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Proof-of-principle experiment for the dark-field detection concept for measuring vacuum birefringence

Michal Šmíd1,*, Pooyan Khademi2,3,4, Naser Ahmadiniaz1, Michal Andrzejewski5, Carsten Baehtz1, Erik Brambrink5, Jakub Bulička6,7, Tomáš Burian6, Samuele Di Dio Cafiso1 et al.

Jaromír Chalupský6, Thomas E. Cowan1,8, Sebastian Göde5, Jörg Grenzer1, Věra Hájková6, Peter Hilz2,3,4, Willi Hippler2,3,4, Hauke Höppner1, Alžběta Horynová6,9, Lingen Huang1, Oliver Humphries5, Šimon Jelínek10,6,7, Libor Juha6, Felix Karbstein2,3,4,†, Christian Kohlfürst1, Alejandro Laso Garcia1, Robert Lötzsch2,3,4, Masruri Masruri5, Aimé Matheron2,3,4, Motoaki Nakatsutsumi5, Gerhard G. Paulus2,3,4, Alexander Pelka5, Thomas R. Preston5, Sripati V. Rahul5, Lisa Randolph5, Alexander Sävert2,3,4, Hans-Peter Schlenvoigt1, Ralf Schützhold1,8, Jan Patrick Schwinkendorf1,5, Thomas Stöhlker2,3,4, Monika Toncian1, Toma Toncian1, Maksim Valialshchikov2,3,4, Vojtěch Vozda6, Edgar Weckert11, Colin Wessel2,3,4, Jan Wild7, Ulf Zastrau5, and Matt Zepf2,3,4,‡

  • *Contact author: m.smid@hzdr.de
  • †Contact author: f.karbstein@hi-jena.gsi.de
  • ‡Contact author: m.zepf@hi-jena.gsi.de
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Phys. Rev. A 112, 063512 – Published 8 December, 2025

DOI: https://doi.org/10.1103/xpxy-ntwz

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 (XFELs) constitute a particularly promising probe, due to their brilliance, the possibility of precise control and favorable 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 dark-field detection concept has recently been proposed. Here we present the results of a proof-of-principle experiment validating this approach by demonstrating that using real-world x-ray optics the background signal can be suppressed sufficiently to measure the weak nonlinear response of the vacuum.

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Phys. Rev. A 112, 063512– Published 8 December, 2025
  • Received 2 July 2025
  • Accepted 10 November 2025
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