  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 (via 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. Our techniques to control the environment can
be used to enable novel quantum-computation, state-engineering, and simulation
paradigms based on dissipation and decoherence.
