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arXiv · 2506.11649

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

Michal Smíd·Pooyan Khademi·Carsten Bähtz·Erik Brambrink·Jindrich Chalupsky·Tom E. Cowan·Samuele Di Dio Cafiso·Sebastian Göde·Jörg Grenzer·Vera Hajkova·Peter Hilz·Willi Hippler·Hauke Höpner·Alzbeta Horynova·Oliver Humphries·Simon Jelinek·Libor Juha·Felix Karbstein·Alejandro Laso-Garcia·Robert Lötzsch·Aimé Mathéron·Gerhard G. Paulus·Lisa Randolph·Alexander Sävert·Hans-Peter Schlenvoigt·Jan Patrick Schwinekendorf·Thomas Stöhlker·Toma Toncian·Maxim Valialshchikov·Edgar Weckert·Colin Wessel·Matt Zepf
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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.

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Michal Smíd, Pooyan Khademi, Carsten Bähtz, Erik Brambrink, Jindrich Chalupsky, Tom E. Cowan, Samuele Di Dio Cafiso, Sebastian Göde, Jörg Grenzer, Vera Hajkova, Peter Hilz, Willi Hippler, Hauke Höpner, Alzbeta Horynova, Oliver Humphries, Simon Jelinek, Libor Juha, Felix Karbstein, Alejandro Laso-Garcia, Robert Lötzsch, Aimé Mathéron, Gerhard G. Paulus, Lisa Randolph, Alexander Sävert, Hans-Peter Schlenvoigt, Jan Patrick Schwinekendorf, Thomas Stöhlker, Toma Toncian, Maxim Valialshchikov, Edgar Weckert, Colin Wessel, Matt Zepf. 2025-06-13. The Darkfield Approach to Measuring Vacuum Birefringence and Light-by-Light Couplings -- A Proof-of-Principle Experiment. https://doi.org/10.1103/xpxy-ntwz

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