Complementarity Reveals Bound Entanglement of Two Twisted Photons
  We witness for the first time the generation of bound entanglement of two photon qutrits, whose existence has been predicted by the Horodecki family in 1998. Detection of these heavily mixed entangled states, from which no pure entanglement can be distilled, is possible using a key concept of Nature: complementarity. This captures one of the most counterintuitive differences between a classical and quantum world, for instance, the well-known wave-particle duality is just an example of complementary observables. Our protocol uses maximum complementarity between observables: the knowledge about the result of one of them precludes any knowledge about the result of the other. It enables ample detection of entanglement in arbitrary high-dimensional systems, including the most challenging case, the detection of bound entanglement. For this we manipulate "twisted" twin photons in their orbital angular momentum degrees of freedom. Our experimentally demonstrated "maximum complementarity protocol" is very general and applies to all dimensions and arbitrary number of particles, thus enables simple entanglement testing in high-dimensional quantum information and opens up the quest of understanding the meaning of this type of entanglement in Nature. 
Beatrix C. Hiesmayr and Wolfgang L\"offler
quant-ph