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Prof. Dr. Stefan Willitsch

Department of Chemistry
Profiles & Affiliations

Cold and Controlled Molecules and Ions

The research activities of the Willitsch group are situated at the interface between chemistry, quantum optics, quantum physics and the nanosciences. Specifically, our research is devoted to the generation of cold molecules and ions, the development of quantum technologies for single trapped molecules for applications in precision molecular spectroscopy and chemistry and the study of chemical reactions under cold and extremely controlled conditions. Owing to the highly interdisciplinary nature of our work, we enjoy various collaborations with experimentalists and theoreticians in both physics and chemistry departments worldwide as well as with industrial partners in the quantum and photonics industries.

Selected Publications

Kilaj, Ardita, Wang, Jia, Stranak, Patrik, Schwilk, Max, Rivero, Uxia, Xu, Lei, von Lilienfeld, O. Anatole, Kupper, Jochen, & Willitsch, Stefan. (2021). Conformer-specific polar cycloaddition of dibromobutadiene with trapped propene ions. Nature Communications, 12(1), 6047. https://doi.org/10.1038/s41467-021-26309-5

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Sinhal, Mudit, Meir, Ziv, Najafian, Kaveh, Hegi, Gregor, & Willitsch, Stefan. (2020). Quantum-nondemolition state detection and spectroscopy of single trapped molecules. Science, 367(6483), 1213–1218. https://doi.org/10.1126/science.aaz9837

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Dörfler, Alexander D., Eberle, Pascal, Koner, Debasish, Tomza, Michał, Meuwly, Markus, & Willitsch, Stefan. (2019). Long-range versus short-range effects in cold molecular ion-neutral collisions. Nature Communications, 10(1), 5429. https://doi.org/10.1038/s41467-019-13218-x

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Germann, Matthias, Tong, Xin, & Willitsch, Stefan. (2014). Observation of electric-dipole-forbidden infrared transitions in cold molecular ions. Nature Physics, 10(11), 820–824. https://doi.org/10.1038/nphys3085

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Chang, Yuan-Pin, Długołęcki, Karol, Küpper, Jochen, Rösch, Daniel, Wild, Dieter, & Willitsch, Stefan. (2013). Specific Chemical Reactivities of Spatially Separated 3-Aminophenol Conformers with Cold Ca+ Ions. Science, 342(6154), 98–101. https://doi.org/10.1126/science.1242271

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Selected Projects & Collaborations

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Exploring and Controlling Chemistry Using Quantum Logic

Research Project  | 1 Project Members

Over the past years, the development of experimental techniques for the coherent manipulation and control of single isolated quantum systems has made impressive progress. Such "quantum-logic" methods are also highly attractive in a chemical context in view of unravelling and controlling the quantum dynamics of molecular collisions and chemical reactions. However, for complex quantum systems like molecules, these techniques are still in their infancy and their considerable potential remains to be unlocked. The aim of the present project is to merge the fields of quantum science and chemical dynamics by advancing quantum technologies to polyatomic molecular ions and applying them to the study of ion-molecule collisions and chemical reactions. For this purpose, we have recently developed a quantum-non-demolition technique which enables the readout and spectroscopy of the quantum state of a single molecular ion without destroying the molecule or even perturbing its quantum state [Science 367 (2020), 1213]. In the present project, we will apply this method to achieve a complete projective state preparation of single molecular ions in specific Zeeman- hyperfine-spin-rovibronic levels as a starting point for collision studies and use the same methods to sen- sitively detect the quantum state of the collision product. In this way, state-to-state experiments on the single-molecule level will be realised using a quantum-logic state readout. In combination with Stark- decelerated beams of neutral molecules, we will be able to study for the first time completely state- and energy-controlled ion-molecule elastic, inelastic and reactive collisions. In particular, we will be able to explore the role of the hyperfine and Zeeman states in collisions involving molecular ions, which is largely uncharted territory. The quantum-non-demolition nature of our detection scheme will allow us to reach measurement sensitivities several orders of magnitude higher compared to previously used destruc- tive methods and thus enable an unprecedented precision in the study of ionic collisional processes. By introducing quantum-logic approaches to the study of molecular collisions, the present project will establish a new paradigm for probing molecular processes and a new frontier in studies of chemical dy- namics. The extension of our methods to polyatomics will also make a broad range of molecular systems available for applications in the quantum sciences including quantum bits, quantum memories, quantum simulations and quantum sensing.

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Precision molecular spectroscopy using a network for distribution of the Swiss primary frequency standard

Research Project  | 5 Project Members

The goal of this project is to exploit recent progress in laser technology, frequency metrology and molecule optics to carry out ultra-precise measurements of energy intervals between electronic, vibrational and rotational states of molecules, in particular molecular ions. The present project aims to achieve a relative measurement accuracy in molecular-ion spectroscopy of order 10−14 −10−15, an improvement of several orders of magnitude in comparison to the present state of the art of 10−9. These advancements will open up a new frontier in precision molecular spectroscopy which will pave the way for using molecules as new high-precision frequency standards and clocks, for addressing fundamental physical problems such as the proton-radius puzzle and a possible temporal variation of fundamental physical constants and for precision tests of quantum electrodynamics. All of these application will be explored in the present project. The dramatic advancement in measurement accuracy targeted here will be enabled by the implementation of new spectroscopic methodologies based on quantum technologies, by the de- velopment of ultranarrow quantum-cascade laser sources tailored to the present needs, and in particular through the implementation of a fibre-optical network for the distribution of the Swiss primary frequency standard maintained by the Federal Institute of Metrology METAS to spec- troscopy laboratories in Basel and Zurich. This network will enable the absolute stabilisation, calibration and frequency comparison of the laser sources employed in the present measurements at a level of up to 10−15 by their referencing to the Swiss primary standard. While several Eu- ropean countries have already set up similar national and international networks for precision frequency distribution, Switzerland thus far possesses no such facilities. For Switzerland not to lose contact and competitiveness in the key future scientific domain of frequency metrology, it is imperative for our country to establish similar infrastructures. The present project will establish and test a prototype network connecting ETH Zurich, the University of Basel and the Federal Institute of Metrology METAS in Bern/Wabern. This prototype is intended to form the nucleus of a Swiss national network for precision frequency- and time distribution linking a broad range of national laboratories and research groups involved in frequency metrology in the future. These objectives can only be reached through the close collaboration of a highly interdisci- plinary team involving physical chemists, laser physicists, metrologists and telecommunication- network engineers which are assembled in the present project.

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Conformationally Controlled Chemistry

Research Project  | 9 Project Members

The relationship between structure and reactivity is one of the central tenets of chemistry. In particular, many molecules exhibit structural isomers that interconvert over low barriers through rotations about covalent bonds (conformers). Conformers are the dominant isomers of complex molecules, and the conformation of a molecule can have pronounced effects on its chemical reactivity. Despite the eminent importance of conformational isomers in chemistry, very few studies have been reported thus far charac- terising conformational effects in chemical reactions under single-collision conditions. Consequently, the role of molecular conformations in fundamental reactions is only poorly understood. This striking lack of data reflects the experimental challenges to isolate und control specific conformers. In a recent proof-of-principle study [ Science 342 (2013), 98], we have spatially separated specific con- formers in a molecular beam through electrostatic deflection and directed them at a spatially localized reaction target of cold ions in a trap. This approach allowed us to study conformation-specific effects in ion-molecule reactions under precisely controlled experimental conditions in the gas phase. Here, we pro- pose a wide-ranging research programme aiming at extending our method to neutral reactions and applying it to a range of chemically relevant problems in order to explore the relationship between structure and reactivity in unprecedented detail. These methodological advances will enable for the first time detailed and systematic studies of the reaction mechanisms and dynamics of isolated conformers. The fundamental mechanistic insights gained will benefit a wide range of fields as diverse as fundamental reaction dynamics, organic synthesis, catalysis, atmospheric chemistry and rational molecule design.