Generating Correlated X-ray Pairs as a Low-Noise Low-Damage Probe

Spontaneous Parametric Down Conversion (SPDC) is a nonlinear, quantum optical process in which a single photon elastically produces a pair of lower-energy photons, generally termed the ‘signal’ and ‘idler’. These pairs serve as a convenient source of correlated optical photons, offering the ability to prepare distinctly quantum mechanical states of radiation. Entangled photon pairs, for instance, can showcase various quantum phenomena, including violating the Bell Inequalities.



We present results of our down conversion measurements at the APS synchrotron, in which we resolve individual photon arrival times and energies in order to demonstrate time and energy correlations which are the hallmark of down conversion. We also discuss considerations that would need to be accounted when transitioning this experiment to an X-ray free electron (XFEL) facility, where a different approach would be required due to the much higher instantaneous photon flux. However, it would also allow ultrafast probing with correlated pairs, which can greatly increase the signal to noise ratio due to the ‘heralding’ of signal photons.



At extreme states, such as in warm dense plasmas, the features of an XFEL source could allow signal to be distinguished from the broad bremsstrahlung background generated by intense laser-matter interactions. This has the potential to allow tomographic reconstruction of individual unique objects, such as organelles, which would be destroyed by the full dose of an XFEL pulse.