X-ray photon pairs with highly suppressed background

We describe an experiment demonstrating the possibility to generate collinear indistinguishable x-ray photon pairs with a 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 2 orders of magnitude. We measure the pure coincidence spectrum without any energy discrimination with a bandwidth of 1.5 keV at the full width at half maximum.

Figure 1

(a) Phase-matching scheme. (b) Schematic of the experimental setup. The indices $p,s,$ and $i$ are for the pump, signal, and idler, respectively, $\vec{G}$ represents the reciprocal lattice vector orthogonal to the (660) atomic planes, and $\widehat{e}$ is a polarization vector. $D1$ and $D2$ indicate the two detectors.

Figure 2

Typical raw spectrum measured by a single detector. The Compton peak is at the calculated photon energy. The possible nature of the second Compton-like peak is discussed in the text. The inset shows the structure at the energies of interest. Several fluorescence peaks originated mainly from elements such as iron and lead are clearly seen in the range of 6-15 keV.

Figure 3

Energy histograms of the measured coincidence count rate at the degeneracy for detectors $D1$ and $D2$ are depicted on (a) and (b), respectively. Raw coincidence data and results of applying the energy conservation filter, i.e., the sum of the energies of each coincidence event equals the pump energy within an energy window of 500 eV. Solid lines are theoretical calculations.

Figure 4

Coincidence diagrams after applying the energy conservation for the off-degeneracy cases (a) ${\omega }_s/{\omega }_{s0}=1.2$ and (b) ${\omega }_s/{\omega }_{s0}=1.4$.