The technique, developed by an international team including physicists from the University of Warsaw, uses a quantum state known as a ‘Schrödinger's cat’ state to turn an almost imperceptible change in the motion of the molecular ion into a measurable signal.

An absorption spectrum acts like a molecular fingerprint. Different wavelengths of light correspond to transitions between energy states, such as the vibration of a chemical bond, allowing scientists to identify a molecule and learn about its structure.

Usually, researchers work with samples containing billions of molecules, producing enough absorbed light to measure. A single molecular ion presents a much greater challenge: it may absorb just one photon, while the resulting loss of light can be hidden by natural fluctuations in the intensity of the beam.

The team, led by Philipp Schindler, a professor at the University of Innsbruck, instead measured the effect of the absorbed photon on the molecule.

When a molecule absorbs a photon, it receives not only energy but also a small amount of momentum, causing it to recoil. The effect is too small to detect directly, but the researchers found a way to transfer the information to a neighbouring calcium ion.

They placed a calcium hydroxide ion (CaOH+) and a single calcium ion (Ca+) in an electromagnetic trap. Their electrostatic interaction coupled the motion of the two particles, forming a system known as a two-ion crystal.

The recoil of the molecular ion therefore also changed the motion of the neighbouring calcium ion.

To make this tiny change easier to detect, the researchers prepared the joint motion of the two ions in a quantum state known as a Schrödinger cat state. The system existed as a superposition of two distinct modes of motion.

When the molecular ion absorbed a photon, the recoil itself did not increase, but the relative phase between the two components of the quantum state changed. This made the effect of the transferred momentum easier to detect.

The researchers then transferred information about the change to the internal state of the calcium ion. Under controlled illumination, the ion either fluoresced or remained dark, allowing it to act as a sensor for the molecular ion.

The method was tested by measuring the vibration of the oxygen-hydrogen bond in CaOH+. The ion was exposed to a sequence of up to 34 precisely synchronised, ultrashort mid-infrared pulses. By varying the frequency of the light and repeating the measurements, the researchers reconstructed the absorption spectrum of the single ion.

The centre of the measured band was close to the theoretically predicted value.

During each sequence, the ion could absorb no more than one photon. The researchers could not yet detect every individual absorption event in a single measurement, so they determined the absorption probability from numerous repetitions.

They now plan to increase the laser intensity and readout sensitivity in an effort to detect individual absorption events without averaging the results.

Marcin Gronowski, PhD, and Professor Michał Tomza from the Faculty of Physics at the University of Warsaw carried out the key theoretical work. They performed quantum-chemical calculations of the electronic structure of CaOH+ and predicted an O–H bond vibration frequency of 3783 cm⁻¹.

The result helped the experimental team select the appropriate laser operating range and interpret the measured spectrum. The calculations used the PLGrid infrastructure at ACK Cyfronet AGH, while the Polish part of the research was funded by the Polish National Science Centre.

The results were published in Nature. Unlike methods in which absorption is detected only after a molecule has broken apart or undergone a chemical reaction, the new technique leaves the molecular ion intact, allowing it to be studied repeatedly.

The researchers say the method could be used to study increasingly complex molecular ions, control their quantum states and monitor rapid processes inside molecules. It also shows that the absorption of a single photon can be detected indirectly by measuring a tiny change in a molecule's motion. (PAP)

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