Experimental Test of the Bell Inequalities

Previous Experiments

I will focus in this overview on recent tests of the Bell inequalities that were mostly based on entangled states between photons. For a more detailed review, see this paper.

In the first experiments, the entangled states were produced via cascade decays of atoms, where the selection rules for dipole allowed transitions lead to an entanglement of the polarization between the two emitted photons. The earliest tests took place in 1972 and were performed by Clauser and coworkers (Clauser, 1972). Later Holt, Clauser and Fry performed additional experiments, the latter of which was the first one based on lasers. In the early 80's these experiments were improved by Aspect and coworkers (Aspect, 1980). They made the first attempt to enforce the locality condition by switching between different analyzers.

More recent experiments produced entanglement through parametric down conversion: a UV photon is split by the non-linear interaction within a crystal into two IR photons. Those experiments were pioneered by Shih (Shih, 1987) and Mandel et al. (Mandel, 1988) in so called type-I sources and developed further by Kwiat, Shih and Zeilinger et al. (Kwiat, 1995). The important step in the latter experiments was to switch to parametric down conversion sources of type-II. In those the entangled pairs are directly produced, resulting in large count rates and hence good signal/noise ratios in the experiment. This is important to achieve good statistics and hence meaningful results. Recently, Gisin and coworkers have experimentally demonstrated that the entanglement between two photons exists over a long time (Gisin, 1998). (Some scientists have explained the results of the photon experiments by the short time scales involved.) They performed a test of the Bell inequalities over a large distance with optical fibers. Unfortunately, due to their specific setup the detection efficiency loophole could not be closed in these experiments. However, they represent an important step foward.

In the latest development in the series of these experiments, Zeilinger and his group enforced the locality conditions by separating the two analyzers by approximately 400 m and randomly switching the orientation of the analyzing polarizers (Zeilinger, 1998). The year 2000 marked another important step forward. Wineland and coworkers performed a Bell inequalities test based on the entanglement of ions in traps, where they achieved a very high detection efficiency. Their detection scheme, however, does not allow them to enforce the locality condition simultaneously (Wineland, 2001).

In summary, tremendous progress has been made in the last couple of years from the conception of the Bell inequalities to the latest tests. Most of the experiments gave agreement with Quantum Mechanics. No experiment to date closes all loopholes simultaneously. A rigorous test of the Bell inequalities has not been performed Opens internal link in current window(c.f. theoretical background).


Prof. Dr. Thomas Walther

Laser und Quantenoptik
Institut für Angewandte Physik
Fachbereich 05 - Physik
Technische Universität Darmstadt
Schlossgartenstr. 7
D-64289 Darmstadt

+49 6151 16-20831 (Sekretariat)

+49 6151 16-20834 (Fax)




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