  Entangled photon pairs are essential for a multitude of photonic quantum
applications. To date, the best performing solid-state quantum emitters of
entangled photons are semiconductor quantum dots operated around liquid-helium
temperatures. To favor the widespread deployment of these sources, it is
important to explore and understand their behavior at temperatures accessible
with compact Stirling coolers. Here we study the polarization entanglement
among photon pairs from the biexciton-exciton cascade in GaAs quantum dots at
temperatures up to 65 K. We observe entanglement degradation accompanied by
changes in decay dynamics, which we ascribe to thermal population and
depopulation of hot and dark states in addition to the four levels relevant for
photon pair generation. Detailed calculations considering the presence and
characteristics of the additional states and phonon-assisted transitions
support the interpretation. We expect these results to guide the optimization
of quantum dots as sources of highly entangled photons at elevated
temperatures.
