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Quantum dot as source of entangled photon pairs

Similarly to atomic systems quantum dots can be driven resonantly. Therefore, the population can be exchanged between the ground and the excited state coherently. This means that by a proper choice of the excitation pulse area one can excite the quantum dot with near unity probability. Quantum dot photons can give us polarization and time-bin entanglement. While the degree of entanglement for the polarization entanglement depends predominantly on quantum dot's symmetry, the time-bin entanglement can be achieved from asymmetric dots too.

What makes quantum dots so special?

While the vast majority of proof-of-principle quantum information experiments were performed using process of spontaneous parametric downconversion this is not the most efficient method to generate single photons and entangled photon pairs. This is so, due to statistics of the pair emission, which is inherently thermal. In the other hand, single quantum emitters feature discrete energy levels that can be excited and driven resonantly and coherently. Furthermore, in a especial excitation regime called two-photon resonant excitation one can generate photon pairs (biexciton-exciton cascade) with near unity probability. As the energy levels of the quantum dot are atom-like the emitted photons exhibit sub-Poissonian statistics.


Figure 1: Using tailored laser pulses we excite quantum dots from the ground to the excited state resonantly. This generates a train of single photons.