ATOMS AS NONLINEAR MIXERS FOR DETECTION OF QUANTUM CORRELATIONS AT ULTRAHIGH FREQUENCIES

Measurements of quantum correlations are reported for a frequency diff erence of 25 THz between the signal and idler output fields generated by a subthreshold nondegenerate optical parametric oscillator. By simu ltaneously exciting a two-photon transition in atomic Cs by a combinat ion of signal, idler, and ''references oscillator'' fields, we record modulation of the excited-state population due to quantum interference between two alternative excitation pathways. The observed phase-sensi tive modulation is proportional to the correlation function [(E) over cap(s)$(E) over cap(i)$] for the quantized signal and idler fields.