This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For decades, light has been used to understand the molecular structures of matter. A sample is irradiated with light, and measurements determine the wavelengths at which it is absorbed.
Since each molecule absorbs light at very specific wavelengths that depend on its structure, the resulting absorption spectrum acts like a molecular fingerprint. For individual molecules, however, this signal is vanishingly small and mostly indistinguishable from noise. Using an elegant quantum physics trick, the team led by Philipp Schindler from the Department of Experimental Physics at the University of Innsbruck has now succeeded in measuring the spectrum of a single calcium hydroxide molecule.
The work is published in the journal Nature. The basic idea draws from quantum information processing. In a technique known as quantum logic spectroscopy, two ions are coupled via their electric repulsion: a hard-to-access molecular ion and an easily controllable auxiliary atom move together in an ion trap, exchanging information in the process.
In this way, techniques originally developed for quantum computers can be applied to molecules that are otherwise difficult to study using conventional methods. When a molecule absorbs a single infrared photon, it gains a tiny amount of momentum—normally far too small to be measured directly. The Innsbruck team achieves the sensitivity required to detect the absorption of individual photons by using a quantum-mechanically entangled atom as a detector.
"The entanglement of the particles in a so-called Schrödinger's cat state makes the system extremely sensitive to the slightest disturbances," explains Zhenlin Wu, the study's lead author. "The tiny recoil of the absorbed photon is made measurable by this cat state." Using this method, the research team identified a characteristic molecular vibration—the O-H stretching vibration—of the singly charged calcium hydroxide molecular ion. The spectrum measured by the group agrees well with theoretical calculations performed by collaborators at the University of Warsaw.
A key advantage of this method is that it is nondestructive: The molecular ion is neither destroyed nor chemically altered during analysis. As a result, it enables the measurement and preparation of quantum states across a wide variety of molecular species, paving the way for precise spectroscopy of complex molecules and a broad range of applications in future quantum technologies. Zhenlin Wu et al, Infrared absorption spectroscopy of a single polyatomic molecular ion, Nature (2026).
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