Quantum perturbation-free study of single molecules

A charged nitrogen molecule is interrogated by a charged calcium atom in an optical lattice. (Image: University of Basel, Department of Chemistry)

Researchers of the University of Basel have developed a new method with which individual isolated molecules can be studied precisely, without destroying the molecule or even influencing its quantum state.

Spectroscopic analyses are based on the interaction of matter with light. In typical spectroscopic experiments, a sample containing a large number of molecules is irradiated directly. The molecules can only absorb light at well-defined wavelengths which correspond to energy differences between two of their quantum states. This is referred to as a spectroscopic excitation.

In the course of these experiments, the molecules are perturbed and change their quantum state. In many cases, the molecules even have to be destroyed to detect the spectroscopic excitations.

Inspired by quantum methods developed for the manipulation of atoms, the research team has developed a new technique which enables spectroscopic measurements on the level of a single molecule.

The molecule is trapped in a radio-frequency trap and cooled down to near the absolute zero point of the temperature scale (approx. -273 °C). To enable cooling, an auxiliary atom (here a single, charged calcium atom) is simultaneously trapped and localized next to the molecule. This spatial proximity is also essential for the subsequent spectroscopic study of the molecule.

Then, a force is generated on the molecule by focusing two laser beams on the particles to form a so-called optical lattice. The strength of this optical force increases with the proximity of the irradiated wavelength to a spectroscopic excitation in the molecule resulting in a vibration of the molecule within the trap instead of its excitation. The strength of the vibration is thus related to the proximity to a spectroscopic transition and is transmitted to the neighboring calcium atom from which it is detected with high sensitivity. In this way, the same information on the molecule can be retrieved as in a conventional spectroscopic experiment.

This highly sensitive technique for probing molecules is widely applicable and paves the way for a range of new applications in the fields of quantum science, spectroscopy and chemistry.

The paper has been published in the journal Science.

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