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PRINCIPAL PUBLICATION

X-ray parametric down-conversion reveals EUV-polariton, D. Krebs et al., Nat. Commun. 16, 5383 (2025); https:/doi.org/10.1038/s41467-025-60845-8

REFERENCES

[1] B. Adams et al., J. Synchrotron Radiat. 7, 81-88 (2000). [2] P.G. Kwiat et al., Phys. Rev. Lett. 75, 4337 (1995).

modulation, with a positive circular pattern surrounded by a negative region (Figure 56a). This contrasts with uniformly positive cones observed in optical down-conversion, indicating previously unexplored physics.

To explain this modulation, a polaritonic model of XPDC was developed. Polaritons are hybrid light–matter states arising from strong coupling between photons and material excitations. In this case, the “idler ” photon at extreme-ultraviolet (EUV) wavelengths couples to electronic excitations, forming a novel EUV polariton. A minimal two-level model reproduces the measured scattering patterns (Figure 56b), linking the observed two-fold modulation to the avoided crossing of the polaritonic dispersion branches (Figure 56d). Fitting a line-out (Figure 56c) enabled the quantification of the light–matter coupling strength, which reached the onset of the strong coupling regime without the need for any enhancing cavity.

The identification of an EUV polariton extends strong-coupling physics into previously unexplored spectral regions. This advance opens opportunities for both fundamental studies of light–matter interaction at high photon energies and for potential applications of EUV polaritons as sensitive probes of electronic structure. The combination of XPDC and polaritonic hybridisation offers a pathway towards generating novel states of light– matter entanglement in the EUV regime. These results demonstrate that nonlinear X-ray optics can reveal exotic quantum phenomena previously inaccessible to experimental observation, establishing a foundation for future research in X-ray quantum optics.

Fig. 56: Polaritonic imprints in XPDC. Comparison of (a) experimental and (b) simulated scattering patterns. c)

Horizontal line-out of the XPDC cone (green diamonds) fitted with the two-level system model to extract coupling strength

V (red/blue lines). d) Simulated polaritonic dispersion, showing upper (red) and lower (blue) polariton branches

contributing to the corresponding parts of the line-out (c). Inset highlights the Rabi-splitting 2V = 1.64 eV.