INTERFERENCE OF TWO PHOTONS OF DIFFERENT COLOR

We consider the previous terminterference of two photons with differentnext term colors in the context of a Hong-Ou-Mandel experiment, in which single previous termphotonsnext term enter each of the input ports of a beam splitter, and exit together in the same, albeit undetermined, output port. Such previous terminterferencenext term is possible if one uses an active (energy-non-conserving) beam splitter. We find scenarios in which one "red" and one "blue" previous termphotonnext term enter the beam splitter, and either previous termtwonext term red or previous termtwonext term blue previous termphotonsnext term exit, but never one of each color. We show how the precise form of the active beam-splitter transformation determines in what way the spectral degrees of freedom of the input previous termphotonsnext term should be related to each other for perfect destructive previous terminterference of the differentnext term-color components in the output. We discuss previous termtwonext term examples of active beam splitters: one is a gedanken experiment involving a moving mirror and the other is a more realistic example involving four-wave mixing in an optical fiber.