I believe this is the paper (letter) they're talking about: http://www.nature.com/nature/journal/v511/n7511/full/nature13559.html http://www.nature.com/nature/

I believe this is the paper (letter) they're talking about: http://www.nature.com/nature/journal/v511/n7511/full/nature13559.html http://www.nature.com/nature/journal/v511/n7511/full/nature1... and here's the version at arxiv.org: http://arxiv.org/abs/1403.4992 http://arxiv.org/abs/1403.4992 Note the use of the Bloch sphere to represent the state space: https://en.wikipedia.org/wiki/Bloch_sphere https://en.wikipedia.org/wiki/Bloch_sphere