Fhttp://qurope.eu/db/publications/matrix-product-operators-and-states-np-hardness-and-undecidability as PRL on the 16th October 2014
The paper, authored by M. Kliesch, D. Gross, and J. Eisert, has been published the 10th October 2014 on Phys. Rev. Lett. 113 (2014);