Zou Xy

Observation of induced coherence in two-photon downconversion

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.

Time-varying induced coherence

A previously reported interference experiment, involving two parametric downconverters with their idlers aligned, is reanalyzed with a time-dependent modulator introduced between the two downconverters. This change modifies the analysis nontrivially. It is found that the visibility of the interference effect is completely determined by the state of the modulator at one moment on the light cone backward from the detected photon, even though no photon may be passing the modulator at that time. The implication for a delayed-choice experiment is discussed briefly.

New technique for controlling the degree of coherence of two light beams

Abstract A new technique for varying the degree of coherence between two light beams through the amplitude of a third beam, without affecting the light intensities, is described. The method makes use of the process of parametric down-conversion in two similar non-linear crystals which are coherently pumped with their idlers aligned, and it allows very precise variation of the degree of coherence.

Optical Phase Information Due to the Vacuum in Two-Photon Down-Conversion

According to QED single photons carry no phase information about the optical field, yet it is possible for a superposition of states, including the vacuum, to carry phase memory. We have demonstrated this principle by showing experimentally that the photon pairs generated in the process of parametric down-conversion,1 through their entanglement with the vacuum state, contain information about the phase of the pump field, as was recently pointed out by Grangier et al.2 However, our experimental technique is different from the one they proposed.