  Photonic qubits are key enablers for quantum-information processing
deployable across a distributed quantum network. An on-demand and truly
scalable source of indistinguishable single photons is the essential component
enabling high-fidelity photonic quantum operations. A main challenge is to
overcome noise and decoherence processes in order to reach the steep benchmarks
on generation efficiency and photon indistinguishability required for scaling
up the source. We report on the realization of a deterministic single-photon
source featuring near-unity indistinguishability using a quantum dot in an
'on-chip' planar nanophotonic waveguide circuit. The device produces long
strings of $>100$ single photons without any observable decrease in the mutual
indistinguishability between photons. A total generation rate of $122$ million
photons per second is achieved corresponding to an 'on-chip' source efficiency
of $84 \%$. These specifications of the single-photon source are benchmarked
for boson sampling and found to enable scaling into the regime of quantum
advantage.
