  We propose an efficient, scalable, and deterministic scheme to generate
multiple indistinguishable photons over independent channels, on demand. Our
design relies on multiple single-photon sources, each coupled to a waveguide,
and all of them interact with a common cavity mode. The cavity synchronizes and
triggers the simultaneous emission of one photon by each source, which are
collected by the waveguides. For a state-of-the-art circuit QED implementation,
this scheme supports the creation of single photons with purity,
indistinguishability, and efficiency of $99\%$ at rates of $\sim $MHz. We also
discuss conditions to produce up to 100 photons simultaneously with generation
rates of hundreds of kHz. This is orders of magnitude more efficient than
previous demultiplexed sources for boson sampling and enables the realization
of deterministic multi-photon sources and scalable quantum information
processing with photons.
