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Journal of the Optical Society of America B

Time-varying induced coherence

L. J. Wang, X. Y. Zou, L. Mandeļ

Published Apr 1, 1992• 11 citations
Publisher

Abstract

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.

DOI
10.1364/josab.9.000605

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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.
Although induced coherence is generally associated with induced or stimulated emission, in the process of spontaneous parametric downconversion of light induced coherence can be generated on its own. This is demonstrated in an experiment in which the signal beams from two coherently pumped downconverters come together and interfere whenever the idler beam from the first downconverter (DC1) passes through and is aligned with the second downconverter (DC2). Misaligning the idlers, or blocking the first idler, makes the interference disappear. More generally, the degree of coherence can be controlled by varying the amplitude of the idler from DC1 that reaches DC2. The interpretation of this nonclassical interference is discussed in terms of the interpretation of the wave function or state vector of the emitted photon.
We perform a time-resolved quantum interference experiment based on the effect of induced coherence without induced emission using two pulsed parametric down-conversion sources. We periodically vary the transmission of the first source's idler beam and measure the time-dependent visibility of fringes in the interference between the two signal beams. The visibility is found to be correlated with the transmissivity with a time delay corresponding to a combined path of the first idler and the second signal beam. The experiment is discussed in the context of the delayed choice experiments.
An interference experiment with signal and idler photons produced by parametric down-conversion in two nonlinear crystals is described and analyzed theoretically. It is found that when the idlers for the two crystals are superposed and aligned, the idler photon from the first crystal can induce coherence between the two signals, but without inducing any additional emission. Blocking the first idler wipes out the interference. The implications for the interpretation of the quantum state vector are discussed, and this leads to the conclusion that the state reflects not only what is known, but to some extent also what is knowable in principle.