Precision measurements in molecules with frequency combs

Project Information

COMB

Grant agreement ID: 101054704

DOI

10.3030/101054704

EC signature date 20 May 2022

Start date 1 December 2022

End date 30 November 2027

Funded under

• European Research Council (ERC)

Total cost

€ 3 218 398,76

EU contribution

€ 3 218 398,75

Coordinated by

FORSCHUNGSVERBUND BERLIN EV
Germany

Periodic Reporting for period 1 - COMB (Precision measurements in molecules with frequency combs)

Reporting period: 2022-12-01 to 2025-05-31

Summary of the context and overall objectives of the project

Precise frequency measurements enable accurate determinations of physical constants, stringent tests of fundamental theories and searches for possible drifts of the fundamental constants. Simple atoms, hydrogen in particular, have long been the dedicated systems for confronting experimental data and accurate quantum electrodynamics theoretical calculations. With continued progress to ab initio calculations, precision measurements in small molecules, such as molecular hydrogen H_2, are gaining much relevance and may be envisioned as an independent way to determine fundamental constants and to test quantum-chemistry theory. In this project, we will develop an instrument of precision molecular spectroscopy, based on frequency combs, broad spectra composed of equidistant narrow lines whose absolute frequency can be known within the accuracy of an atomic clock. Building on our unique know-how, this revolutionary ultraviolet spectrometer will simultaneously combine broad spectral coverage, Doppler-free resolution and extreme accuracy for precise studies of small molecules. Using two-photon excitation and dual-comb spectroscopy with comb lasers of low repetition frequency, we will devise an optical analogue of the Ramsey-fringe method where many molecular transitions will be simultaneously and unambiguously observed and assigned. While such a spectrometer will enable significant progress in our understanding of the structure of many small molecules, it will first be applied to absolute-frequency measurements of rovibronic transitions in the EF – X system of H_2 around 3000 THz. The measured frequencies can be used to benchmark molecular theory in the involved ground and excited states. They may contribute to an improved determination of the dissociation energy of H2, set new basis for an independent determination of the proton-charge radius and for searches of variations of the proton-electron mass ratio via comparison to astrophysical measurements.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A major achievement has been the possibility to demonstrate to perform broadband spectroscopy at high resolution and at very low light levels. Indeed dual-comb spectroscopy typically required relatively intense laser beams (microwatts at the detector), making it less suitable for scenarios where low light levels are critical, such as harmonic generation in the ultraviolet range. Within the ERC COMB project, we have now shown experimentally that dual-comb spectroscopy can be effectively used in starved-light conditions, at power levels more than a million times weaker than those typically used. The interference signals can be observed in the statistics of the clicks of a photon-counting detector, even when the power is so low, that, on average, only one click is registered over the time of 100 laser pulses. Under such circumstances it is extremely unlikely that two photons, one from each laser, are simultaneously present in the detection path. The experiment cannot be explained intuitively by assuming that a photon exists before detection.
Our results in the ERC COMB project are showcased using two distinct experimental setups with different types of frequency-comb generators, with a signal-to-noise ratio at the fundamental limit. Our achievement highlights the optimal use of available light for experiments, and opens up prospects in challenging scenarios where low light levels are essential. One of our experiments was performed in the near-ultraviolet region, where spectra with resolved-comb lines could be obtained for the first time, as a step towards shorter wavelengths. Furthermore, this establishes a very serious strategy for our future application that is precise vacuum- and extreme-ultraviolet molecular spectroscopy over broad spectral spans. Currently, broadband extreme-UV spectroscopy is limited in resolution and accuracy, and relies on unique instrumentation at specialized facilities. Dual-comb spectroscopy at short wavelengths is particularly challenging and our work provides a promising answer to the pressing problem of dealing with the low power of ultraviolet frequency comb generators produced by non-linear frequency conversion of near-infrared sources. More generally, our results extend the full capabilities of dual-comb spectroscopy to low-light conditions, unlocking novel applications in precision spectroscopy, biomedical sensing, and environmental atmospheric sounding. These results have been published in Nature 627, 289–294 (2024).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our experiments report the first implementation of high-resolution linear-absorption dual-comb spectroscopy in the ultraviolet spectral range. Importantly, they also establish, using two distinct experimental setups with different types of frequency-comb generators, that the full capabilities of dual-comb spectroscopy are extended to starved-light conditions, at power levels more than a million-fold weaker than those commonly employed in dual-comb spectroscopy. By repeatedly achieving a quantum-noise-limited signal-to-noise ratio, an optimal use of the light available for the experiments is achieved. Our photon-level interferometer accurately reproduces the statistics of photon counting, as shown by the signal-to-noise ratio at the fundamental limit, the instrumental line-shape that follows the theoretical expectation and the direct referencing of the frequency scale to a radio-frequency clock. The prospect of harnessing dual-comb spectroscopy at very low light levels may seem counterintuitive. We have experimentally realised such a milestone, which will unlock novel applications.