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.