  We theoretically study the polarization entanglement of photons generated by
the biexciton cascade in a GaAs/InAs semiconductor quantum dot (QD), located in
a nano cavity. A detailed analysis of the complex interplay between photon- and
carrier coherences and phonons which occurs during the cascade allows us to
clearly identify where the entanglement is generated and destroyed. A quantum
state tomography is performed for varying exciton fine structure splittings. By
constructing an effective multi-phonon Hamiltonian which couples the continuum
of the wetting layer states to the QD we investigate the relaxation of the
biexciton and exciton states. This consistently introduces a temperature
dependence to the cascade. Considering typical Stranski-Karastanov grown QDs,
for temperatures around 80 K the degree of entanglement starts to be affected
by the dephasing of the exciton states and is ultimately lost above 120 K.
