Advanced X-ray polarimeter design for nuclear resonant scattering.

This work presents the improvements in the design and testing of polarimeters based on channel-cut crystals for nuclear resonant scattering experiments at the 14.4 keV resonance of 57Fe. By using four asymmetric reflections at asymmetry angles of α1 = -28°, α2 = 28°, α3 = -28° and α4 = 28°, the degree of polarization purity could be improved to 2.2 × 10-9. For users, an advanced polarimeter without beam offset is now available at beamline P01 of the storage ring PETRA III.

Influence of higher harmonics of the undulator in X-ray polarimetry and crystal monochromator design.

The spectrum of the undulator radiation of beamline P01 at Petra III has been measured after passing a multiple reflection channel-cut polarimeter. Odd and even harmonics up to the 15th order, as well as Compton peaks which were produced by the high harmonics in the spectrum, could been measured. These additional contributions can have a tremendous influence on the performance of the polarimeter and have to be taken into account for further polarimeter designs.

Tunable Subluminal Propagation of Narrow-band X-Ray Pulses.

Group velocity control is demonstrated for x-ray photons of 14.4 keV energy via a direct measurement of the temporal delay imposed on spectrally narrow x-ray pulses. Subluminal light propagation is achieved by inducing a steep positive linear dispersion in the optical response of 57Fe Mössbauer nuclei embedded in a thin film planar x-ray cavity. The direct detection of the temporal pulse delay is enabled by generating frequency-tunable spectrally narrow x-ray pulses from broadband pulsed synchrotron radiation. Our theoretical model is in good agreement with the experimental data.

Vacuum-assisted generation and control of atomic coherences at x-ray energies.

The control of light-matter interaction at the quantum level usually requires coherent laser fields. But already an exchange of virtual photons with the electromagnetic vacuum field alone can lead to quantum coherences, which subsequently suppress spontaneous emission. We demonstrate such spontaneously generated coherences (SGC) in a large ensemble of nuclei operating in the x-ray regime, resonantly coupled to a common cavity environment. The observed SGC originates from two fundamentally different mechanisms related to cooperative emission and magnetically controlled anisotropy of the cavity vacuum. This approach opens new perspectives for quantum control, quantum state engineering and simulation of quantum many-body physics in an essentially decoherence-free setting.