Cavity with Iron Nuclei Slows Down X Rays

Abstract

In vacuum, the propagation of photons proceeds with the same speed for each color (or frequency) of light. A laser pulse, corresponding to a broad range of frequencies, will thus propagate in vacuum without any deformation or delay. However, if the pulse enters a medium with an index of refraction that depends strongly on the frequency, then some colors will travel faster than others, causing a change in the overall pulse. In some cases this dispersion can slow down light [1, 2], so that photons at the end of the pulse catch up to those at the front of the pulse. In this way, slow light propagation can yield higher densities of photons, i.e., higher intensities, inside the medium, which can be used to explore nonlinear and/or quantum effects in the light-matter interactions. Slow light has been studied extensively at visible frequencies, but recent work has begun to explore related phenomena in x rays [3]. Kilian Heeg of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and his colleagues have measured slow x rays in a dispersive system [4]. The team directed x rays from a synchrotron source into a specially designed cavity and recorded a time delay in the outgoing light, which implied a speed decrease by 4 orders of magnitude. Further progress will help to concentrate high-energy light for nonlinear x-ray optics with possible applications, e.g., in super-resolved imaging or in quantum optics and quantum cryptography in the x-ray regime.