Plate Nº 32 · recorded November 11, 2026
PhysicsReported finding
Researchers Control 'Forbidden' Molecular Vibrations Without Destroying Them
Basel physicists repeatedly measured 'forbidden' vibrations in single nitrogen molecules, boosting precision tenfold and paving the way for molecular clocks and qubits.
By Priya Raman3 min read699 words
In brief
- University of Basel researchers controlled forbidden vibrations in single nitrogen molecular ions, published in Nature Communications (2026).
- The new quantum-logic method measures the same molecule repeatedly without destroying it, unlike the group's 2014 experiment.
- The fundamental vibrational frequency was determined 10 times more precisely than in previous measurements.
- Forbidden transitions can be more than a billion times weaker than allowed transitions.
- Measurements combined data from several hundred individually prepared molecules.
Physicists at the University of Basel have, for the first time, identified and coherently controlled "forbidden" vibrations in single isolated nitrogen molecules — and measured the same molecule again and again without destroying it. The team determined the molecule's fundamental vibrational frequency ten times more precisely than previous measurements allowed, according to a study published in the journal Nature Communications in 2026.
The achievement opens a path toward molecular clocks, quantum bits and precision tests of the fundamental laws of physics, the researchers say.
What makes a vibration "forbidden"?
The green shimmer of the northern lights offers a familiar example of the phenomenon. That glow comes from a quantum jump that physicists label "forbidden" — a term that is somewhat misleading. Such transitions between the energy levels of atoms and molecules are not impossible, only extremely unlikely. Forbidden excitations can be more than a billion times weaker than ordinary "allowed" transitions.
That very weakness is what makes them valuable. A forbidden transition can possess an extraordinarily well-defined frequency (the rate at which a quantum system's state repeats). In atoms, such transitions already underpin today's most precise clocks. Molecules, with their far more complex energy levels, have resisted this kind of control.
How did the Basel team do it?
Professor Stefan Willitsch, who leads the work at Basel's Department of Chemistry, has chased these signals for over a decade. In 2014, his group became the first to observe forbidden vibrations in a molecular ion (a molecule carrying a positive electrical charge). The chemical detection method available then, however, destroyed the sample in every measurement cycle.
"To achieve the highest precision, it is much better to perform experiments on single molecules, and not lose them in the process," Willitsch said.
The solution is a technique called quantum-logic spectroscopy, which uses a second ion as a messenger. The researchers trap a single charged nitrogen molecule together with a charged calcium atom. The nitrogen molecule does not emit light that the team can readily detect; the calcium atom, by contrast, can be made to glow under laser light.
A carefully tuned laser sets the pair in motion only if the nitrogen molecule sits in its lowest state of vibration and rotation. By watching the calcium ion's response, the researchers read the nitrogen molecule's state without changing it.
To locate the light frequencies that excite the molecule's forbidden vibrations, the team varied the frequency of a second laser. At the right frequency, one pulse excites the vibration and a second pulse returns the molecule to its initial state. Because the process is reversible, the same molecule can be measured repeatedly.
"Using the same molecule again and again was crucial. It made the search for such faint lines feasible," said Dr. Aleksandr Shlykov, co-first author of the study. The spectral lines in question mark the frequencies at which the molecule absorbs light.
By combining measurements on several hundred individually prepared molecules, the team identified different components of the transition. This is what yielded the tenfold improvement in precision for the molecule's fundamental vibrational frequency.
Why does it matter?
Several of the transitions the team studied are predicted to be remarkably insensitive to external disturbances. That stability makes them useful as fixed references for measuring time.
"This makes them highly promising candidates for future molecular clocks and qubits," Willitsch said. A qubit (quantum bit) is the basic unit of information in a quantum computer.
Precisely measuring molecular vibrations could also help test fundamental physics. Researchers could probe, for example, whether the masses of protons and electrons stay constant over time — a question that touches on the foundations of physical law.
The findings remain early-stage, and translating single-molecule control into working clocks or quantum devices will require substantial further work. The result nonetheless demonstrates that fragile, faint quantum signals can be hunted in individual molecules and pinned down with repeatability rather than destruction.
One molecule, measured a hundred ways, kept its silence intact.
Publication: Aleksandr Shlykov et al., "Quantum-logic spectroscopy of forbidden vibrational transitions in single nitrogen molecular ions," Nature Communications (2026). DOI: 10.1038/s41467-026-78680-w. Preprint: arxiv.org/abs/2603.10553
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Senior reporter covering people, launches and day-to-day practice in general science news at SciBeat.
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