April 08, 2026 -- Understanding how the simplest constituents of matter interact with each other is the foundation of modern physics. Comparing quantities that can be both measured and calculated to high precision, enables tests of the most foundational models that describe the universe. The molecular hydrogen ion HD+, a bound system containing just a proton, a deuteron, and a single electron, provides an excellent platform for such tests. In HD+, the spins of the constituents interact with each other, giving rise to what is known as hyperfine structure. Researchers at the ALPHATRAP Penning trap experiment at the Max-Planck-Institut für Kernphysik (MPIK) have isolated and trapped a single HD+ ion and were able to precisely investigate its hyperfine structure.

The team was able to determine the magnetic properties of the electron bound to the proton and deuteron, the bound electron g factor, to a relative precision of 2 parts in 10 billion. They then compared this value to newly published theoretical predictions with even slightly better precision and found excellent agreement. Their measurements could also probe the interactions between the electron and the proton as well as between the electron and deuteron which quantum theory can also predict.

In recent years, precision measurements with the simplest molecules, such as HD+, have become an important testbed for quantum theory and enable researchers to determine fundamental quantities such as the mass of the proton relative to the mass of the electron, which are important pillars of the underlying theories.

Unlike previous investigations into single-electron molecules, Matthew Bohman from MPIK remarks “we were able to use a single HD+ ion and detect its state by measuring the tiny currents the charged particle induces on nearby trapping electrodes”. “An important aspect”, adds lead author of the study, Charlotte König, “is that the same measurement sequence can be used for antimatter molecules. Once it becomes possible to create these molecules, they can be trapped and detected in exactly the same way.”

For now, though, the group hopes their work will inspire further theoretical and experimental studies of simple molecular ions. “We’ve been able to compare experiment and theory to unprecedented precision in the molecular ground state of HD+” says ALPHATRAP group leader Prof. Sven Sturm. “We soon hope to perform similar measurements when the ion is excited so that it vibrates and rotates. We can induce these excitations using highly stable lasers to achieve even higher precision and test different aspects of the theory”.

The collaborative effort involved researchers from not only MPIK but from the Heinrich-Heine-Universität Düsseldorf (Düsseldorf, DE), Laboratoire Kastler Brossel (Paris, FR), the University of Kassel (Kassel, DE), and the Bulgarian Academy of Sciences (Sofia, BG). The publication describing this work appears in Physical Review Letters.