NONLOCAL CANCELLATION OF DISPERSIVE BROADENING IN MACH-ZEHNDER INTERFEROMETERS

We present theoretical and experimental second- and fourth-order inter ference patterns for entangled photons of different colors entering si ngle and dual Mach-Zehnder interferometers in which dispersive element s have been deliberately placed. Although photon wave packets are gene rally broadened, as well as delayed, by passage through dispersive opt ical,elements, coincidence measurements made with entangled photon pai rs can be free of such broadening. This occurs for materials with part icular dispersive behavior, as well as when the dispersion is balanced in both arms. This nonlocal behavior arises from the entanglement in the frequencies of the down-converted pair photons. We also show that nonlocal pump-frequency oscillations are present in the coincidence ra te patterns for long path-length-difference times, confirming the robu stness of this phenomenon in the presence of dispersion. However, the magnitude of these oscillations is reduced when an arbitrary dispersiv e material is used in an unbalanced configuration. Difference-frequenc y oscillations are also robust in the presence of dispersion, although they decay for large path-length-difference times.