Entangled X-ray Photon Pair Generation by Free Electron Lasers

Abstract: Einstein, Podolsky and Rosen's prediction on incompleteness of quantum mechanics was overturned by experimental tests on Bell's inequality that confirmed the existence of quantum entanglement. In X-ray optics, entangled photon pairs can be generated by X-ray parametric down conversion (XPDC), which has certain wavelength window. Meanwhile, free electron laser (FEL) has successfully lased at X-ray frequencies recently. However, FEL is usually seen as a classical light source, and its quantum effects are considered minor corrections to the classical theory. Here we investigate entangled X-ray photon pair emissions in FEL. We establish a theory for coherently amplified entangled photon pair emission from microbunched electron pulses in the undulator. We numerically demonstrate the properties of entangled emission, and provide a scheme to generate highly entangled X-ray photon pairs, which is of great importance in X-ray quantum optics. Our work shows a unique advantage of FELs over synchrotrons in entangled X-ray photon pair generation.