Polarization-Frequency Entangled Narrowband Photon Pairs

We demonstrate a robust scheme to produce narrowband biphotons with polarization-frequency-coupled entanglement from spontaneous four-wave mixing (SFWM) in laser cooled atoms with a right-angle geometry. Making use of an acousto-optic modulator and polarization-dependent two-photon interference, we create a coupled hyperentanglement between the polarization and frequency domains from unentangled SFWM photon pairs. We further demonstrate that, making use of the coupling effect, we can transfer the phase of a complex polarizer into the frequency entanglement and create a tunable two-mode frequency entangled Bell state.