Measuring a Single Molecule Without Disturbing Its State

Researchers at the University of Innsbruck, in collaboration with Dr Marcin Gronowski and Professor Michał Tomza from the Faculty of Physics at the University of Warsaw, have published a paper in Nature describing an innovative method for studying individual molecular ions. They demonstrated that the absorption of a single photon by a single polyatomic CaOH⁺ ion can be detected using precise quantum techniques.

From left: researchers from the Faculty of Physics, University of Warsaw: Prof. Michał Tomza and Dr Marcin Gronowski. Photo: Patrycja Chuchała/UW

The researchers developed an approach in which the signal produced by the absorbed photon is not observed directly. Instead, it is detected through the extremely subtle recoil imparted to the CaOH⁺ molecule. This effect is then amplified using a nonclassical motional state of two ions confined in the same trap, the so-called Schrödinger cat state. Such a state is a quantum superposition of two distinct states of motion and is exceptionally sensitive to even the smallest disturbances. The information about photon absorption is subsequently mapped onto a change in the state of the second ion, an atomic Ca+ ion, which acts as a highly sensitive detector. The resulting signal is then read out using laser-induced fluorescence. The authors emphasize that this is the first application of this method to detecting the absorption of a single photon by a molecular ion.

The experiment was designed and carried out by Professor Philipp Schindler’s group in Innsbruck, while the researchers from the University of Warsaw developed a detailed theoretical description of the properties of the molecular ion. This description was essential for analysing and interpreting the experimental results. The method was used to investigate the O–H bond-stretching vibrations of individual CaOH⁺ ions excited by femtosecond pulses in the mid-infrared spectral range.

The published results represent an important step towards nondestructive measurements of complex polyatomic molecules and more advanced control of their quantum states. They open up new possibilities for both precision molecular spectroscopy and the development of molecule-based quantum technologies.

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