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Quantum entanglement is one of the central resources underlying many emerging quantum technologies, including quantum-enhanced sensing. Transferring established concepts from optical quantum sensing to electron microscopy promises new routes towards enhanced measurement sensitivity and imaging beyond the standard quantum limit. Coherent inelastic interactions between free electrons and optical excitations are expected to induce the required entanglement [1,2]. However, the generation and verification of free-electron-photon entanglement remains an outstanding challenge [3-5].
Here, we demonstrate quantum entanglement between the path of a swift free electron and the polarisation of a generated photon [6]. In our experiment, two partial electron beams are prepared in a coherent superposition using an amplitude grating and directed near the two edges of a sub-micron-sized, metal-coated glass prism. At these edges, electrons can spontaneously emit photons whose polarisation is tied to the electron's path. These photons are collected and analysed in a polarisation-resolved, single-photon-sensitive setup, while the recombined electron beams are recorded on an event-based detector. Observing electron interference in coincidence with measurements of the photon polarisation in different bases, we implement state tomography of the complete electron-photon two-qubit quantum state. We find that the reconstructed state violates the Peres-Horodecki entanglement criterion by more than seven standard deviations, constituting an unequivocal demonstration of free-electron-photon entanglement.
This observation establishes free-electron-photon entanglement as a cornerstone of free-electron quantum optics and, harnessing entanglement swapping, provides a pathway towards quantum-enhanced electron microscopy based on entangled free electrons [4].
[1] O. Kfir, Phys. Rev. Lett. 123, 103602 (2019)
[2] A. Konecna, F. Iyikanat, and F. J. García de Abajo, Sci. Adv. 8, eabo7853 (2022)
[3] E. Kazakevich, H. Aharon & O. Kfir, Phys. Rev. Res. 6, 043033 (2024)
[4] J.-W. Henke, H. Jeng & C. Ropers, Phys. Rev. A 111, 012610 (2025)
[5] P. Rembold et al., Quantum Sci. Technol. 10, 045003 (2025)
[6] J.-W. Henke, H. Jeng, M. Sivis, and C. Ropers, arXiv:2504.13047 (2025)