Down-Converted X-ray Pair Generation at an X-ray Free Electron Laser

Spontaneous Parametric Down Conversion (SPDC) is a quantum optical process in which a pair of photons are produced by the interaction of single photon in a nonlinear crystal. In the optical regime, such down-converted pairs have been used to showcase various quantum entanglement phenomena, including violating the Bell Inequalities. In the X-ray regime, a similar process can happen in a detuned perfect crystal, but other interactions give a much larger background noise to contend with.



In experiments at both SACLA and LCLS, we have successfully observed X-rays generated from spontaneous parametric down conversion in a diamond crystal. Compared to previous synchrotron results, these deliver a far higher instantaneous flux, making it more challenging to distinguish the signal and noise, but potentially allowing time-resolved measurements with the ultrashort pulses.



Our XFEL results demonstrate that we observed ~1000s of pairs per hour, which is over an order of magnitude higher than any previously published result. Future experiments will look at how to reduce the noise on the detector, allowing the pairs to be more easily distinguished. This would allow work towards imaging, initially of static structures and then of transient states, where the quantum correlations could allow signal to be distinguished from the background generated by laser-matter interactions, or to allow imaging of fragile objects which would be destroyed by the full dose of an XFEL pulse.