Fourth-order interference effects at large distances

We present an experimental demonstration of a fourth-order interference effect first suggested by Franson Phys. Rev. Lett. 62, 2205 (1989), where pair photons are directed to two separated Mach-Zehnder interferometers. Apparent nonlocal control of the output ports in which coincident photons are detected can be demonstrated. We show that this effect is present when one of the photons has passed through 170 m of multimode optical fiber. Such a system could form the basis of an interception-proof communication system.