  A deterministic source of coherent single photons is an enabling device of
quantum-information processing for quantum simulators, and ultimately a
full-fledged quantum internet. Quantum dots (QDs) in nanophotonic structures
have been employed as excellent sources of single photons, and planar
waveguides are well suited for scaling up to multiple photons and emitters
exploring near-unity photon-emitter coupling and advanced active on-chip
functionalities. An ideal single-photon source requires suppressing noise and
decoherence, which notably has been demonstrated in electrically-contacted
heterostructures. It remains a challenge to implement deterministic resonant
excitation of the QD required for generating coherent single photons, since
residual light from the excitation laser should be suppressed without
compromising source efficiency and scalability. Here, we present the design and
realization of a novel planar nanophotonic device that enables deterministic
pulsed resonant excitation of QDs through the waveguide. Through nanostructure
engineering, the excitation light and collected photons are guided in two
orthogonal waveguide modes enabling deterministic operation. We demonstrate a
coherent single-photon source that simultaneously achieves high-purity
($g^{(2)}(0)$ = 0.020 $\pm$ 0.005), high-indistinguishability ($V$ = 96 $\pm$ 2
%), and $>$80 % coupling efficiency into the waveguide. The novel
`plug-and-play' coherent single-photon source could be operated unmanned for
several days and will find immediate applications, e.g., for constructing
heralded multi-photon entanglement sources for photonic quantum computing or
sensing.
