  The generation and long-haul transmission of highly entangled photon pairs is
a cornerstone of emerging photonic quantum technologies, with key applications
such as quantum key distribution and distributed quantum computing. However, a
natural limit for the maximum transmission distance is inevitably set by
attenuation in the medium. A network of quantum repeaters containing multiple
sources of entangled photons would allow to overcome this limit. For this
purpose, the requirements on the source's brightness and the photon pairs'
degree of entanglement and indistinguishability are stringent. Despite the
impressive progress made so far, a definitive scalable photon source fulfilling
such requirements is still being sought for. Semiconductor quantum dots excel
in this context as sub-poissonian sources of polarization entangled photon
pairs. In this work we present the state-of-the-art set by GaAs based quantum
dots and use them as a benchmark to discuss the challenges to overcome towards
the realization of practical quantum networks.
