  Planar nanostructures allow near-ideal extraction of emission from a quantum
emitter embedded within, thereby realizing deterministic single-photon sources.
Such a source can be transformed into M single-photon sources by implementing
active temporal-to-spatial mode demultiplexing. We report on the realization of
such a demultiplexed source based on a quantum dot embedded in a nanophotonic
waveguide. Efficient outcoupling (>60%) from the waveguide into a single mode
optical fiber is obtained with high-efficiency grating couplers. As a
proof-of-concept, active demultiplexing into M=4 spatial channels is
demonstrated by the use of electro-optic modulators with an end-to-end
efficiency of >81% into single-mode fibers. Overall we demonstrate four-photon
coincidence rates of >1 Hz even under non-resonant excitation of the quantum
dot. The main limitation of the current source is the residual population of
other exciton transitions that corresponds to a finite preparation efficiency
of the desired transition. We quantitatively extract a preparation efficiency
of 15% using the second-order correlation function measurements. The experiment
highlights the applicability of planar nanostructures as efficient multiphoton
sources through temporal-to-spatial demultiplexing and lays out a clear path
way of how to scale up towards demonstrating quantum advantages with the
quantum dot sources.
