Bright Entanglement via Dark States

One of the most mature quantum technologies is quantum communication and, in particular, quantum cryptography, which is about absolutely secure communication. For this as well as for the networking of quantum computers one needs so-called entangled quantum states of two light particles (photons). Bright sources of these special quantum states of light are important for practical applications. Nanometer-sized structures in semiconductors called quantum dots are among the best of these sources. In a predecessor project we were able to make great progress in the optical control of quantum dots, in particular we were successful in identifying and targeted excitation of so-called dark states. These are important for the controlled creation of time-entangled photon pairs. Time-entanglement means that the emission time of the two photons is not determined, but the two photons are always emitted perfectly simultaneously. In our project we aim at the first realization and investigation of this type of entanglement from semiconductor quantum dots. We expect to be able to achieve larger transmission rates and -distances in quantum communication. In contrast to other, traditional approaches, we will be able to extend to higher-dimensional entangled states of more than two photons and more than two time states. This project is carried out by a collaboration of four teams, two based in Austria, one in Germany and one in Poland. It treats all aspects including the theory of quantum dots and their interaction with light, the growth and fabrication of quantum dots in photonic nanostructures and their application in quantum communication experiments.