  Two-photon resonant excitation of the biexciton-exciton cascade in a quantum
dot generates highly polarization-entangled photon pairs in a
near-deterministic way. However, the ultimate level of achievable entanglement
is still debated. Here, we observe the impact of the laser-induced AC-Stark
effect on the quantum dot emission spectra and on entanglement. For increasing
pulse-duration/lifetime ratios and pump powers, decreasing values of
concurrence are recorded. Nonetheless, additional contributions are still
required to fully account for the observed below-unity concurrence.
