  Deterministic photon sources allow long-term advancements in quantum optics.
A single quantum emitter embedded in a photonic resonator or waveguide may be
triggered to emit one photon at a time into a desired optical mode. By
coherently controlling a single spin in the emitter, multi-photon entanglement
can be realized. We demonstrate a deterministic source of three-qubit
entanglement based on a single electron spin trapped in a quantum dot embedded
in a planar nanophotonic waveguide. We implement nuclear spin narrowing to
increase the spin dephasing time to $T_2^* \simeq 33$ ns, which enables
high-fidelity coherent optical spin rotations, and realize a spin-echo pulse
sequence for sequential generation of high-fidelity spin-photon and
spin-photon-photon entanglement. The emitted photons are highly
indistinguishable, which is a key requirement for subsequent photon fusions to
realize larger entangled states. This work presents a scalable deterministic
source of multi-photon entanglement with a clear pathway for further
improvements, offering promising applications in photonic quantum computing or
quantum networks.
