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Entanglement in a quantum system can be demonstrated experimentally by performing the measurements prescribed by an appropriate entanglement witness. However, the unavoidable mismatch between the implementation of measurements in practical devices and their precise theoretical modelling generally results in the undesired possibility of false-positive entanglement detection. Such scenarios can be avoided by using the recently developed device-independent entanglement witnesses (DIEWs) for genuine multipartite entanglement. Similarly to Bell inequalities, the only assumption of DIEWs is that consistent measurements are performed locally on each subsystem. No precise description of the measurement devices is required. Here we report an experimental test of DIEWs on up to six entangled 40Ca+ ions. We also demonstrate genuine multipartite quantum nonlocality between up to six parties with the detection loophole closed. [ABSTRACT FROM AUTHOR]

Multiparticle entanglement leads to richer correlations than two-particle entanglement and gives rise to striking contradictions with local realism, inequivalent classes of entanglement and applications such as one-way or topological quantum computing. When exposed to decohering or dissipative environments, multiparticle entanglement yields subtle dynamical features and access to new classes of states and applications. Here, using a string of trapped ions, we experimentally characterize the dynamics of entanglement of a multiparticle state under the influence of decoherence. By embedding an entangled state of four qubits in a decohering environment (through spontaneous decay), we observe a rich dynamics crossing distinctive domains: Bell-inequality violation, entanglement superactivation, bound entanglement and full separability. We also develop new theoretical tools for characterizing entanglement in quantum states. Recent quantum-computing, state-engineering and simulation paradigms driven by dissipative or decohering environments can benefit from the environment engineering techniques demonstrated here. [ABSTRACT FROM AUTHOR]