Tabletop creation of entangled multi-keV photon pairs and the Unruh effect

Electrons moving in a strong periodic electromagnetic field (e.g., laser or undulator) may convert quantum vacuum fluctuations into pairs of entangled photons, which can be understood in terms of the Unruh effect. Apart from verifying this striking phenomenon, the considered effect may allow the construction of a tabletop source for entangled photons (photon pair laser) and the associated quantum-optics applications in the multi-keV regime with near-future facilities.