  Near-infrared Hong-Ou-Mandel quantum interference is observed in silicon
nanophotonic directional couplers with raw visibilities on-chip at 90.5%.
Spectrally-bright 1557-nm two-photon states are generated in a
periodically-poled KTiOPO4 waveguide chip, serving as the entangled photon
source and pumped with a self-injection locked laser, for the photon
statistical measurements. Efficient four-port coupling in the communications
C-band and in the high-index-contrast silicon photonics platform is
demonstrated, with matching theoretical predictions of the quantum interference
visibility. Constituents for the residual quantum visibility imperfection are
examined, supported with theoretical analysis of the sequentially-triggered
multipair biphoton contribution and techniques for visibility compensation,
towards scalable high-bitrate quantum information processing and
communications.
