  We investigate the production of entangled X-ray photons by incoherent double
Compton scattering (IDCS) which seems to be an interesting alternative to
coherent double Compton scattering used in earlier work. Very recently it has
been proposed to use the Unruh effect as a source for photon pairs
(R.Sch\"utzhold et al.,Phys.Rev.Lett. 100, 091301 (2008)). We will discuss this
mechanism in comparison with inverse IDCS.
