Single spins confined in semiconductor quantum dots - artificial atoms in the solid-state - are attractive candidates for quantum mechanical bits, the fundamental units and building blocks of a quantum computer. The ability to address quantum dot spins optically allows us to initialise and manipulate the state of the quantum bit. Gaining information on the qubit, for example by reading out its state, not only requires state-selective optical excitation, but also access to the single photons scattered in response by the quantum dot. Further, for a distributed computer architecture where nodes of few quantum bits are interlinked via optical communication channels photonic quantum bits are required to faithfully transmit the quantum information.In this thesis we advocate resonant excitation of quantum dot transitions and collection of the resonance fluorescence to address two outstanding challenges: generating dephasing-free single photons for use as flying quantum bits and single-shot spin readout. To this end we investigate the spectral and first-order coherence properties of quantum dot resonance fluorescence. In particular, we directly observe highly coherent scattering in the low Rabi frequency limit which has remained unexplored for solid-state single photon emitters so far.At the same time, interactions with the semiconductor environment are revealed and quantified through their optical signatures: exciton-phonon coupling, nuclear spin dynamics and local electric field fluctuations signal a departure from the ideal atom-like behaviour.Taking advantage of the laser-like coherence of single phase-locked quantum dot photons in the Heitler regime, we demonstrate near-ideal two-photon quantum interference. This benchmark measurement is a precursor for the photonic entanglement of distant quantum dot spins in a quantum optical network, and the results here predict a high fidelity operation.Finally, moving to tunnel-coupled quantum dot molecules we show that the overlap of carrier wave functions in two closely spaced quantum dots forms new spin-selective optical transitions not available in single quantum dots. Then, the presence or absence of scattered photons reveals the electron spin. Intermittency in the quantum dot resonance fluorescence allowed us, for the first time, to observe spin quantum jumps in real-time.Both achievements - highly coherent photons and spin readout - provide the missing link to attempt creation of a small-scale quantum network now.
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