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Physical Review Letters

Observation of nonlocal interference in separated photon channels

Z. Y. Ou, X. Y. Zou, L. J. Wang, L. Mandeļ

Published Jul 16, 1990• 222 citations
Publisher

Abstract

A two-photon coincidence experiment of the kind recently proposed by J. D. Franson [Phys. Rev. Lett. 62, 2205 (1989)] has been carried out with signal and idelr photons produced in the process of parametric down-conversion. The coincidence rate registered by the two detectors is found to exhibit a cosine variation with the optical path difference, with periodicity equal to the wavelength.

DOI
10.1103/physrevlett.65.321

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A two-photon coincidence experiment of the kind recently proposed by J. D. Franson [Phys. Rev. Lett. 62, 2205 (1989)] has been carried out with signal and idelr photons produced in the process of parametric down-conversion. The coincidence rate registered by the two detectors is found to exhibit a cosine variation with the optical path difference, with periodicity equal to the wavelength.
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An experiment has been performed in which signal and idler photons produced in the process of parametric downconversion are mixed at a beam splitter, and the mixed beam splitter outputs are directed to two photodetectors feeding a coincidence counter. Two optical filters with passbands centered at frequencies ω1, and ω2 are inserted just before the detectors. When the two-photon coincidence rate is measured as a function of the time delay δτ introduced between the two photons, it is found that the rate exhibits a cosine variation with δτ at the difference frequency |ω1− ω2|. The experimental results are consistent with a quantum analysis of the experiment but not with a classical treatment. We have observed a beat note with a period of 37 fs, which is several orders of magnitude shorter than the resolving time of the photodetectors. In principle a beat note should be observable at optical frequencies with this technique.
We report on two-photon quantum interference experiments in the standard telecommunication band. Two identical photons in the 1.5 mm wavelength band were generated in spatially separated modes by a type-I spontaneous parametric down-conversion process, and injected into a fiber-optic Hong-Ou-Mandel interferometer. Two-photon interference patterns of dip and spatial beating in the coincidence counting rate were observed by varying the difference in optical path lengths. The visibilities obtained in the net coincidences were close to the theoretical value of 100%. The raw visibilities were also well above the classical limit.
A two-photon correlation experiment of the kind that was proposed by Franson and has recently been carried out by two groups is analyzed. In this experiment two simultaneously produced photons are directed along two paths to two photodetectors without mixing. An unbalanced interferometer with delay time that is much longer than the coherence time of the light is inserted in each path. Yet the rate of two-photon detection by the two detectors in coincidence exhibits interference as the path difference is changed. The question of whether classical field theory can account for these observations is discussed. We show that no ergodic classical field is able to reproduce the experimental results, although a nonergodic field can give rise to interference through the artifice of generating an ensemble by initializing repeatedly.
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.