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arXiv:quant-ph/0302042 · Physical Review Letters

Experimental Observation of Four-Photon Entanglement from Parametric Down-Conversion

Manfred Eibl, Sascha Gaertner, Mohamed Bourennane, Christian Kurtsiefer, Marek Żukowski, Harald Weinfurter

Published May 22, 2003• 181 citations
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Abstract

We observe polarization entanglement between four photons produced from a single down-conversion source. The nonclassical correlations between the measurement results violate a generalized Bell inequality for four qubits. The characteristic properties and its easy generation with high interferometric contrast make the observed four-photon state well suited for implementing advanced quantum communication schemes such as multiparty quantum key distribution, secret sharing, and telecloning.

DOI
10.1103/physrevlett.90.200403

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We observe polarization entanglement between four photons produced from a single down-conversion source. The nonclassical correlations between the measurement results violate a generalized Bell inequality for four qubits. The characteristic properties and its easy generation with high interferometric contrast make the observed four-photon state well suited for implementing advanced quantum communication schemes such as multiparty quantum key distribution, secret sharing, and telecloning.
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Multipartite entangled states are key elements for quantum information processing. Here we experimentally investigate the particular properties of a polarization-entangled four-photon state, which can be generated directly by second order parametric down-conversion. The perfect correlations and the invariance under local transformations enable one to encode one qubit of quantum information in a decoherence-free subspace and thus to communicate it safely over noisy quantum channels. Furthermore, we present an experimental method to detect genuine fourpartite entanglement using entanglement witness operators. The implementation of such operators requires only of few local polarization measurements but uniquely proves the genuine multipartite entanglement.
We obtain a four-photon polarization-entangled state with a visibility as high as $(95.35\ifmmode\pm\else\textpm\fi{}0.45)%$ directly from a single down-conversion source. A success probability of $(81.54\ifmmode\pm\else\textpm\fi{}1.38)%$ is observed by applying this entangled state to realize a four-party quantum communication complexity scenario, which comfortably surpasses the classical limit of 50%. As a comparison, two Einstein-Podolsky-Rosen pairs are shown to implement the scenario with a success probability of $(73.89\ifmmode\pm\else\textpm\fi{}1.33)%$. This four-photon state can also be used to fulfill decoherence-free quantum information processing and other advanced quantum communication schemes.
We report on a high-intensity source of polarization-entangled photon pairs with high momentum definition. Type-II noncollinear phase matching in parametric down conversion produces true entanglement: No part of the wave function must be discarded, in contrast to previous schemes. With two-photon fringe visibilities in excess of 97%, we demonstrated a violation of Bell's inequality by over 100 standard deviations in less than 5 min. The new source allowed ready preparation of all four of the EPR-Bell states.