Polish researchers from the University of Warsaw, in collaboration with researchers from the Czech Republic and Austria, have developed a method for detecting single-photon absorption by a single molecular ion. Their research findings have just been published in Nature.
Prof. Michał Tomza and Dr Marcin Gronowski from the Faculty of Physics at the University of Warsaw addressed the theoretical part of the research, while experiments were conducted by the research group headed by Prof. Philipp Schindler from the University in Innsbruck, with the participation of a researcher from the Palacký University Olomouc. The National Science Centre funded the Polish part of the research.
Light absorption is usually studied by measuring how much light is lost as it passes through a sample. It does not work with a single molecular ion because the signal is too weak. Instead of detecting the photon, the research team measured the small recoil—or momentum transfer—received by the molecule once a photon is absorbed. The recoil is too small to be measured directly. To detect it, the researchers trapped a molecular ion alongside a calcium atomic ion (Ca+). Their strong Coulomb interaction, i.e. the electrostatic force between charged particles, couples their motion, causing them to form collective modes of motion. The researchers then prepared the system in a non-classical quantum state, known as a Schrödinger cat state. When the molecular ion absorbs a photon, the resulting recoil shifts this state, greatly amplifying the signal. This makes it possible to detect the absorption event by precisely measuring the neighbouring calcium ion through its fluorescence. The method was demonstrated on the O–H stretching vibration in a CaOH+ molecular ion, excited by ultrashort femtosecond laser pulses in the mid-infrared range.
The published results represent an important step towards non-destructive 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.
Marcin Gronowski and Michał from the University of Warsaw were responsible for the theoretical part of the project. They carried out advanced quantum-chemical calculations of the ion’s electronic structure, predicting an O–H bond vibrational frequency of 3783 cm⁻¹, which enabled the Insbruck team to precisely tune the laser pulses and interpret the experimental results. The calculations were carried out using the Polish PLGrid supercomputing infrastructure hosted at the Academic Computer Centre CYFRONET AGH.
The two researchers are winners of multiple NCN calls for proposals. In 2020, Prof. Michał Tomza received the NCN Award in Physical Sciences and Engineering. This is yet another paper featuring Michał Tomza that has been published in Nature. In 2021, his research team together with an experimental team led by Professor Tobias Schaetz from the University of Freiburg, were the first to observe Feshbach resonances between a single ion and ultracold atoms.
Prof. Tomza discussed his research a few months ago in the #NCNGeneration series.
This is the second publication by the representatives of the #NCNGeneration in Nature this month.
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