Measurement of quantum states in a neutron interferometer

The characterization of quantum states of neutrons in an interferometer is the main topic of this thesis. All related experiments were performed at the instrument S18 of the Institut Laue Langevin (ILL) in Grenoble, France. This instrument is dedicated to neutron interferometry as well as Ultra Small Angle Neutron Scattering (USANS). In principle, the properties of the quantum states entering the interferometer are mainly affected by the neutron-guide (super mirror guide H25) and the perfect crystal monochromator. A coherence volume can be assigned to these states where phase relations are fixed and interference occurs. The coherence behaviour is strongly anisotropic, since only transversal momentum distribution is influenced by dynamical diffraction. The longitudinal and vertical directions remain unaffected. The neutron interferometer is a unique tool for probing the coherence properties. It allows us to split a neutron beam in two coherent paths, separated by several centimeters and to recombine them afterwards. Due to introduction of a longitudinal or a vertical phase shift, respectively, it was possible to measure the related coherence function, which is determined by the auto-correlation function of the wave-function. By inserting a phase shifter in one of the two beam paths, the initial state is converted to a new state by superposition with the phase shifted state. This causes a change in the momentum distribution, which can be measured behind the interferometer with an additional crystal. With this analyzer-crystal the smoothed out interference properties at high interference orders can be restored behind the interferometer, if proper spectral filtering is applied. To measure the spatial distribution of this newly created state, a 2-loop-interferometer is needed. This kind of interferometer permits to create any state in the first loop and analyze it in the second loop. Therefore, it is possible for the first time to measure the spatial and the momentum distribution simultaneously. Finally, the influence of fluctuating magnetic fields on the neutron states in the interferometer are investigated at the end of this work. (author)