  Integrated photonic circuits are one of the most promising platforms for
large-scale photonic quantum information systems due to their small physical
size and stable interferometers with near-perfect lateral-mode overlaps. Since
many quantum information protocols are based on qubits defined by the
polarization of photons, we must develop integrated building blocks to
generate, manipulate, and measure the polarization-encoded quantum state on a
chip. The generation unit is particularly important. Here we show the first
integrated polarization-entangled photon pair source on a chip. We have
implemented the source as a simple and stable silicon-on-insulator photonic
circuit that generates an entangled state with 91 \pm 2% fidelity. The source
is equipped with versatile interfaces for silica-on-silicon or other types of
waveguide platforms that accommodate the polarization manipulation and
projection devices as well as pump light sources. Therefore, we are ready for
the full-scale implementation of photonic quantum information systems on a
chip.
