Waveguides offer a means to controllably couple atomic ensembles to the electromagnetic field therein. Here, we demonstrate x-ray propagation in planar thin-film waveguides coupled to M\"ossbauer nuclei under collective resonant excitation by short pulses of synchrotron radiation. We record x-ray photons that have been emitted into resonant modes of the waveguide. Depending on the geometry and mode of excitation, two fundamentally different signatures of the collective emission are observed, for which we present a unifying theoretical model. Our results form a new platform for waveguide quantum electrodynamics in the hard x-ray regime with the potential to provide a coherent narrowband source of x-rays on the nanometer scale.
@article{arxiv.2403.06508,
title = {Collective nuclear excitation and pulse propagation in single-mode x-ray waveguides},
author = {Leon M. Lohse and Petar Andrejić and Sven Velten and Malte Vassholz and Charlotte Neuhaus and Ankita Negi and Anjali Panchwanee and Ilya Sergeev and Adriana Pálffy and Tim Salditt and Ralf Röhlsberger},
journal= {arXiv preprint arXiv:2403.06508},
year = {2026}
}