Quantentechnologie

We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. Elastic two-body collisions lead to a rapid thermalization of the impurity inside the Rb gas, representing the first realization of an ultracold gas doped with a precisely known number of impurity atoms interacting via s-wave collisions. Inelastic interactions are restricted to a single three-body recombination channel in a highly controlled and pure setting, which allows to determine the Rb-Rb-Cs three-body loss rate with unprecedented precision. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum many-body system.

We report on the controlled insertion of individual Cs atoms into an ultracold Rb gas at ≈400 nK. This requires one to combine the techniques necessary for cooling, trapping and manipulating single laser cooled atoms around the Doppler temperature with an experiment to produce ultracold degenerate quantum gases. In our approach, both systems are prepared in separated traps and then combined. Our results pave the way for coherent interaction between a quantum gas and a single or few neutral atoms of another species.

My thesis will present the experimental realisation and characterisation of all prerequisites necessary for the investigation of the coherence dynamics of single atoms immersed in a quantum gas: In the first chapter, the traps used to combine the single Cs atom with the ultracold Rb cloud are presented and characterised. Afterwards the methods used to control the internal degree of freedom of the Cs atom, e.g. microwave spectroscopy, are introduced. In the last chapter the coherence time of the generated superposition states is measured. A species selective lattice was built and implemented in the current setup. A transfer efficiency from the single atom MOT into the lattice and back close to unity has been achieved. This is an essential prerequisite for future experiments, because all measurements rely on counting the atom number in the MOT before and after manipulation of the atom. It was verified that the measured lattice parameters agree with the theoretically predicted values. A microwave setup has been built and used to generate a superposition state of two Zeeman states in a single Cs atom. From a Ramsey spectroscopy measurement performed in the lattice, the inhomogeneous coherence time of nearly 60 μs has been inferred and the differential light shift has been found to be the limiting factor. Within the dipole trap we measured the homogeneous coherence time of approximately 1200 μs using different trap depths, observing that the time stays the same in each measurement. The limiting processes are mainly ascribed to uctuations of the current owing through the coils generating the magnetic guiding field. Unfortunately it was not possible to perform a combined measurement of a single Cs atom and an ultracold Rb cloud because of a major vacuum leak.