08 Sep Infrared spectroscopy of a single molecule
Measuring the absorption spectrum of a single molecule is challenging: The signal is so weak that conventional methods can barely distinguish it from noise. Researchers at the University of Innsbruck have now detected the absorption of a single infrared photon and used it to measure the spectrum of an individual calcium hydroxide molecular ion.
According to the researchers, the team used a method originating from quantum information processing. In a technique known as quantum logic spectroscopy, a molecular ion and a well-controlled auxiliary atom are coupled in an ion trap. Through their shared motion, information can be transferred from the difficult-to-measure molecule to the auxiliary atom, where it can be read out.
Photon recoil becomes measurable
When the molecule absorbs a single infrared photon, the photon transfers a tiny amount of momentum to the molecule. This recoil is normally too small to be measured directly. To detect it, the researchers quantum-mechanically entangled the molecule with an atom that serves as a detector.
They placed the coupled system in a so-called Schrödinger cat state. This quantum state makes the system particularly sensitive to even the smallest disturbances. As a result, even the tiny recoil from a single absorbed photon becomes measurable.
Using this method, the team determined a characteristic molecular vibration – the O-H stretching vibration – of a singly positively charged calcium hydroxide molecular ion (CaOH⁺). The measured spectrum agreed well with theoretical calculations performed by researchers at the University of Warsaw.
Spectroscopy without destroying the molecule
One advantage of the method is that the molecular ion being studied is neither destroyed nor chemically altered during the measurement. According to the researchers, the method therefore makes it possible to measure and selectively prepare the quantum states of different types of molecules.
The method could enable high-precision spectroscopy of molecular species that are difficult to investigate using conventional techniques. The researchers also see potential for studying more complex molecules and for future quantum technologies that use molecules as quantum systems.
Original publication:
[Zhenlin Wu et al.: Infrared absorption spectroscopy of a single polyatomic molecular ion; Nature, 2026, DOI: https://www.nature.com/articles/s41586-026-10915-8 [arXiv: 2511.19687]]
Source: www.uibk.ac.at
Image: Helene Heinzer