The possibility to control “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 now 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 little misleading 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.
Yet this weakness is also what makes them so valuable. 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.
Now, a research team led by Professor Stefan Willitsch at the University of Basel’s Department of Chemistry has identified and coherently controlled such 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 Stefan Willitsch.
To achieve this, the researchers turned to a technique called quantum-logic spectroscopy, in which a second ion acts as a messenger. They trap 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.
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, co-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 ten times more precisely than previous measurements.
From molecular clocks to new physics
Several of these transitions studied 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 Stefan 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.
Nature Communications
Quantum-logic spectroscopy of forbidden vibrational transitions in single nitrogen molecular ions.