Polarization-Entangled Photons from a Warm Atomic Ensemble Using a Sagnac Interferometer.

We report a polarization-entangled photon-pair source obtained via spontaneous four-wave mixing (SFWM) in a Doppler-broadened atomic ensemble of ^{87}Rb atoms using a Sagnac interferometer. Collective two-photon coherence occurs in the Doppler-broadened ladder-type atomic system with bidirectional counterpropagating two-photon resonant pump and coupling fields; hence, polarization-entangled photon pairs are collectively radiated in the phase-matched direction. Without phase stabilization of the interferometry for polarization entanglement, we robustly produce all four Bell states via a polarization Sagnac configuration. The brightness, stability, and temporal purity advantages provided by our polarization-entangled SFWM photon-pair source have very important applications in the context of a practical scalable quantum network.