The interference pattern of a circular photon source has long been used to define the optical diffraction limit. Here we show the breakdown of conventional x-ray diffraction theory for the fundamental case of a "source," consisting of a back-illuminated thin film in a circular aperture. When the conventional spontaneous x-ray scattering by atoms in the film is replaced at high incident intensity by stimulated resonant scattering, the film becomes the source of cloned photon twins and the diffraction pattern becomes self-focussed beyond the diffraction limit. The case of cloned x-ray biphotons is compared to and distinguished from the much studied case of entangled optical biphotons.
Using higher order coherence of thermal light sources, we can achieve enhancement of resolution of standard x-ray imaging techniques, such as x-ray diffraction and phase contrast imaging. The cost of implementing such schemes is minimal comparing to the schemes using entangled two-photon pairs. The proposed diffractive quantum crystallography using multipath interference of thermal light can be eventually free of radiation damage, because the diffraction pattern could be formed by using low energy photons of optical wavelength. Thus it is promising to apply the proposed quantum crystallography scheme to nanocrystalline or non-crystalline samples that are too difficult to be crystallized.