Coherent manipulation of ultracold atoms with microwave near-fields

The spectacular progress in the eld of ultracold quantum gases is intimately connected with the availability of sophisticated techniques for quantum-level control of internal states, motional states, and collisional interactions. Atom chips provide such control in compact, robust, and scalable setups, which makes them attractive for both applications and fundamental studies. In this thesis I report on experiments that use a new method for coherent manipulation of ultracold atoms. The method is based on microwave nearelds, provided by a waveguide structure that is fully integrated on an atom chip. We generate microwave neareld potentials that combine the versatility of optical traps with the robustness and tailorability of static magnetic microtraps. These potentials depend on the internal atomic state, and we use them for state-selective splitting of Rb Bose-Einstein condensates. We show for the rst time combined coherent manipulation of internal and motional states on an atom chip, realizing a trapped-atom interferometer with internal state labeling of the interferometer paths. Moreover, we use microwave neareld potentials for the preparation of spin-squeezed states for quantum-enhanced metrology through controlling statedependent collisional interactions. In addition, very promising proposals exist for the implementation of a quantum phase gate using these potentials. Measuring microwave elds is important for engineering of microwave devices as well as in science, e.g. to characterize the eld homogeneity in the interaction regions of an atomic clock. We develop a novel technique that uses clouds of uncondensed ultracold atoms as sensitive, tunable and non-invasive probes for microwave eld imaging with micrometer spatial resolution. The microwave magnetic eld components drive Rabi oscillations on atomic hyper ne transitions whose frequency can be tuned with a static magnetic eld. Readout is accomplished using state-selective absorption imaging. Quantitative data extraction is simple and it is possible to reconstruct the amplitudes and phases of the di erent microwave magnetic eld components. While we demonstrate 2D imaging, an extension to 3D imaging is straightforward. We use the method to determine the microwave neareld distribution around the on-chip waveguide and reconstruct the corresponding current distribution. For our experimental parameters, the method provides a microwave magnetic eld sensitivity of ∼ 2 × 10−4 G, which can even be improved further with variants discussed. The experiments presented in this thesis open the path for the realization of portable quantum-enhanced interferometer devices, the implementation of a quantum phase gate as well as for a new generation of microwave eld sensors.