Entangled photons from quantum dot devices

Single semiconductor quantum dots provide a viable source for entangled photons, as needed for quantum computation and quantum cryptography: a biexciton, consisting of two electron-hole pairs with opposite spin orientations, decays radiatively through two intermediate optically active exciton states. If the exciton states are degenerate, the two decay paths of the cascade differ in polarization but are indistinguishable otherwise. Therefore, the emitted photons are entangled in polarization. Such ideal performance is usually spoiled by the electron-hole exchange interaction, which splits the intermediate exciton states by a small amount and attaches a "which-path" information to the photon frequencies. Therefore this process deteriorates the photon entanglement. In addition, exciton states in semiconductor quantum dots are subject to various interactions with the solid-state environment of the embedding host material, which might threaten the performance of quantum-dot based photon sources. The primary goal of this project is to theoretically evaluate the viability of existing proposals for quantum-dot based photon sources, to critically examine the detailed nature of dephasing losses to the solid state environment, and to seek for more refined protocols which might allow for a fully deterministic and flexible creation of specific photon wave packets. We will (1) develop a microscopic description of cross dephasing and phonon dephasing, (2) provide a realistic modelling of light-matter coupling in microcavities and plasmonic structures, (3) address the issue of strong coupling in quantum-dot based photon sources, and (4) evaluate the possibility to design specific photon states and wave packets by means of coherent control of exciton states. The results will help us to clarify the potential of quantum-dot based entangled-photon sources, and will provide protocols for creating specific photon states on demand.