents a significant milestone in molecular spectroscopy and computational chemistry.

Accurate line intensities are critical across a wide range of scientific and industrial applications:

• 🌍 Environmental Monitoring: Enhanced precision in measuring atmospheric CO improves climate models and pollution tracking.

• 🏭 Industrial Applications: Supports advanced sensors for combustion analysis, safety monitoring, and process control in industrial settings.

• 🔭 Astrophysics: Enables better interpretation of CO spectra in exoplanet and stellar atmospheres, aiding in the search for habitable worlds.

• ⚖️ Metrology: Provides first-principles reference data for developing primary standards for temperature and pressure, reducing dependence on empirical calibrations and supporting traceability in optical gas sensing.

• ⚛️ Scientific Research: Provides a reliable benchmark for quantum chemistry methods, reducing the need for empirical fittings and supporting future studies on other molecules

PriSpecTemp partners PTB, UMK, and DLR participated in an international measurement campaign (pilot study CCQM-P229) organized under the auspices of the Consultative Committee for Amount of Substance (CCQM).

Six laboratories carried out independent measurements of ro-vibrational line intensities in the 3-0 vibrational band of ¹²C¹⁶O using three different spectroscopic techniques: CRDS, CMDS and FTIR. This study showed consistency between the experimental and theoretical band intensities at the sub per mille level- reducing uncertainty in this quantity by more than an order of magnitude.

PriSpecTemp collaborator Dr. Joseph T. Hodges, Leader of the Optical Measurements Group at NIST, who coordinated this pilot study within the CCQM-GAWG Task Group on Advanced Spectroscopy (TGAS) said:  "The main goal of TGAS is to demonstrate and realize the enormous potential of rotationally resolved, linear absorption spectroscopy as a primary measurement method to make SI-traceable measurements of amount-of-substance and isotopologue composition as well as pressure and temperature.  Spectra of this type depend on intrinsic quantum-mechanical properties of molecules such as transition moments, molecular Hamiltonians etc., which can be accurately determined from complementary laboratory measurements and ab initio calculations.  Moreover for maximizing progress, the TGAS brings together expertise in gas standards metrology and the molecular spectroscopy communities- thus leveraging best-practices in the measurement science of gas samples with the most advanced spectroscopic experiments and theoretical models available."

This work is published in the journal Metrologia:

J T Hodges, K Bielska, M Birk, R Guo, G Li, J S Lim, D Lisak, Z D Reed and G Wagner, Metrologia 62 08006 (2025) https://iopscience.iop.org/article/10.1088/0026-1394/62/1A/08006

PriSpecTemp's project partner, the University of Helsinki, published an article titled "Reference-free dual-comb spectroscopy with inbuilt coherence," in the prestigious journal Optics Letters [1]. 

Dual-comb Fourier-transform spectroscopy (FTS) combines the advantages of laser spectroscopy and classic FTS, offering high precision, speed, and broadband coverage. The first successful demonstration of dual-comb spectroscopy in gas thermometry was reported in 2018 by Shimizu et al. at the National Metrology Institute of Japan [Shimizu et al. 2018]. However, improving the signal-to-noise ratio of the measurements is critical in order to enhance the accuracy of derived gas temperatures. Another important area for further development is to simplify the experimental implementation and ensure its robustness for practical applications.

Prof. Markku Vainio, the last author of the Optics Letters article, explains the key enabler for this scientific advance: "Dual-comb spectroscopy has proven a powerful technology for high-precision gas spectroscopy, but its implementation is often technically involved, requiring optical phase locking and fast control electronics. In our work, we have shown that the experimental implementation can be significantly simplified without sacrificing the measurement precision. This is a step towards practical and accurate gas thermometry using dual-comb spectrometers."

This work is published in the journal Optics Letters.

M. Roiz, S. Larnimaa, T. Uotila, M. Närhi, and M. Vainio, "Reference-free dual-comb spectroscopy with inbuilt coherence," Opt. Lett. 49, 2473-2476 (2024). https://doi.org/10.1364/OL.521866

Reference:  Y. Shimizu et al., “Molecular gas thermometry on acetylene using dual-comb spectroscopy: analysis of rotational energy distribution,” Appl. Phys. B 124, 71 (2018). https://doi.org/10.1007/s00340-018-6933-x