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- Häffner, H., Schmidt-Kaler, F., Hänsel, W. et al. Appl. Phys. B (2005) 81: 151. doi:10.1007/s00340-005-1917-z
It is common belief among physicists that entangled states of quantum systems lose their coherence rather quickly. The reason is that any interaction with the environment which distinguishes between the entangled sub-systems collapses the quantum state. Here we investigate entangled states of two trapped Ca+ ions and observe robust entanglement lasting for more than 20 s.