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 $\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}{\mathrm{O}}_4$ yields a measured flux of 5000 polarization-entangled pairs/s/mW of pump power in a $1\text{−}\mathrm{nm}$ 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.

• Received 29 September 2005

DOI:https://doi.org/10.1103/PhysRevA.73.012316