Interferometric measurement of nuclear resonant phase shift with a nanoscale Young double waveguide

Abstract The phase shift of an electromagnetic wave, imprinted by its interaction with atomic scatterers, is a central quantity in optics and photonics. In particular, it encodes information about optical resonances and photon–matter interaction. Although being a routine task in the optical regime, interferometric measurements of phase shifts in the X-ray frequency regime are notoriously challenging due to the short wavelengths and associated stability requirements. As a result, existing X-ray interferometers are ill-suited for light confined to the nanometre scale. Here we demonstrate a nanoscale interferometer, inspired by Young’s double-slit experiment, to measure the dispersive phase shift due to the 14.4-keV nuclear resonance of the Mössbauer isotope 57 Fe coupled to an X-ray waveguide. From the single-photon interference patterns, we precisely extract the phase shifts in the vicinity of the nuclear resonance resolved in photon energy by using Bayesian inference. We find that the combined information from phase shift and absorbance reveals microscopic coupling parameters, which are not accessible from the absorption measurements alone. The demonstrated principle lays the basis for integrated X-ray interferometric sensors.