From quantum dots to quantum communication

A lot has been written and said already about why epitaxial semiconductor quantum dots (QDs) are excellent single photon sources with remarkable applications in the field of quantum information. However, if we really want to use the single photons emitted by optically excited QDs to exchange quantum information for distributed computing and secure communication, certain requirements must be fulfilled, as the properties of the photons have to be tailored to the needs of the quantum information application. Quantum communication for example requires a vanishing contribution of multi-photon emission events in combination with a high single photon yield, while interference based protocols need especially photons with high indistinguishability. These properties can be optimized by choosing appropriate optical excitation schemes such as Two-Photon-Resonant excitation (TPE) to minimize multi-photon emission in combination with a stimulation laser pulse to achieve high indistinguishability. In this context the quantum state in which the light is emitted in the Fock basis (pure vs. mixed) is investigated in this work. The emission into a pure state in the Fock basis leads to Photon-Number Coherence (PNC), which has crucial implications for the cryptographic protocols performed. Practical requirements for real-world applications also require ideally on-demand single photons from plug-and-play sources, excited with robust excitation schemes, which can be realized via chirped excitation. Thus, the influence of excitation power fluctuations on the performance of different protocols is also analyzed here. To ensure long-range transmission in optical fibers, single photons in the so-called Telecom wavelength range of lowest fiber loss around 1550 nm is desirable. While this can be achieved in different ways, direct emission of indistinguishable and on-demand Telecom single photons is crucial as is achieved in this work. InAs/InP QDs in randomly integrated mesa structures are excited coherently and compared to their deterministically integrated counterparts in Circular Bragg Gratings (CBGs). The potential of CBGs as the best photonic structure is further emphasized by demonstrating the strictly resonant excitation of QDs deterministically integrated into CBG structures, yielding ultra-short lifetimes, indistinguishability enhancement and traces of dynamically dressed states. Furthermore, the possibility to excite such fast decaying QDs at GHz rates is shown. For some quantum information protocols also entangled photons are needed, which are typically obtained from the biexciton-exciton emission cascade. Here, we go another route by using sequential TPE to generate energy-time entanglement encoded in the Fock basis. Together with extensive numerical and analytical modeling, the entanglement mode structure of the emitted photonic state is reconstructed. Finally, an implementation of strong quantum coin flipping showcases the compatibility of QD single photons and fast polarization modulators and yields a single photon quantum advantage.