X-ray interferometry with microelectronvolt resolution

We demonstrate an interferometer for hard x rays with two back-reflecting sapphire crystal mirrors-a prototype x-ray Fabry-Perot interferometer. A finesse of 15 and 0.76 μeV broad Fabry-Perot transmission resonances are measured by the time response of the interferometer. Interference patterns are observed directly in spectral dependences of reflectivity.