Number of x-ray photons (color) as a function of time (vertical) and detuning (horizontal) shows that near-resonant pulses are delayed. Selected for an Editors’ Suggestion and a Viewpoint in Physics.

From the article:

Tunable Subluminal Propagation of Narrow-band X-Ray Pulses
Kilian P. Heeg, Johann Haber, Daniel Schumacher, Lars Bocklage, Hans-Christian Wille, Kai S. Schulze, Robert Loetzsch, Ingo Uschmann, Gerhard G. Paulus, Rudolf Rüffer, Ralf Röhlsberger, and Jörg Evers
Phys. Rev. Lett. 114, 203601 (2015)

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