Coherent two-photon excitation of quantum dots

Single semiconductor quantum dots, due to their discrete energy structure, form single photon and twin photon sources that are characterized by a well-defined frequency of the emitted photons and inherently sub-Poissonian statistics. The single photons are generated through a recombination of an electron-hole pair formed by an electron from the conduction band and a hole from the valence band. When excited to the biexciton state quantum dots can provide pairs of photons emitted in a cascade. It has been shown that this biexciton-exciton cascade can deliver entangled pairs of photons. To achieve a deterministic generation of photon pairs from a quantum dot system one requires exciting it using a two-photon resonant excitation of the biexciton. Particularly, an efficient and coherent excitation of the biexciton requires the elimination of the single exciton probability amplitude in the excitation pulse and reaching the lowest possible degree of dephasing caused by the laser excitation. These two conditions impose contradictory demands on the excitation pulse-length and its intensity. We addressed this problem from a point of view that does not include interaction of the quantum dot with the semiconductor environment. We found an optimized operation regime for the system under consideration and provide guidelines on how to extend this study to other similar systems. In particular, our study shows that an optimal excitation process requires a trade-off between the biexciton binding energy and the excitation laser pulse length.