Greenberger-Horne-Zeilinger's have only one twin!


For four qubits (=two-level systems) one can find a basis of 16 GHZ states (GHZ...Greenberger-Horne-Zeilinger). Surprisingly, choosing one GHZ state one always finds a twin state in this basis such that an equal mixing results in a separable state, the genuinely multipartite entanglement property is fully lost. If, however, one mixes with another GHZ state in the basis, the resulting states does not lose all entanglement properties. This has been observed with two physical photons entangled in their polarisation and angular momentum degree of freedom (published in Scientific Reports).

In a joint collaboration between theory (Vienna) and experiment (Rome) researchers studied the geomety of mixtures of GHZ states published in Scientific Reports. Mixing GHZ states unmasks different entanglement features based on their particular local geometrical connectedness. Exploiting these local geometrical relations provides a toolbox for generating specific types of multipartite entanglement, each providing different benefits in outperforming classical devices. Controlling the different types of entanglement properties of the finally generated state will be the key for interesting applications. Alternatively, from the theoretical perspective it is also interesting to ask what is the minimum number of pure states in a convex combination needed for a state to have specific properties concerning entanglement.

This work was based on our theoretical studies on the magic simplex where a ``Schön ist so ein Ringelspiel (famous Viennese song by Hermann Leopoldi about the pleasure of using the Carousel in the Viennese Prater)'' (Merry Go Round) was found (Phys.Lett. A). And on our HMGH-framework (Phys.Rev.Lett.) that provided the experimental tool to distinguish different types of multipartite entanglement.