Markus Deiß

We review recent progress in the field of cold trapped molecular ions. A new generation of collision and cold chemistry experiments between atoms and ions has emerged, where cold atoms and ions are brought into contact in a controlled way in novel hybrid atom-ion platforms. Furthermore, new possibilities for the preparation and detection of molecular quantum states with high sensitivity and precision have been demonstrated based on quantum-logic schemes. These advances represent important stepping stones for new directions in fundamental research and technological applications across various domains including precision measurements, quantum technologies and chemical dynamics.

We report on Rydberg spectroscopy of ultracold atoms in an atom-ion hybrid trap for probing the electric fields in a mixture of atoms and ions. We obtain spectra which exhibit excitation gaps corresponding to avoided level crossings in the Stark map. From these measurements we can conclude that the ground state atoms experience electrical fields of up to 250 V/cm. There is, however, a difficulty in interpreting the results, because some data indicate that the electrical fields are produced by the ions while other data indicate that they stem from the Paul trap. We discuss possible scenarios for explaining the measured data, provide first measurements to check these scenarios, and propose methods to finally solve this puzzle.

We present a method to control collisions between ultracold neutral atoms in the electronic ground state and trapped ions. During the collision, the neutral atom is resonantly excited by a laser to a low-field-seeking Rydberg state, which is repelled by the ion. As the atom is reflected from the ion, it is de-excited back into its electronic ground level. The efficiency of shielding is analyzed as a function of laser frequency and power, initial atom-ion collision energy, and collision angle. The suitability of several Rydberg levels of Na and Rb for shielding is discussed. Useful applications of shielding include the suppression of unwanted chemical reactions between atoms and ions, a prerequisite for controlled atom-ion interactions.

We report on the compensation of excess micromotion due to parasitic rf-electric fields in a Paul trap. The parasitic rf-electric fields stem from the Paul trap drive but cause excess micromotion, e.g. due to imperfections in the setup of the Paul trap. We compensate these fields by applying rf-voltages of the same frequency but adequate phases and amplitudes to Paul trap electrodes. The magnitude of micromotion is probed by studying elastic collision rates of the trapped ion with a gas of ultracold neutral atoms. Furthermore, we demonstrate that also reactive collisions can be used to quantify micromotion. We achieve compensation efficiencies of about 1$\:\text{Vm}^{-1}$, which is comparable to other conventional methods.

Three-body recombination is a prime example of the fundamental interaction between three particles. Due to the complexity of this process it has resisted a comprehensive description. Experimental investigations have mainly focussed on the observation of corresponding loss rates without revealing information on the reaction products. Here, we provide the first general experimental study on the population distribution of molecular quantum states after three-body recombination in a non-resonant regime. We have developed a highly sensitive detection scheme which combines photoionization of the molecules with subsequent ion trapping. By analyzing the ionization spectrum, we identify the population of energy levels with binding energies up to $h\times 750\:$GHz. We find a broad population of electronic and nuclear spin states and determine a range of populated vibrational and rotational states. The method presented here can be expanded to provide a full survey of the products of the recombination process. This may be pivotal in developing an in-depth model that can qualitatively and quantitatively predict the reaction products of three-body recombination.