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Highest degree of purity achieved for polarized X-rays


Helmholtz Institute Jena opens up new possibilities at the European X-ray laser European XFEL.
A research team was able to generate polarized X-rays with unprecedented  purity at the European XFEL in Hamburg. The experiments involved  scientists from the Helmholtz Institute Jena, a branch of GSI, Friedrich  Schiller University Jena and the Helmholtz Center Dresden-Rossendorf.  The method is supposed to be used in the coming years to show that even  vacuum behaves like a material under certain circumstances — a  prediction from quantum electrodynamics.
The polarization of electromagnetic radiation describes in which plane  in space a wave oscillates. While everyday electromagnetic radiation,  such as sunlight, is unpolarized, lasers produce polarized radiation.  This is an important requirement for a wide range of experiments from  solid-state physics to quantum optics.
Additional polarizers, such as those being developed at the Helmholtz  Institute in Jena, have the purpose of further improving polarization  purity, but for a long time the limit of a few 10^-10, i.e., out of ten  billion photons, only a handful have the unwanted polarization, could  not be pushed any further. In 2018, Kai Schulze, first author of the  paper now published in Physical Review Research, found that the  divergence of synchrotron radiation is the reason for this limit. "So to  get a further improvement in purity, we needed a source with better  divergence," says the physicist, who leads work on vacuum birefringence  at HI Jena and is jointly responsible for related DFG research projects  at the University of Jena. "The commissioning of the European X-ray  laser, European XFEL, in Schenefeld near Hamburg set the course for  this."
Together with scientists from the Friedrich Schiller University of Jena  and the Helmholtz Center Dresden-Rossendorf, Schulze and his team  developed an experiment setup at the European XFEL that set a new purity  record of 8×10^-11 thanks to special polarizer crystals, a very precise  alignment and a stable setup. This new purity record has already  enabled a number of experiments on quantum optics in the X-ray range and  on charge distribution in solids. However, special interest is devoted  to the detection of the so-called vacuum birefringence.
The interaction of light with light was described as early as 1936 by  Werner Heisenberg and Hans Euler, but has not yet been directly observed  on Earth. "Vacuum birefringence is currently the most promising effect  to directly detect light-light interaction," Schulze explains. "In this  process, the polarization of a sample beam changes when it collides in  vacuum with a very intense second light beam. The vacuum thus acts like a  birefringent crystal, which also affects the polarization; hence the  name. The effect is extremely small, but grows with decreasing  wavelength of the sample beam. Precise polarizers in the X-ray range  therefore provide a good tool to detect the effect."
The High Energy Density instrument at the European XFEL will provide the  ideal conditions for such an experiment in the future, Schulze further  explains. And the research team now has a setup with which the smallest  polarization changes can be measured. The detection of vacuum  birefringence would not only further underpin the foundations of quantum  electrodynamics, but, if deviations from theoretical expectations  emerge, also provide clues to previously unknown elementary particles  (such as axions, or millicharged particles). "We hope to be able to  launch the first experiments in the next few years."
Detection of the phenomenon would also be interesting for future  experiments at the FAIR particle accelerator center. "If we succeed in  measuring vacuum birefringence, this will help interpret the measurement  data from FAIR. Among other things, vacuum polarization will play a  role there, which is closely linked to vacuum birefringence," Schulze  said.