In a remarkable leap forward for precision physics, researchers at Heinrich Heine University Düsseldorf (HHU), led by Professor Stephan Schiller Ph.D., have harnessed an advanced technique known as Doppler-free laser spectroscopy to probe the molecular hydrogen ion, H₂⁺, with an unprecedented level of accuracy. This breakthrough has enabled them to measure fundamental constants, such as the proton-to-electron mass ratio, with a precision never before achieved. Their findings, published in the prestigious journal Nature, herald a new era in precision measurement and open promising avenues for exploring potential physics beyond the Standard Model.
The molecular hydrogen ion H₂⁺, consisting of just two protons bound with a single electron, represents the simplest molecular system. Its elegant simplicity affords theorists the unique advantage of calculating its properties—particularly its energy levels—with exceptional precision. This theoretical exactitude creates an ideal platform for experimental physicists to perform rigorous tests: by comparing high-precision experimental measurements of H₂⁺ transitions with equally precise theoretical predictions, deviations can be critically examined. Such discrepancies could signal unknown physics or provide clues about the fundamental forces shaping our universe.
Professor Stephan Schiller’s team at HHU has pursued increasingly refined measurement techniques aimed at pushing the boundaries of experimental accuracy. The core motivation behind this quest lies in the detection of ‘new physics’—phenomena that elude the explanatory power of the Standard Model of particle physics. “Our goal,” Schiller elucidates, “is to identify minute discrepancies between theory and experiment by conducting ultra-precise spectroscopy on the H₂⁺ ion. Any such mismatch could provide insight into forces or particles yet undiscovered.”
Dr. Soroosh Alighanbari, a postdoctoral researcher and lead author of the study, elaborates on the broader implications: “Variations in the spectroscopic data may hint at the presence of a hypothetical fifth fundamental force, supplementing the known four forces of nature. Alternatively, these measurements could shed light on hidden extra spatial dimensions that potentially modify gravitational interactions at microscopic scales.” Such profound possibilities elevate the significance of their precise spectroscopic measurements.
The experimental approach at HHU intricately combines ion trapping techniques with laser cooling and laser frequency metrology to probe transition frequencies in trapped H₂⁺ ions. Previously, the team succeeded in performing direct laser spectroscopy on a vibrational transition of H₂⁺; however, this earlier work suffered from measurement imprecision due primarily to Doppler broadening—an effect that arises from the thermal motion of ions, which distorts the spectral lines and limits resolution.
To overcome these limitations, the Düsseldorf physicists innovated a Doppler-free laser spectroscopy method, effectively nullifying Doppler-induced line broadening. This formidable technical achievement demanded simultaneously addressing other perturbing influences such as stray electric and magnetic fields. “We trap molecular ions alongside atomic ions that can be laser cooled,” Dr. Alighanbari explains, “and these cold atoms sympathetically cool the molecular ions, drastically reducing their kinetic energy and motion. But to fully eradicate Doppler broadening, we also implemented a specialized spectroscopy geometry tailored to this purpose.”
The resulting data quality is extraordinary. By accurately measuring vibrational transition frequencies in H₂⁺ devoid of Doppler distortions, the team could infer fundamental constants embedded deeply within quantum mechanics. Since quantum mechanical equations dictate the energy-level structure of atoms and molecules, these constants govern phenomena such as molecular vibration and rotational spectra, and consequently the frequencies of absorbed or emitted electromagnetic radiation during transitions.
Of particular significance is the precise determination of the proton-to-electron mass ratio (m_p/m_e), a dimensionless constant central to molecular physics. Unlike atomic spectroscopy, where electronic transitions dominate, molecular vibrations and rotations are critically dependent on nuclear masses, making molecular ions like H₂⁺ uniquely sensitive probes for m_p/m_e. Professor Schiller emphasizes, “Our molecule-based spectroscopy provides a powerful tool for measuring the proton-to-electron mass ratio with astonishing accuracy—this ratio fundamentally scales particle-mass effects in molecular structures.”
Their results have shattered previous precision records, achieving uncertainty as low as 26 parts per trillion—a three orders of magnitude improvement over former measurements. Notably, this surpasses precision levels attained by Penning-trap mass spectrometry, one of the most advanced mass measurement techniques in existence. Dr. Alighanbari remarks, “Our findings not only confirm prior high-precision determinations but exceed them, demonstrating the robustness and huge potential of molecular ion spectroscopy.”
Beyond refining fundamental constants, these measurements pave the way toward testing fundamental symmetries of nature, notably CPT invariance—the principle that charge conjugation (C), parity transformation (P), and time reversal (T) combined should leave physical laws unchanged. Professor Schiller notes, “The methodology we’ve developed could eventually enable an extraordinarily sensitive CPT test by comparing transitions in H₂⁺ to those in its antimatter counterpart, anti-H₂⁺. Realizing this will hinge on successfully synthesizing the anti-H₂⁺ ion, an endeavor underway at CERN’s antimatter research programs.”
The significance of such CPT tests cannot be overstated. Any violation of CPT invariance would demand a revision of the Standard Model and reshape our understanding of matter-antimatter asymmetry—the enduring mystery of why the universe is composed predominantly of matter rather than equal parts matter and antimatter. Investigating these questions offers a direct window into the origins of the cosmos and the fundamental architecture of physical law.
The HHU team’s work resides at the intersection of quantum technology and fundamental physics. By integrating ion trapping, sympathetic laser cooling, and advanced laser frequency metrology, they have established a novel experimental paradigm. This platform not only enhances measurement precision but also facilitates probing subtle interactions and hypothetical phenomena potentially linked to dark matter, dark energy, or extra spatial dimensions suggested by some unification theories.
In sum, the research carried out by Professor Stephan Schiller and Dr Soroosh Alighanbari represents a landmark achievement in molecular physics and precision metrology. Their Doppler-free laser spectroscopy of H₂⁺ refines a cornerstone fundamental constant with unprecedented exactness and primes the scientific community for future explorations into the universe’s deepest secrets. The horizon is bright for uncovering new physics through the lens of the most elemental molecular system known.
Subject of Research: Precision measurement of molecular hydrogen ion (H₂⁺) transitions for determining fundamental constants and exploring new physics
Article Title: High-accuracy laser spectroscopy of H₂⁺ and the proton-electron mass ratio
News Publication Date: 2025
Web References: https://www.nature.com/articles/s41586-025-09306-2
References: S. Alighanbari, M. R. Schenkel, V. I. Korobov & S. Schiller. High-accuracy laser spectroscopy of H₂⁺ and the proton-electron mass ratio. Nature 644, 69-75 (2025). DOI: 10.1038/s41586-025-09306-2
Image Credits: HHU/Nicolas Stumpe
Keywords
Laser spectroscopy, molecular hydrogen ion, proton-to-electron mass ratio, Doppler-free spectroscopy, fundamental constants, precision measurement, quantum metrology, CPT invariance, antimatter, new physics, ion trapping
