# University of Basel controls forbidden vibrations in single N2+ ions with quantum-logic spectroscopy

- Published: 2026-11-12 21:28 UTC
- Section: Science › Physics
- Tags: forbidden vibrational transitions, quantum-logic spectroscopy, N2+ molecular ion, single molecule spectroscopy, calcium ion, molecular clock, rotational-vibrational transitions, precision spectroscopy, University of Basel, Stefan Willitsch
- URL: https://realtech.media/news/rt-49d58559ce21dec60406f6d8_university_of_basel/

**Physicists identified and coherently drove forbidden vibrational transitions in single trapped nitrogen ions, refining the fundamental vibration frequency tenfold.**

![University of Basel controls forbidden vibrations in single N2+ ions with quantum-logic spectroscopy](https://realtech.media/media/2026/11/8ecee4324aa5e42d.jpg)
*National Institute of Standards and Technology, Wikimedia Commons, Public domain*

Researchers at the University of Basel, led by Professor Stefan Willitsch, have identified and coherently manipulated electric-quadrupole rotational-vibrational transitions in single trapped molecules, according to a paper in Nature Communications. The press release is dated 10 November 2026. The object was the nitrogen molecular ion N2+. The team resolved individual hyperfine-Zeeman-rotational components of its fundamental vibrational transition and performed coherent population transfer between energy levels [1].

## Why forbidden transitions matter

A "forbidden" transition is not impossible, only extremely unlikely. Such excitations can be more than a billion times weaker than ordinary allowed ones. The same weakness allows an extraordinarily well-defined frequency. In atoms, these transitions already underlie advanced quantum computers, atomic clocks and quantum sensors [1].

In molecules, the complex energy-level structure has made such transitions hard to find and control. In 2014, Willitsch's group was the first to observe forbidden vibrations in a molecular ion. The chemical detection used then destroyed the sample in every measurement cycle.

## Quantum-logic spectroscopy on one molecule

The new work uses quantum-logic spectroscopy. A single charged nitrogen molecule is trapped together with a calcium ion. The nitrogen does not emit light that is easy to detect, but the calcium ion can be made to glow under laser light. A tuned laser sets the pair in motion only if the nitrogen molecule is in its lowest vibrational and rotational state. The response of the calcium ion therefore reveals the molecular state without changing it [1].

To find the transition frequencies, the team varied the frequency of a second laser. At the right frequency, one pulse excites the vibration and another returns the molecule to its initial state. Because the process is reversible, the same molecule can be measured repeatedly, which the authors say made the search for such faint lines feasible.

Combining measurements on several hundred individually prepared molecules, the team identified different components of the transition. According to the university, this gave the fundamental vibrational frequency of the molecule ten times more precisely than previous measurements.

Several of the studied transitions are predicted to be insensitive to external disturbances. The authors describe them as candidates for molecular clocks and qubits. They also point to precision molecular spectroscopy and searches for new physics, for example tests of whether the proton and electron masses stay constant over time [1].

## Sources

1. [Nature Communications](https://doi.org/10.1038/s41467-026-78680-w)
2. [EurekAlert](https://www.eurekalert.org/news-releases/1151067)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-49d58559ce21dec60406f6d8_university_of_basel/