  The ideal photon-pair source for building up multi-qubit states needs to
produce indistinguishable photons with high efficiency. Indistinguishability is
crucial for minimising errors in two-photon interference, central to building
larger states, while high heralding rates will be needed to overcome
unfavourable loss scaling. Domain engineering in parametric down-conversion
sources negates the need for lossy spectral filtering allowing one to satisfy
these conditions inherently within the source design. Here, we present a
telecom-wavelength parametric down-conversion photon source that operates on
the achievable limit of domain engineering. We generate photons from
independent sources which achieve two-photon interference visibilities of up to
$98.6\pm1.1\%$ without narrow-band filtering. As a consequence, we reach net
heralding efficiencies of up to $67.5\%$, which corresponds to collection
efficiencies exceeding $90\%$.
