X-ray photon pairs with highly suppressed background

The effect of Parametric Down Conversion (PDC) has been used widely as a source of non-classical states of electromagnetic radiation, such as entangled photons, squeezed light, single photons. The extension of quantum optics into the x-ray regime would open new possibilities for research. Despite the fact, that PDC in X-ray regime has been proposed about 45 years ago, no evidence of non-classical states of light in this regime has been reported yet. The main challenge is Compton scattering. Typically it is many orders of magnitude larger than PDC and completely outweighs the signal from PDC. Perhaps, even more serious problem is that Compton scattering limits the input flux to a level at which Compton scattering starts saturating the detectors.

I will describe an experiment demonstrating the possibility to generate collinear x-ray photon pairs with highly suppressed background. By choosing angles near 90 degrees between the detectors and the pump beam, where the pump polarization is in the scattering plane, we improve the signal-to-noise ratio by nearly three orders of magnitude. We measure about two coincidence counts per second with a bandwidth of 1.5 keV at the full width at half maximum.