  A quantum emitter efficiently coupled to a nanophotonic waveguide constitutes
a promising system for the realization of single-photon transistors,
quantum-logic gates based on giant single-photon nonlinearities, and high
bit-rate deterministic single-photon sources. The key figure of merit for such
devices is the $\beta$-factor, which is the probability for an emitted single
photon to be channeled into a desired waveguide mode. We report on the
experimental achievement of $\beta = 98.43 \pm 0.04\%$ for a quantum dot
coupled to a photonic-crystal waveguide, corresponding to a single-emitter
cooperativity of $\eta = 62.7 \pm 1.5$. This constitutes a nearly ideal
photon-matter interface where the quantum dot acts effectively as a 1D
"artificial" atom, since it interacts almost exclusively with just a single
propagating optical mode. The $\beta$-factor is found to be remarkably robust
to variations in position and emission wavelength of the quantum dots. Our work
demonstrates the extraordinary potential of photonic-crystal waveguides for
highly efficient single-photon generation and on-chip photon-photon
interaction.
