X-ray interferometry

has been achieved in a cavity. Due to their high energies, x rays are notoriously difficult to reflect at high angles to a surface. Indeed, x-ray telescopes in orbit use grazing-incidence mirrors to gradually focus x rays onto a detector. Recently, however, physicists at the University of Hamburg, Germany, succeeded in reflecting x rays directly back from special sapphire crystal mirrors. The price for achieving normal-incidence reflectivity is that it operates only over a few-meV spectral range near a fixed energy determined by Bragg’s law. The group used the mirrors to build a prototype Fabry–Pérot interferometer (resonator) for hard x rays. In their instrument’s 50-mm cavity, the physicists observed as many as 60 reflections, and measured 0.76-µeV-wide resonances for 14.3 keV x rays. The interference shows up as a modulation, in both time and wavelength, of the radiation that exits the cavity. The work could lead to high-resolution...