Via carefully designed entanglement witnesses it become for the first time possible to observe genuine multipartite entanglement in a system of four photons. The photons were angled in their orbital angular momentum. This was a joint work between Vienna (theory) and Leiden (experiment).
A synopsis written by the PRL-editor can be found at physics.aps.org/synopsis-for/10.1103/PhysRevLett.116.073601
More information: Observation of four-photon orbital angular momentum entanglement, B. C. Hiesmayr, M, J. A. de Dood, W. Löffler, Physical Review Letters, 2016. On Arxiv: arxiv.org/abs/1508.01480
Read more at: https://phys.org/news/2016-02-leiden-physicists-entangle-rotating-photons.html#jCp
More information: Observation of four-photon orbital angular momentum entanglement, B. C. Hiesmayr, M, J. A. de Dood, W. Löffler, Physical Review Letters, 2016. On Arxiv: arxiv.org/abs/1508.01480
Read more at: https://phys.org/news/2016-02-leiden-physicists-entangle-rotating-photons.html#jCp
More information: Observation of four-photon orbital angular momentum entanglement, B. C. Hiesmayr, M, J. A. de Dood, W. Löffler, Physical Review Letters, 2016. On Arxiv: arxiv.org/abs/1508.01480
Read more at: https://phys.org/news/2016-02-leiden-physicists-entangle-rotating-photons.html#jCp
Publication: Observation of four-photon orbital angular momentum entanglement, B. C. Hiesmayr, M, J. A. de Dood, W. Löffler, Physical Review Letters, 2016. On Arxiv: arxiv.org/abs/1508.01480