Preservation of energy-time entanglement in a slow light medium

We demonstrate the preservation of entanglement of an energy-time entangled biphoton through a slow light medium. Using the D1 and D2 fine structure resonances of Rubidium, we delay one photon of the 1.5 THz biphoton by 1.3 correlation lengths and measure the fourth order correlation fringes. After the group delay the fringe visibility is reduced from 97.0±4.4% to 80.0±4.8%, but is still sufficient to violate a Bell inequality. We show that temporal broadening is the primary mechanism for reducing the fringe visibility and that smaller bandwidths lead to greatly reduced broadening.