Parametric Down-Conversion and Polarization Entangled Photons at X-Ray Wavelengths

In the past few decades, parametric down- conversion has been widely used as a source

for generating entangled photons in the optical regime (infrared, visible, ultraviolet).

Experiments using entangled photons have provided remarkable insight into numerous

quantum phenomena. Recent and expected improvements in brilliance and beam quality

of x-ray sources, together with new facilities such as the x-ray free-electron laser, offer

the possibility of extending the concepts of nonlinear and quantum optics to x-ray

energies. This talk will focus on the first steps toward this extension.

I will describe the physical mechanism of the x-ray second order nonlinearities, and my

current study of nonlinear and quantum optics at x-ray wavelengths. I will present my

recent experimental results and describe the first proposal for generating polarizationentangled

photons at x-ray energies via spontaneous parametric-down conversion. I will

show that each of the four Bell states can be generated by choosing the angle of incidence

and polarization of the pumping beam. I will also describe our experimental observations

and theory showing the generation of x-ray photon pairs when the absorption in the

generating crystal is substantial. Our results show that it is essential to retain the

Langevin terms to obtain agreement with experiment.