Franson interferometer for characterizing highly non-degenerate correlated biphoton sources

We report the demonstration of a Franson interferometer for a highly non-degenerate time-energy entangled biphoton source. A Franson interferometer realizes a Bell test for time-energy entanglement and can thus be used to verify entanglement distribution in quantum networks and security in quantum key distribution. The highly non-degenerate source has signal and idler photons at 810 nm and 1550 nm, respectively, making this source a potential interconnect between free-space and fiber optic quantum networks. We observe a single channel visibility of V=0.992(6) with an average visibility over the four output channels of V=0.984(3). These visibilities overcome the visibility threshold necessary in closing the postselection loophole in the Franson interferometer Bell test.