Abstract: The present disclosure relates to a monolithically integrated photon pair source, in particular a monolithically constructed waveguide stack arranged to provide quantum-mechanically entangled photon pairs by means of Spontaneous Parametric Down Conversion (SPDC). A monolithically integrated photon pair source according to the present disclosure comprises a dielectric waveguide (DWL); and a Bragg reflection waveguide (BRWL) arranged to provide quantum-mechanically entangled photon pairs by means of spontaneous parametric down conversion, wherein the BRWL is disposed below the DWL and they are interacting with one another in a common coupling portion (KA) such that pump radiation guided in the DWL is cross-coupled into the BRWL.