1. Institut für Experimentalphysik,
2. Institut für Theoretische Physik, Universität Innsbruck, Technikerstrae 25, A–6020 Innsbruck, Austria
3. Institut für Quantenoptik und Quanteninformation der Österreichischen Akademie der Wissenschaften, Technikerstrae 21a, A–6020 Innsbruck, Austria
4. †Present address: Fachrichtung Technische Physik, Universität des Saarlandes, Postfach 151150, D-66041 Saarbrücken, Germany

Correspondence to: H. Häffner^1,3 Correspondence and requests for materials should be addressed to H.H. (Email: Hartmut.Haeffner@uibk.ac.at).

Abstract

The generation, manipulation and fundamental understanding of entanglement lies at the very heart of quantum mechanics. Entangled particles are non-interacting but are described by a common wavefunction; consequently, individual particles are not independent of each other and their quantum properties are inextricably interwoven^{1, 2, 3}. The intriguing features of entanglement become particularly evident if the particles can be individually controlled and physically separated. However, both the experimental realization and characterization of entanglement become exceedingly difficult for systems with many particles. The main difficulty is to manipulate and detect the quantum state of individual particles as well as to control the interaction between them. So far, entanglement of four ions⁴ or five photons⁵ has been demonstrated experimentally. The creation of scalable multiparticle entanglement demands a non-exponential scaling of resources with particle number. Among the various kinds of entangled states, the 'W state'^{6, 7, 8} plays an important role as its entanglement is maximally persistent and robust even under particle loss. Such states are central as a resource in quantum information processing⁹ and multiparty quantum communication. Here we report the scalable and deterministic generation of four-, five-, six-, seven- and eight-particle entangled states of the W type with trapped ions. We obtain the maximum possible information on these states by performing full characterization via state tomography¹⁰, using individual control and detection of the ions. A detailed analysis proves that the entanglement is genuine. The availability of such multiparticle entangled states, together with full information in the form of their density matrices, creates a test-bed for theoretical studies of multiparticle entanglement. Independently, 'Greenberger–Horne–Zeilinger' entangled states¹¹ with up to six ions have been created and analysed in Boulder¹².

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