Multipair-free source of entangled photons in the solid state

We investigate the effect of multiphoton emission on polarization-entangled photon pairs from a coherently driven quantum dot by comparing quantum state tomography and second-order autocorrelation measurements as a function of the excitation power. We observe that the relative (absolute) multiphoton emission probability is as low as pm=(5.6±0.6)×10−4 [p2=(1.5±0.3)×10−4] at the maximum source brightness, with a negligible effect on the degree of entanglement. In contrast with probabilistic sources of entangled photons, the multiphoton emission probability and the degree of entanglement remain practically unchanged against the excitation power over multiple Rabi cycles, while observing oscillations in the second-order autocorrelation function by more than one order of magnitude. Our results, explained by a model which links the second-order autocorrelation function to the actual multiphoton contribution in the two-photon density matrix, highlight that quantum dots can be regarded as a multipair-free source of entangled photons in the solid state.