Sub-Doppler Frequency Metrology in HD for Tests of Fundamental Physics

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Weak transitions in the (2,0) overtone band of the hydrogen deuteride molecule at λ=1.38 μm were measured in saturated absorption using the technique of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Narrow Doppler-free lines were interrogated with a spectroscopy laser locked to a frequency comb laser referenced to an atomic clock to yield transition frequencies [R(1)=217105181895(20) kHz; R(2)=219042856621(28) kHz; R(3)=220704304951(28) kHz] at three orders of magnitude improved accuracy. These benchmark values provide a test of QED in the smallest neutral molecule, and they open up an avenue to resolve the proton radius puzzle, as well as constrain putative fifth forces and extra dimensions.

Original languageEnglish
Article number153002
Pages (from-to)1-5
Number of pages5
JournalPhysical Review Letters
Issue number15
Early online date9 Apr 2018
Publication statusPublished - 13 Apr 2018


The authors wish to thank Prof. J. Gauss (Mainz) for a calculation of hyperfine constants in HD and levels. P. D. is supported by the CNRS. W. U. acknowledges the European Research Council for an ERC-Advanced Grant under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 670168).

FundersFunder number
Horizon 2020 Framework Programme670168
European Research Council
Centre National de la Recherche Scientifique


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