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 x-ray spectral filters, phase imaging with enhanced sensitivity, x-ray clocks, and a new way of calibrating length measurements at the atomic scale. The addition of a metallic film to the sapphire mirrors could also lead to new combined optical–x-ray devices. (Yu. V. Shvyd’ko et al., Phys. Rev. Lett. 90, 013904, 2003 .)