Indistinguishability and Quantum Pathways in Strong-field Resonant X-ray Scattering

X-ray scattering driven by intense attosecond pulses is emerging as a powerful mechanism for enhancing ultrafast diffraction beyond limits imposed by photoionization and electronic bleaching. Ultrashort resonant x-ray pulses can drive electronic dynamics on attosecond time scales, generating transient core-hole configurations whose scattering signatures deviate strongly from the linear, single-photon regime [2]. Motivated by recent observations of nonlinear resonance enhancement in rapidly ionizing Xe nanoparticles [3] and Rabi-driven excitation reversal in Ne that suppresses absorption while boosting coherent scattering [4], we developed a time-dependent QED framework [4–6] to describe x-ray resonant absorption, emission, scattering, and coherent dynamics in atoms and clusters.  Our theory shows that strong-field resonant driving opens multiple interference pathways that extend ground-state contrast and amplify the effective structure factor far beyond linear scaling. Only a subset of these pathways remains indistinguishable and contributes to first-order interference, with fringe visibility governed by the pulse area, polarization geometry, and the initial electronic state. In the linear limit, resonant channels yield the highest fringe contrast and a differential signal resembling a conventional structure factor, but with substantially enhanced photon counts relative to nonresonant scattering. These effects can be exploited to enhance contrast and reduce radiation damage in ultrafast x-ray imaging.

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[2] P. J. Ho, et. al., Nat. Comm. 11, 167 (2020).

[3] S. Kuschel, et. al., Nat. Comm. 16, 847 (2025).

[4] A. Ulmer, et. al., submitted, arXiv:2506.19394 (2025).

[5] A. Venkatesh and P. J. Ho, Phys. Rev. A 111, L021101 (2025); Phys. Rev. A 111, 023101 (2025).

[6] A. Venkatesh and P. J. Ho, arXiv:2506.06585 (2025).