  We perform full time resolved tomographic measurements of the polarization
state of pairs of photons emitted during the radiative cascade of the confined
biexciton in a semiconductor quantum dot. The biexciton was deterministically
initiated using a $\pi$-area pulse into the biexciton two-photon absorption
resonance. Our measurements demonstrate that the polarization states of the
emitted photon pair are maximally entangled. We show that the measured degree
of entanglement depends solely on the temporal resolution by which the time
difference between the emissions of the photon pair is determined. A route for
fabricating an on demand source of maximally polarization entangled photon
pairs is thereby provided.
