MULTIPHOTON ABSORPTION CROSS-SECTION AND VIRTUAL-STATE SPECTROSCOPY FOR THE ENTANGLED N-PHOTON STATE

Using a microscopic theory, we determine the entangled n-photon state generated both by the nonlinear process of spontaneous n-photon parame tric down-conversion and in a cascade of n - 1 crystals with two-photo n parametric down-conversions. Expressions for the entangled n-photon absorption cross sections are also obtained. The absorption cross sect ions exhibit a linear dependence on the photon flux density, and depen d on the entanglement times characterizing photons. The effect of rela tive path delay in the beams is discussed. It forms the basis for extr acting the quadrupole moments between states involved in the interacti on from the absorption cross section measured as a function of path de lay. Effects competitive with entangled three-photon absorption are di scussed.