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Annual Meeting Optical Society of America

Quantum effects in optical interference

L. Mandeļ, Z. Y. Ou

Published Jan 1, 1989• 1 citation
Publisher

Abstract

A number of unusual optical interference effects have been observed that involve pairs of photons and cannot be understood or described in terms of classical electromagnetic waves. They include fourth-order interference of >50% visibility, which is unchanged when one of the beams is attenuated tenfold; measurement of the time interval between two photons with femtosecond accuracy; demonstration of phase memory in the two-photon down-conversion process due to vacuum entanglement.

DOI
10.1364/oam.1989.wh4

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A number of unusual optical interference effects have been observed that involve pairs of photons and cannot be understood or described in terms of classical electromagnetic waves. They include fourth-order interference of >50% visibility, which is unchanged when one of the beams is attenuated tenfold; measurement of the time interval between two photons with femtosecond accuracy; demonstration of phase memory in the two-photon down-conversion process due to vacuum entanglement.
Although we tend to think of optical interference as a classical wave phenomenon, recent experiments have revealed a number of effects that are not describable in classical terms. This is particularly true of interference effects involving the detection of a photon pair. We shall refer to them as fourth order interference, on the grounds that the joint probability density for the detection of one photon at r sub 1 at time t and another r sub 2 at time t is proportional to the fourth order correlation function of the field. This probability is readily measured when two photodetectors are positioned at r sub 1 and r sub 2 and the signals from the two detectors are fed to a coincidence counter that registers 'simultaneous' detections by the two detectors in coincidence. The topics covered include: fourth order interference measurements; the Franson experiment; and experimental test of the de Broglie guided wave theory.
Although interference of light is commonly regarded as a classical wave phenomenon, interesting fourth‐order interference effects are exhibited by correlated photon pairs, which violate classical probability. The effects manifest themselves in that the visibility is larger than allowed by classical theory, or the interference is independent of the light intensity, in violation of classical theory. In some situations classical optics predicts no interference at all, whereas quantum optics does. The interference technique has been used to measure time intervals between two photons with fsec resolution. Several examples of experiments in which non‐classical interference effects were observed will be described.
The quantum theory of fourth-order interference of light is presented in a general format and compared with classical wave theory. The conditions under which nonclassical phenomena occur are discussed. In particular, the interference between the quantum field and classical field may give rise to a nonclassical effect. For some special states of light, the interference pattern does not disappear even though one field is much stronger than the other, for which no classical analog exists. Fourth-order effects in the interference between two independent fields are analyzed in detail. It is pointed out that the fourth-order interference between independent fields will not disappear when the integration time of detection is of the order of the reciprocal bandwidth of the two light fields as long as the spectra of the two fields are symmetric around the same center frequency, and for some correlated fields, the interference does not vanish even if the detection time is much larger than the reciprocal bandwidth of the fields. A new type of fourth-order interference experiment involving a beam splitter is proposed in which local realism of the Einstein-Podolsky-Rosen form is violated for quantum mechanics. This general argument is then applied to the interference between two photons generated in the parametric down-conversion process. The possibility of violations of Bell's inequalities in interference experiments is investigated.
A new fourth-order interference experiment has been carried out and analyzed theoretically in classical and in quantum terms. Two photons produced in the process of parametric down-conversion provide the two inputs to a Mach-Zehnder type of interferometer, while two photodetectors coupled to a coincidence counter measure the output. The coincidence rate, after subtraction of accidentals, exhibits a cosine variation with the optical path difference, in agreement with quantum mechanics, but in disagreement with a classical analysis. By contrast, when two coherent light beams from a He:Ne laser are used as inputs to the interferometer, no fourth-order interference is observed.