Description
Modern physics views the vacuum not as the absence of everything, but as a highly non-trivial quantum state characterized by the omnipresence of vacuum fluctuations. Because electromagnetic fields couple to charges, vacuum fluctuations of charged particles are predicted to give rise to nonlinear self-couplings of electromagnetic fields. This fundamental tenet, anticipated 90 years ago in the seminal theoretical work of Heisenberg and Euler [doi:10.1007/bf01343663], has remained experimentally challenging and is yet to be tested in the laboratory. X-ray free-electron lasers (XFELs) constitute a particularly promising probe, due to their brilliance, the possibility of precise control and favorable frequency scaling. However, the effect is very small even when probing a tightly focused high-intensity laser field with XFEL radiation. Achieving a sufficiently good signal-to-background separation is key to its first successful detection in a controlled laboratory experiment. To master this challenge, recently a dark-field detection concept has been proposed [doi:10.1103/PhysRevLett.129.061802]. In this talk, I will detail the theoretical foundations of the dark-field concept, provide predictions for the attainable signals and discuss the status and perspectives of this approach for the measurement of light-by-light scattering signals in a dedicated experiment at the HED/HIBEF instrument of the European XFEL [doi:10.1017/hpl.2024.70].
