  We demonstrate a simple, robust, and ultrabright parametric down-conversion
source of polarization-entangled photons based on a polarization Sagnac
interferometer. Bidirectional pumping in type-II phase-matched periodically
poled KTiOPO4 yields a measured flux of 5 000 polarization-entangled pairs/s
per mW of pump power in a 1-nm bandwidth at 96.8% quantum-interference
visibility. The common-path arrangement of the Sagnac interferometer eliminates
the need for phase stabilization for the biphoton output state.
