| In the process of spontaneous parametric down-conversion, an incident pump photon splits into a pair of signal and idler photons in a nonlinear medium. Some correlation effects between conjugate photons are investigated theoretically and experimentally.; In a newly revised theory the state of the quantized radiation field is represented by a continuous superposition of states of the pump, signal, and idler field modes. A model interaction Hamiltonian is assumed for the description of the coupling between the fields. The final state of the signal and idler fields which evolves from a one-pump-photon initial state, is found to be a state of two photons whose momenta are strongly correlated. It follows from this that signal and idler photons are always generated together, and that they exhibit Einstein-Podolsky-Rosen type nonlocal correlation in the domains of space, time, and energy. The theory shows that the degrees of spatial and temporal correlation between conjugate photons depend on the characteristics of the nonlinear medium. In particular, the correlation time, which can be much shorter than the coherence time of the pump light, is determined by the length of the medium through the spectrum.; Some correlation effects between signal and idler photons were investigated experimentally. The normalized intensity cross-correlation function of the signal and idler fields was found to be inversely proportional to the pump intensity. Time intervals between the detections of signal and idler photons were measured with 100-ps accuarcy. By making use of fourth-order interference effects between conjugate signal and idler photons, we were able to measure the correlation time of order 100 fs with 1-fs accuracy. It was shown that localized one-photon states of the idler field were realized experimentally. Direct counting of idler photons, conditioned on the detection of a signal photon, showed that there is only one photon in the idler field within a short time interval with a probability close to unity. Finally, it was demonstrated that signal and idler photons generated in spontaneous parametric down-conversion could be utilized in an optical communication channel based on coincidence photon counting, that allows very effective discrimination against background. (Abstract shortened with permission of author.)... |
