Precision spectroscopy for the fundamental questions of physics
Physicists seek answers to a number of fundamental questions: Are natural constants really constant? Is nature governed by four and only four forces? What is dark matter? Researchers hope to find the answers by studying simple quantum systems very meticulously. In recent years, a new family of such quantum systems has attracted particular attention: molecular hydrogen ions, the simplest of all molecules. One member of this family is the molecular ion HD⁺, which consists of one proton and one deuteron bonded by a single electron. As such, this is a very simple molecule that can on the one hand be used as a model system for more complex molecules and on the other serve as a convenient system for conducting fundamental research into the properties and interactions of the electron, the proton and the deuteron. Using this molecule, the Helmholtz Prize winners at the Heinrich Heine University Düsseldorf have developed and tested an innovative high-precision spectroscopy technique that can now be applied to other molecular ions. They further succeeded in enhancing the precision of some aspects of fundamental physics.

One characteristic shared by the vast majority of molecular ions is that cooling them directly with laser light is a complex and costly undertaking. But overcoming this challenge is well worth the effort because the precision spectroscopy of molecular ions promises to deliver insights that complement our knowledge of atoms and neutral molecules. In the early 2000s, researchers at Heinrich Heine University were the first to venture into this area of research. Their approach is to capture two ensembles of different ion types – one consisting of molecular ions and one of atomic ions – in a single ion trap. Here, the atomic ions are cooled by laser (as is done in optical atomic clocks and trapped-ion-based quantum computers) and they, in turn, cool the molecular ions through electrical interaction. This cooling dramatically slows the motion of the molecular ions, allowing their properties to be characterized with high accuracy.

Helmholtz Prize in the category of “Precision Measurements in Fundamental Research”
Dr. Soroosh Alighanbari, Dr. Gouri Shankar Giri, Ivan Kortunov, Magnus Roman Schenkel, Prof. Dr. Stephan Schiller For their paper entitled “Most precise determination of a fundamental mass ratio and novel tests of fundamental physics with the HD⁺ molecule using a new technique for precision laser spectroscopy of ions”

Scientific publication
I. V. Kortunov, S. Alighanbari, M. G. Hansen, G. S. Giri, V. I. Korobov, S. Schiller: Proton–electron mass ratio by high-resolution optical spectroscopy of ion ensembles in the resolved-carrier regime. Nature Physics 17, 569–573 (2021)

Contact
Dr. Soroosh Alighanbari, Institute for Experimental Physics, Heinrich Heine University Düsseldorf, 40225 Düsseldorf, Germany phone: +49-211 811-2465, e-mail: soroosh.alighanbari(at)hhu.de