Observation of induced coherence in two-photon downconversion

Abstract

By using two mutually coherent light beams of adjustable intensity to induce downconversions of the signal beams in two similar nonlinear crystals and then allowing the two idlers to interfere, we measure the variation of the induced degree of coherence as a function of inducing intensity. The key parameter N is the average photon occupation number per mode of the inducing field. We show experimentally that the induced degree of coherence is given by N/(N+1), as is expected theoretically.