A finely tuned laser pulse excites “forbidden” vibrations in a single nitrogen molecule. A calcium ion is used as a sensor to detect the state of the nitrogen molecule using quantum-logic spectroscopy.
Image: Aleksandr Shlykov, University of Basel
Scientific Frontline: Extended "At a Glance" Summary: Forbidden Molecular Vibrations
The Core Concept: Forbidden molecular vibrations are highly improbable quantum jumps between the internal energy levels of a molecule. Although these transitions are remarkably weak, their exceptionally well-defined frequencies make them ideal for ultra-precise quantum measurements.
Key Distinction/Mechanism: Unlike standard "allowed" transitions, forbidden vibrations can be over a billion times weaker. Instead of using traditional chemical detection methods that destroy the sample, researchers employ quantum-logic spectroscopy to trap a charged nitrogen molecule alongside a calcium messenger ion, using tuned lasers to measure the nitrogen molecule's state through the calcium ion's response without altering the target molecule.
Origin/History: Researchers at the University of Basel first observed forbidden vibrations in a molecular ion in 2014, though the measurement process destroyed the molecules. In 2026, the same research team successfully controlled these transitions in single trapped molecules using non-destructive techniques.
Major Frameworks/Components:
- Quantum-Logic Spectroscopy: A non-destructive method that utilizes a glowing messenger ion to read the quantum state of a dark target molecule.
- Reversible Excitation: The use of sequential laser pulses to excite a molecule's vibration and return it to its baseline state, allowing for repeated measurements on the same molecule.
- Trapped Molecular Ions: Positively charged nitrogen molecules isolated in a vacuum to study complex rotational and vibrational energy levels free from external interference.
Branches of Science: Quantum Chemistry, Molecular Physics, Quantum Metrology, Spectroscopy, Atomic Physics, and Optical Physics.
Future Application: Controlling forbidden excitations provides a foundation for developing extremely accurate infrared molecular clocks and stable quantum bits for advanced quantum technology.
Why It Matters: Achieving unprecedented precision in measuring molecular vibrations enables physicists to rigorously test the fundamental laws of nature, such as determining whether the masses of protons and electrons remain constant over time.
Controlling "forbidden" excitations of vibrations in molecules could form the basis of extremely accurate clocks, quantum bits, and precise tests of the fundamental laws of physics. Researchers at the University of Basel have identified and controlled such "forbidden" transitions in single, isolated nitrogen molecules without destroying them.
The green shimmer of the northern lights comes from a quantum jump that physicists call "forbidden." The term is a misnomer, as such transitions between the energy levels of atoms and molecules are not impossible, only extremely unlikely. Such excitations can be more than a billion times weaker than ordinary "allowed" transitions.
This weakness, however, provides utility for precision measurement. A forbidden transition can have an extraordinarily well-defined frequency. In atoms, such transitions already underpin today’s most precise clocks. In molecules, with their far more complex energy levels, they have remained very difficult to find and control.
A research team led by Professor Stefan Willitsch at the University of Basel’s Department of Chemistry has coherently controlled these transitions in single trapped molecules. The results have been published in the journal Nature Communications.
A Calcium Ion as a Messenger
The team studied individual nitrogen molecules that carry a positive electrical charge, known as molecular ions. In 2014, Willitsch’s group was the first to observe forbidden vibrations in a molecular ion, but the chemical detection method used at the time destroyed the sample of molecules 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,” says Willitsch.
To achieve this, the researchers utilized quantum-logic spectroscopy, in which a second ion acts as a messenger. They trapped a single charged nitrogen molecule together with a charged calcium atom. The nitrogen molecule does not emit light that the researchers can readily detect, but the calcium atom can be made to glow with laser light.
A carefully tuned laser sets the pair in motion only if the nitrogen molecule is in its lowest energy state of vibration and rotation. By observing the calcium ion’s response, the researchers gain insight into the nitrogen molecule’s state without changing it.
Measuring the Same Molecule Repeatedly
To find the light frequencies that excite the molecule’s "forbidden" vibrations, the team varied the frequency of another laser. At the right frequency, a laser 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,” says Dr. Aleksandr Shlykov, first author of the study. These spectral lines mark the frequencies at which the molecule absorbs light.
Combining measurements on several hundred individually prepared molecules, the team identified different components of the transition. This enabled them to determine the fundamental vibrational frequency of the molecule 10 times more precisely than previous measurements.
From Molecular Clocks to New Physics
Several of the studied transitions are predicted to be remarkably insensitive to external disturbances. This stability makes them useful as precise references for measuring time.
“This makes them highly promising candidates for future molecular clocks and qubits,” says Willitsch.
Precisely measuring molecular vibrations could also help test fundamental laws of physics. Researchers could test, for example, whether the masses of protons and electrons stay constant over time.
NCCR Precision and Basel Quantum Center
Professor Stefan Willitsch is co-director of the National Centre of Competence in Research (NCCR) "Precision," which runs a comprehensive research program on precision measurements, focusing on the physical quantities of time and frequency. Its goal is to raise the accuracy and sensitivity of measurements to unprecedented levels and, in doing so, to explore previously unknown physical phenomena beyond the Standard Model of particle physics.
He is also part of the Basel Quantum Center at the University of Basel, which integrates and coordinates the quantum science and technology activities in the Basel trinational region. It brings together over 200 researchers from 18 professorships in theoretical and experimental quantum science in the physics and chemistry departments at the University of Basel.
Funding: This work was supported by the Swiss National Science Foundation, grant nrs. 200021_204123, TMAG-2_209193, the European Partnership on Metrology (Funder ID: 10.13039/100019599, grant nr. 23FUN04 COMOMET) and the University of Basel. Open access funding provided by University of Basel
Published in journal: Nature Communications
Title: Quantum-logic spectroscopy of forbidden vibrational transitions in single nitrogen molecular ions
Authors: Aleksandr Shlykov, Meissa L. Diouf, Richard Karl, Mikolaj Roguski, Umesh C. Joshi, and Stefan Willitsch
Source/Credit: University of Basel | Klara Bangert
Edited by: Scientific Frontline
Reference Number: qs111026_01
