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Physical Review A

Violations of local realism in multiphoton interference experiments

Marek Żukowski

Published Jan 12, 2000• 8 citations
Publisher

Abstract

Spontaneous parametric down-conversion is a convenient source of pairs of entangled photons. Within special interferometric configurations involving either two sources of this kind or double emissions from such a source, one can observe three-photon Greenberger-Horne-Zeilenger--type interference effects. Unfortunately, events of some other types necessarily occur. It is shown that under specific operational conditions, thus far not met by the experiments, the entirety of the phenomena observed in such experiments has no local realistic model. However, one can show this only by analyzing the entire pattern of all events predicted by quantum mechanics.

DOI
10.1103/physreva.61.022109

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Spontaneous parametric down-conversion is a convenient source of pairs of entangled photons. Within special interferometric configurations involving either two sources of this kind or double emissions from such a source, one can observe three-photon Greenberger-Horne-Zeilenger--type interference effects. Unfortunately, events of some other types necessarily occur. It is shown that under specific operational conditions, thus far not met by the experiments, the entirety of the phenomena observed in such experiments has no local realistic model. However, one can show this only by analyzing the entire pattern of all events predicted by quantum mechanics.
A certain class of parametric down-conversion Bell-type experiments has the following features. In the idealized perfect situation it is in only 50% of the cases that each observer receives a photon; in the other 5050%50 one observer receives both photons of a pair while the other observer receives none. The standard approach is to discard the events of the second type. Only the remaining ones are used as the data input to some Bell inequalities. This raises justified doubts about whether such experiments could ever be genuine tests of local realism. We propose to take into account these ``unfavorable'' cases and to analyze the entire pattern of polarization and localization correlations. This departure from the standard reasoning enables one to show that the experiments are indeed a true test of local realism.
In recent years parametric-down-conversion photon sources have been proposed and used to test local realism via Bell-type inequalities. This study shows that this technique fails to discriminate between quantum mechanics and local realism in all those tests in which polarization correlations are measured, since dichotomic observables cannot be associated satisfactorily with the state produced in these experiments. A more adequate description of these photon sources leads to an alternative inequality for Einstein locality, which is always satisfied by quantum-mechanical predictions. The result can be extended to all the experiments in which only one-half of the emitted pairs are detected.
Spontaneous parametric downconversion in combination with linear optics was successfully used to observe a variety of multiphoton entangled states. Yet, experiments performed so far lacked flexibility, as each of the various setups was useful for only a particular multiphoton entangled state. In this paper, we describe how, by using multiphoton interference, one can observe entire families of multiphoton entangled states in the very same linear optical setup. Our method thus goes beyond the commonly used two-photon interference and turns out to be a very useful tool for state observation. We will discuss the interference of four and six photons at different types of beam splitters and show which families of entangled states are observable. The benefits of this approach are demonstrated in a four-photon interference experiment by observing a variety of highly entangled multiphoton states.
It is shown that both the visibility V =1/2 predicted for two-photon interference experiments with two independent sources and the visibility V = 1 predicted for two-photon interference experiments with a parametric down-conversion source can be explained in terms of a locally causal, adaptive, corpuscular, classical (non-Hamiltonian) dynamical system. Hence, there is no need to invoke quantum theory to explain the so-called nonclassical effects in the interference of signal and idler photons in parametric down conversion and a revision of the commonly accepted criterion of the nonclassical nature of light is called for.
A corpuscular simulation model for second-order intensity interference phenomena is discussed. It is shown that both the visibility V=1/2{\cal V}=1/2V=1/2 predicted for two-photon interference experiments with two independent sources and the visibility V=1{\cal V}=1V=1 predicted for two-photon interference experiments with a parametric down-conversion source can be explained in terms of a locally causal, modular, adaptive, corpuscular, classical (non-Hamiltonian) dynamical system. Hence, there is no need to invoke quantum theory to explain the so-called nonclassical effects in the interference of signal and idler photons in parametric-down conversion. A revision of the commonly accepted criterion of the nonclassical nature of light is needed.
We report a two-photon double-slit interference experiment. Two correlated photons, generated by spontaneous parametric downconversion and traveling in different directions, are made to pass through a double slit in such a way that the path of one photon can be identified with that of the other photon. No single-photon interference pattern is observed even if the path is not actually identified, but a spatial interference pattern appears in the joint detection counting rates of the photon pairs. It can easily be shown that these nonlocal phenomena cannot be exhibited by any kind of classical field.