  More than 80 years passed since the first publication on entangled quantum
states. In this period of time the concept of spookily interacting quantum
states became an emerging field of science. After various experiments proving
the existence of such non-classical states, visionary ideas were put forward to
exploit entanglement in quantum information science and technology. These novel
concepts have not yet come out of the experimental stage, mostly because of the
lack of suitable, deterministic sources of entangled quantum states. Among many
systems under investigation, semiconductor quantum dots are particularly
appealing emitters of on-demand, single polarization-entangled photon-pairs.
Although, it was originally believed that quantum dots must exhibit a limited
degree of entanglement related to numerous decoherence effects present in the
solid-state. Recent studies invalidated the premise of unavoidable entanglement
degrading effects. We review the relevant experiments which have led to these
important discoveries and discuss the remaining challenges for the anticipated
quantum technologies.
