  High visibility on-chip quantum interference among indistinguishable
single-photons from multiples sources is a key prerequisite for integrated
linear optical quantum computing. Resonant enhancement in micro-ring resonators
naturally enables brighter, purer and more indistinguishable single-photon
production without any tight spectral filtering. The indistinguishability of
heralded single-photons from multiple micro-ring resonators has not been
measured in any photonic platform. Here, we report on-chip indistinguishability
measurements of heralded single-photons generated from independent micro-ring
resonators by using an on-chip Mach-Zehnder interferometer and spectral
demultiplexer. We measured the raw heralded two-photon interference fringe
visibility as 72 +/- 3%. This result agrees with our model, which includes
device imperfections, spectral impurity and multi-pair emissions. We identify
multi-pair emissions as the main factor limiting the nonclassical interference
visibility, and show a route towards achieving near unity visibility in future
experiments.
