  Solid-state quantum dots are promising candidates for efficient light-matter
interfaces connecting internal spin degrees of freedom to the states of emitted
photons. However, selection rules prevent the combination of efficient spin
control and optical cyclicity in this platform. By utilizing a photonic crystal
waveguide we here experimentally demonstrate optical cyclicity up to
$\approx15$ through photonic state engineering while achieving high fidelity
spin initialization and coherent optical spin control. These capabilities pave
the way towards scalable multi-photon entanglement generation and on-chip
spin-photon gates.
