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Europhysics Letters (EPL)

Experimental Bell-State Analysis

Harald Weinfurter

Published Mar 10, 1994• 221 citations
Publisher

Abstract

State analysis of the joint system of two two-state particles is necessary for new types of quantum communication and experiments on Bell's theorem. It is shown how perfect quantum correlations occurring in two-photon interference effects can be utilized for the analysis of the so-called Bell states. The proposed system can give an efficiency as high as η = (1/4) using only linear optical elements.

DOI
10.1209/0295-5075/25/8/001

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State analysis of the joint system of two two-state particles is necessary for new types of quantum communication and experiments on Bell's theorem. It is shown how perfect quantum correlations occurring in two-photon interference effects can be utilized for the analysis of the so-called Bell states. The proposed system can give an efficiency as high as η = (1/4) using only linear optical elements.
Quantum correlations and entanglement in many-particle systems can be used to significantly increase and extend the performance of standard communication and computation techniques. Common to these methods is the joint measurement of two two-state particles in the so-called EPR-Bell-state basis, consisting of the four maximally entangled states of this two-particle system. In our experimental realizations we use interferometric EPR-Bell-state analysis, which allows us to transfer one of three messages per two-state particle (compared with the classical maximum of one of two) and to perform the teleportation with an efficiency of 50%.
Efficient teleportation is a crucial step for quantum computation and quantum networking. In the case of qubits, four different entangled Bell states have to be distinguished. We have realized a probabilistic, but in principle deterministic, Bell-state analyzer for two photonic quantum bits by the use of a nondestructive controlled-NOT gate based on entirely linear optical elements. This gate was capable of distinguishing between all of the Bell states with higher than 75% fidelity without any noise substraction due to utilizing quantum interference effects.
Bell's theorem reveals contradictions between the predictions of quantum mechanics and the EPR postulates for a pair of particles only in situations involving imperfect statistical correlations. However, with three or more particles, contradictions emerge even for perfect correlations. We describe an experiment which can be realized in the laboratory, using four-photon entangled states generated by parametric down-conversion, to demonstrate this contradiction at the level of perfect correlations.