The Darkfield Approach to Measuring Vacuum Birefringence and Light-by-Light Couplings -- A Proof-of-Principle Experiment

TL;DR

This paper demonstrates a proof-of-principle experiment using a darkfield detection method to measure vacuum birefringence and light-by-light couplings with XFEL radiation, advancing quantum vacuum research.

Contribution

It validates the darkfield detection approach for measuring nonlinear vacuum effects using XFEL, a novel technique for such experiments.

Findings

Successful proof-of-principle validation of darkfield detection

Enhanced signal-to-background separation achieved

Potential for precise measurement of quantum vacuum properties

Abstract

Vacuum fluctuations give rise to effective nonlinear interactions between electromagnetic fields. These generically modify the characteristics of light traversing a strong-field region. X-ray free-electron lasers constitute a particularly promising probe, due to their brilliance, the possibility of precise control and favourable frequency scaling. However, the nonlinear vacuum response is very small even when probing a tightly focused high-intensity laser field with XFEL radiation and direct measurement of light-by-light scattering of real photons and the associated fundamental physics constants of the quantum vacuum has not been possible to date. Achieving a sufficiently good signal-to-background separation is key to a successful quantum vacuum experiment. To master this challenge, a darkfield detection concept has recently been proposed. Here we present the results of a proof-of-principle experiment validating this approach at the High Energy Density scientific instrument of the European X-Ray Free Electron Laser.