Forced indistinguishability in ‘‘induced coherence without stimulated emission’’

The nonclassical phenomenon of ``induced coherence without simulated emission'' is analyzed under the condition where two nonlinear χ⁽²⁾ crystals are aligned in such a way that their idler emissions are collinear and inside a ringlike cavity with loss. This forced indistinguishability of the idler photons produces an interference between the free traveling conjugated signal beams when they are superposed. This interference is independent of the cavity loss, within the limits of a single-mode theory, and highly dependent on the position of the crystals inside the cavity. The maximum obtainable visibility of 100% reflects an induced coherence in the two crystals by the cavity field.